catamaran planing hull

Service Locator

Angler Endorsement
Boat Towing Coverage
Mechanical Breakdown
Insurance Requirements in Mexico
Agreed Hull Value
Actual Cash Value
Liability Only
Insurance Payment Options
Claims Information
Towing Service Agreement
Membership Plans
Boat Show Tickets
BoatUS Boats For Sale
Membership Payment Options
Consumer Affairs
Boat Documentation Requirements
Installation Instructions
Shipping & Handling Information
Contact Boat Lettering
End User Agreement
Frequently Asked Questions
Vessel Documentation
BoatUS Foundation
Government Affairs
Powercruisers
Buying & Selling Advice
Maintenance
Tow Vehicles
Make & Create
Makeovers & Refitting
Accessories
Electronics
Skills, Tips, Tools
Spring Preparation
Winterization
Boaters’ Rights
Environment & Clean Water
Boat Safety
Navigational Hazards
Personal Safety
Batteries & Onboard Power
Motors, Engines, Propulsion
Books & Movies
Cockpit Confessions
Communication & Etiquette
Contests & Sweepstakes
Colleges & Tech Schools
Food, Drink, Entertainment
New To Boating
Travel & Destinations
Watersports
Anchors & Anchoring
Boat Handling
← Seamanship

The Planing Power Catamaran: A Different Kind Of Cat

Planing powercats deliver the high speeds dayboaters and weekend anglers crave — but without so much pounding in choppy seas.

Rear view of a dual hull catamaran with two 200 horsepower outboard engines, a bimini top with fishing rods attched to it moored in turquoise blue water

The air cushion created between the two hulls dramatically reduces wave impact at running speeds. (Photo: World Cat)

Powercats are different beasts than sailing cats, and the powercats you're most likely to see on your local waters are those in the 20- to 40-foot range (like my 22-foot Glacier Bay). Unlike the big cruising powercats, which are more like cat trawlers with top ends maybe a little over 20 mph, smaller cats have planing hulls that perform much like today's modern powerboats.

Depending on the engine package, there are a few cats that top out in the lower 30s, lots in the lower 40s, some in the 50s, and a few that break 70 or even 80 mph.

While a similar length monohull may have a 40-mph cruising speed in a 2-foot chop, the monohull captain will pull back the throttles and cruise at 30 to avoid being beaten up. The cat guy, on the other hand, may be able to keep on doing 40 thanks to the smoother ride. But having two hulls underfoot does create some interesting similarities in how these different types of boats react to input from the helm. So you'll see a few of the tips here mirror those used for sailing or cruising catamarans. Whatever type of cat you may be captaining, remember the following:

Center the wheel and use only the throttles to control the boat. Powercats have their engines exceptionally widely spaced apart, and are far more responsive than monohulls when steered via throttles. Generally speaking, turning the steering wheel will only serve to reduce the effectiveness of working the throttles. This, of course, is assuming you have two engines. There are a few rare cats with one engine.
At identical rpm, the engine in forward will create more thrust than the engine in reverse. So even if the throttles are set evenly when opposed, the boat will likely slide forward a bit as opposed to spinning in its own length. As a result, when attempting to speed up the maneuver it's usually best to favor giving the reversed engine extra oomph as opposed to the one in forward (assuming you don't want to move forward while turning the boat).
Check the speed and direction of the wind before docking , and remember that some cats, particularly those with low draft, can be blown around more easily than many monohulls as there may be less hull below the waterline.
When docking in a new slip for the first time with lines that haven't been preset, bear in mind that once you're docked, securing the boat can be difficult in some situations because few powercats have centered cleats. Most will have a single cleat on either side, in some cases obstructed by a bow rail and/or pulpit, which can make crossing lines difficult.
Never shut those engines down until all the lines are secured . Again, remember that many cats can get blown out of kilter faster than the average monohull, and if you don't have lines preset, it may take a moment to figure out how to best secure them. Many a captain has done a perfect docking job and then shut off the engines, only for a gust of wind to push the boat right back out of the slip before the lines can be tied. Keep those engines running until the boat is 100% secure so you can apply power, if necessary, to maintain position.

Why Two Hulls?

Like all boats, catamarans come with distinct advantages (smooth ride, draft), and areas of compromise (docking, turning). Regardless of design aesthetics, the first question is usually: Why two hulls?

Mike Myers, vice president of product development for World Cat explains: "Catamaran hulls experience little to no drag or resistance to get on plane, resulting in greater fuel economy. They have a steady rise in speed and fuel burn with little to no spikes in fuel consumption."Planing powercats have a unique trait — which many cat lovers consider the top advantage over monohulls — the impact-absorbing cushion of air created by a compression tunnel between hulls.

And when it comes to beam, catamarans' parallel hulls create reliable stability, which helps to avoid heeling and capsizing, and greatly reduces the vessel roll at rest and at trolling speeds.

"Many boats are primarily designed around comfort for the captain. This usually means anyone at the front or sides of the boat takes most of the jostling,"Myers says. "The catamaran-style hull delivers ride comfort, smoothness, load distribution, and stability."That stability draws anglers to powercats of typically 20 to 40 feet; and cruisers to sailing cats 40 to 60 feet and beyond.

— Rich Armstrong

Taming The Cat

When it comes to handling powercats in open waters, the most important thing to remember is that all boats are different. Just as you wouldn't lump the handling characteristics of all monohulls together, the same goes for powercats. But many have a few common traits to consider.

Some powercats have relatively low buoyancy in the bow compared to monohulls, as many have very narrow hull entries . As a result, in some cases, idling into a sea can allow waves to break over the bow. Gaining some headway so the bow rises a bit and packs air into the tunnel can alleviate the issue.
Some planing powercats will run smoother at faster speeds than slower speeds, as they compress air in the tunnel between the two hulls. In these cases, speeding up may actually provide a more comfortable ride in some sea states as compared to slowing down. Depending on your boat, its tunnel may result in other differences from the monohull that you may be familiar with. Learning about these will improve you experience.
Some powercats display a "snap roll,"which is a very fast righting motion that can rock the boat uncomfortably, especially when drifting in a beam sea. In these cases, people who may want to drift often (such as anglers) will sometimes deploy a drift sock off the bow to reduce rocking and rolling.

Man wearing a white long-sleeve t-shirt fishing off the bow of a power catamaran as it cruises through the water

Photo: World Cat

In general, powercats are often more weight-sensitive than monohulls, especially when the bow is loaded down . It's always best to be aware of how you're loading your boat, and if the tunnel is slapping or the bow is digging into waves, consider shifting weight aft.
Some powercats, particularly older models, lean out in a turn rather than banking in. There's no way to eliminate this phenomenon (although trimming up an outboard engine when initiating a turn may reduce it a bit), so it's important to give passengers a warning to hold on before making any aggressive maneuvers.
"Sneezing,"or blowing a puff of mist out the front of the tunnel that the boat then runs through (getting everyone aboard damp), is a phenomenon associated with some powercats. In many cases, trimming the bow up a bit will significantly reduce or even eliminate sneezing.

The truth about ceramic coatings for boats.

Our editor investigates the marketing claims of consumer-grade ceramic coatings.

Fine-Tune Your Side Scan Fishfinder

Take your side-scanning fishfinder off auto mode, and you’ll be spotting your prey from afar in no time

DIY Boat Foam Decking

Closed-cell foam flooring helps make boating more comfortable. Here’s how to install it on your vessel

Click to explore related articles

Lenny Rudow

New Boats, Fishing & Electronics Editor, BoatUS Magazine

Top tech writer and accomplished sports fisherman, BoatUS Magazine Contributing Editor Lenny Rudow has written seven practical boating books, won 30 awards from Boating Writers International — many for his marine electronics articles – and two for excellence from the Outdoor Writers Association of America. He judges the NMMA Innovation Awards, and is Angler in Chief at FishTalk, his own Chesapeake-based publication. A great teacher and inspirational writer, Lenny hosts many of BoatUS Magazine’s very-popular how-to videos, which can be found on the BoatUS YouTube channel, or at BoatUS.com

BoatUS Magazine Is A Benefit Of BoatUS Membership

Membership Benefits Include:

Subscription to the print version of BoatUS Magazine

4% back on purchases from West Marine stores or online at WestMarine.com

Discounts on fuel, transient slips, repairs and more at over 1,200 businesses

Deals on cruises, charters, car rentals, hotel stays and more…

All for only $25/year!

We use cookies to enhance your visit to our website and to improve your experience. By continuing to use our website, you’re agreeing to our cookie policy.

PERFORMANCE TOPICS

Optimising Hull Lines for Performance

This article was inspired by a question about the rocker line in the new 8.5m cat Design 256 and I want to stick to the point, so we won’t turn it into a book, but I’ll discuss two issues, hull fineness ratio and some aspects of the rocker profile.

When you manipulate the hull form you’re adjusting the lines in three planes, waterplanes (plan view), buttocks (side view including the keel rocker) and the section shapes. So you need to be aware of how the shapes are changing in the other two planes as you manipulate any one of these three, or all three globally as is now possible with computer modelling.

There are two fundamental constants that you start with and don’t change throughout the process. The big one is the displacement or the amount of buoyancy you need.

If you make the hull finer by narrowing the waterlines you have to increase the draft or make the ends fuller to get back to the required displacement number.

If you flatten the rocker line you have to increase the hull width, fill out the ends, or square up the section shapes rather than having a V or rounded V.

The other constant is the longitudinal centre of buoyancy. You really can’t do any meaningful shaping of the hull form until you have settled on the these two constants.

A third number that we can plug in as a constant if we want to is the prismatic coefficient which describes bow much volume there is end the ends relative to the cross section shape in the middle of the boat, but in sailing boats this is of less importance compared to other factors.

The hull lines for Design 256, 8.5m Cat. It's that hump in the rocker line - right under the back of the cabin that brought up the question and is one of the key points discussed here.

Hull fineness.

Fine hulls are fast, but only in the higher speed range. There’s a misconception I come across quite a bit that you can add weight and windage and you’ll still be fast as long as your hulls are fine.

Well you won’t be. Your boat will simply sink to find the new state of equilibrium. If your transoms are submerged you’ll have more drag. If your bridge deck is too close to the water you’ll have slamming. Much better to be conservative with your displacement figure in the design stage than overly optimistic.

And fine hulls have more wetted area so you have more drag in light air where friction resistance is the primary drag factor.

I’ve seen promotional material for catamarans stating that the boat has less wetted area because it has fine hulls. For a given displacement the minimum wetted area is described by a sphere (or a semi sphere in the case of a floating object). The more you stretch it out in length, keeping the displacement constant, the more wetted area you have.

The more you make the section shape into a deep V or a broad U with tight corners, as opposed to a semicircle, the more wetted area you have. Add into the equation finer hulls are slower to tack.

So fine hulls are only an advantage if your boat is light and has enough sail area to ensure you’re travelling at speeds where form resistance is greater than skin resistance.

In my view the advantage of fine hulls is often overrated as it applies to cruising cats.

At the other end of the scale the resistance curve is fairly flat up to about 1:9 which is still quite fast in most conditions. From there the resistance rises steeply as the hull gets fatter and at 1:8 and fatter you’re suffering from some serious form drag.

Illustration of buoyancy distribution in catamaran hull

This is the rocker line isolated from the lines plan above (in blue) and and the red line shows a more moderate rocker line that achieves the same buoyancy and maintains the centre of buoyancy in the same position. The bow is to the right.

In the image lower right I've squashed it up and increased the height to make the difference in the lines more obvious.

The difference in the two lines is quite subtle, but races are often won or lost by seconds.

Rocker Profile

So if we’re looking for low wetted area we would want a rocker profile that was even and rounded, relatively deep in the middle and rising smoothly to the surface at each end. But this would give us a low prismatic which is not ideal in the higher speed range, and it’s not ideal for pitch damping which in my view is the critical design factor that is often underrated.

Pitching is slow. It destroys the airflow in your sails and the flow around the hulls, and your performance is suffering from slamming loads.

The single most effective way to counter pitching is with asymmetry in the water planes. You can achieve that in the with a fine bow and broad transom. Or you can achieve it with V sections forward and a flattened U shape aft. Or you can achieve it in the profile view with a very straight run forward and a bump in the aft sections. A flatter rocker line is better for resisting pitching than an evenly curved one with deeper draft in the middle.

The final result is a combination of all three of these factors.

On a cat like Design 256 the weight is concentrated well aft so we need to get buoyancy well aft.

The kink you see in the rocker profile helps to do this. It also helps to keep the rocker straight for most of its length and smooth the water flow exiting the hull aft at higher speeds, possibly promoting some planing effect.

If we had a more even rocker line we would slightly reduce the wetted area, but we would increase the pitching and the water would exit the hull aft at a steeper angle, increasing form drag in the higher speed range.

How much of a bump can you put in there without creating a flow separation, and how damaging would that flow separation be? I really don’t know. The way all of these factors interplay in the various conditions we sail in is very complex.

Ultimately a lot of this work is gut feel nurtured by experience, observing things in nature and most importantly experimenting and trying new ideas.

Is the new Groupama AC45 a breakthrough that will influence the form of racing catamarans into the future? I don’t think anyone has a computer that can answer that. We have to wait and see.

Catamaran hull lines showing symmetric and asymmetric waterplanes

Symmetric and non symmetric water-planes. The blue line with grey fill is the DWL from the design above. As is typical with modern cat hulls the bow is long and fine, the stern is full and rounded. This is the asymmetry that has a damping effect on pitching. The red line on the other hand is more like you would see on a double ended monohull and quite a few multihulls have also used this shape in the past. It's quite symmetric about the pitch axis and does not have good pitch resistance.

Lines plan for the Livewire 28 Catamaran

The hull lines of the new 8.5m Sports Cat Design 256

Mad Max , Previously Carbon Copy . She was designed in 1997 but she's the current (2016) title holder of the Australian Multihull Chamionships (2 successive years) and the fastest inshore racing boat in Australian waters.

Scroll to top

Hull Hydrodynamics and Design

2.1 Resistance Components on a Hull

Since the main objective of this project involves investigating the resistance of a hull and how it is affected by the introduction of hydrofoils, it is important to understand the components of this hydrodynamic resistance. Additional components will be added with the addition of the lifting foils and the interaction between the hulls and foils will also need to be accounted for. These will be discussed in chapter 3.

2.1.1 Viscous Resistance

The first and most obvious resistance comportent is the visco 3 resistance resulting from the hull skin friction and viscous pressure drag. Viscous resistance is^ç pendent on Reynolds number and so for equivalent scaling of the model we would require Reynolds similaijify.

2.1.2 Form Resistance

Another component is the form resistance. This resistance component results from the shape of the hull as the fluid is forced to change direction (and speed as a result) in order to travel around the hull. This is modelled by simply adding a form factor (1+k) to the viscous resistance, which is roughly equivalent to the average increase in speed required to travel around the hull.

2.1.3 Wave Resistance

The next component is wave resistance. This resistance component results from the additional energy required to create the waves resulting from the hull travelling through the water. These waves may be divided into the following categories - diverging (from bow and stern) and transverse waves. For equivalent scaling, Froude similarity is required. Note: fluctuations in wave resistance are as a result of both interference of the bow and stern wave system (as discussed by Bertram [Ber02]) and the wave interference of the two demi-hulls to be discussed in Chapter 2.1.9)

2.1.4 Spray Resistance

This resistance component is a result of the energy required to produce the spray. It is however expected that since the given hull shape is semi-displacement and the speed range is not very high, this is not expected to be a very large component.

2.1.5 Interference Resistance due to foils

This is the added resistance due to interference of the boundary layers of the hull and foils where they join. This is discussed in section 3.2.4 where the low pressure above the foil creates a downward suction on the hull, which in turn cancels some of the lift generated by the foils. In terms of hull resistance, the reduced pressure and associated increase in flow in the gap between the hull and foils would result in an increase in viscous resistance. This may be factored by a change in the form factor on the hull due to foils. Since only a percentage of the hull is affected and at speeds where the effect is strong, the hull will be elevated from the water substantially, this is not expected to be large, but may account for errors in assuming minimal interference between hull and foils.

The wave interference of the foils and hull niay be pMisidtereo as acting on either the hydrofoils or the hull. It is a function of speed and positioning of foils. Migjiotte [Mig9^] indicates that these effects are both significant but no means of determining them theoretically or empirically are available.

2.1.6 Heel Resistance

This is simply the change in resistance (primarily wave and viscous) due to heel. An empirical equation describing the coefficient of resistance and its corresponding formula is found in Larsson et al. [LE02] however this is for a monohull, which behaves very differently to a catamaran. The main difference is that for a catamaran, the WSA is remarkably reduced with heel as the one hull emerges from the water, thus reducing the viscous resistance. It is however interesting to note that from the formula, the effect of heel resistance increases with the square of the Froude number. It must also be borne in mind that heel has other implications also as the force vector on the sail gains a vertical component with heel. Normally this is assumed to cancel with the upward component on rudders and daggerboards [LE02] but with the addition of lifting foils there is an increase in sideward component due to the heel angle and a righting moment due to the free surface effects.

2.1.7 Induced Resistance

Normally this resistance is associated with hydrofoils and is explained in more detail in chapter 3.2.1. If the hull is thin, it may be thought of as a symmetric hydrofoil of low aspect ratio, operating in the vertical plane. If there is an angle of attack or leeway angle on the hull, there will be a resulting side force and induced resistance. Since the effective aspect ratio and area of the hull are relatively small, the daggerboards and rudders will offer most of the resistance to leeway, and induced resistance on the hull is expected to be very small.

2.1.8 Resistance due to Submerged Transom

Transom sterns have several advantages, the most important being that the wetted area (and hence viscous resistance) is less than that of a hull with a streamlined stern while still producing an equivalent wave pattern. [DD97]

When the transom of the hull is submerged at low speeds, the flow does not separate cleanly off the transom and therefore produces a large stern wave due to the sudden change in flow direction. At higher speeds, the velocity pressure of the flow under the transom is sufficient for separation resulting in 'smooth' flow off the stern and reduced resistance (see figure 2.1 below). This means.that the LCG may be placed further aft for high speeds, but the transom must be preferably in Ii tie with the water level for low speeds.

Doctors et al. [Doc98] quantified the speed at vhich flow off the submerged transom becomes 'cleanly separated' in terms of a Froude number based on depth of the transom below the static waterline (d). This critical Froude number is defined as follows...

2.1.9 Resistance due to Interference between Demihulls

• Induced resistance on hulls due to asymmetric flow around demi-hulls This results from the flow over the two demihulls affecting one another. This effect is a function of Froude number and separation distance. As described by Couser et al. [CWM97] the flow about the demi-hulls centrelines is not symmetric resulting in a relative angle of attack and in turn a side force and induced drag force resulting on each hull (lift and induced drag as described above). The side forces on either hull cancel one another out. However, the induced drag on both demi-hulls acts in the same direction and therefore they add. It is logical to assume (as demonstrated by [CWM97]) that the greater the separation distance and the lower the speed, the less of an effect this has. It was also noted that even for smaller separation distances and narrow hulls (high effective aspect ratio) the resulting induced drag coefficient is much smaller than the drag coefficient for the demi-hulls alone and may therefore be ignored. This is again supported by [CMAP97] who states that the side force on the demihulls decreases rapidly with increasing separation while the drag remains relatively constant. This implies induced drag is not significant. The generation of side force is however significant and may be required for structural calculations. It was also noted that this side force is almost always outwards however for small separations the venturi effect may dominate over the impinging bow wave effect and cause

, suction between the hulls.

Wave and viscous interference between demi-hulls i—

Figure 2.2 a) Graph of interference factor (t) vs Froude (Fr) number taken from IM91

The wave patterns of the two demihulls may also affect each other depending on speed (Fr) and separation distance. A further effect is that asymmetric flow around the demi-hulls effects the viscous flow i.e. boundary layer formation. Referring to [IM91], this type of interference resistance is a function of separation distance and Froude number. The results of [IM91] show that wave interference causes large fluctuations in wave resistance below Fr = 0.5 and a virtually constant and small interference factor above Fr = 0.6. This effect is diminished with increasing separation. Referring ahead to Chapter 2.8, the Froude number (FrL) range of the hull which was ultimately designed as our representative hull (RH1) is 0 -1.06. This corresponds to the range examined by [IM91].

Alternatively Turner et al. [TT68] provide a series of results from model testing to determine parameters affecting interference drag. The dependence on separation distance and Froude number is again shown and the following graph uses the data presented in [TT68]. It shows that at FrL ~ 0.27 and 0.33 there is minimum interference and at (B-2b)/L = 0.266 (same as RH1 - see 2.8) there are large fluctuations in interference with Froude number. These fluctuations are greatly reduced at higher separations.

2.2 Stability

Included in the main objective of this thesis is that the hydrofoil support system that is used in the final result will provide stable support throughout the given speed range. It is therefore important to consider the effect that the introduction of a hydrofoil support system will have on the stability of the sailing catamaran.

2.2.1 Pitch stability (Porpoising and Pitchpole)

As mentioned in chapter 1, pitch stability tends to be a problem for sailing catamarans due to the elevated thrust position and the fine demi-hull bows which offer very little buoyancy and planing effects to resist a forward pitching motion.

With the addition of the lifting foils, the COD would be lowered thus increasing the pitching moment arm between the thrust force and the hydrodynamic resistance. On the other hand the addition of the foils should reduce the resistance so it is unclear whether the pitching moment will be increased or reduced for a particular speed, depending on the positioning of the foils. What is clear is that the higher the hulls are raised by the lifting foils, the greater the angle at which pitchpole occurs. In addition to that the lifting foils should increase the speed of the boat for a given condition so these two factors will increase the severity of the pitchpole if it were to occur on foil support.

The terminal velocity of a conventional sailing catamaran is determined by the speed at which the large pitching moment has lowered the bows sufficiently so that the deck starts to flood. This results in an unstable condition where the bows dig in completely and the boat pitch-poles.

Porpoising is a dynamic instability caused by the combined oscillations of boat pitch and heave, which either remains constant (not a comfortable ride) or increasing in amplitude (Risk of pitch-pole). The general rule for avoiding porpoising is to reduce the trim angle i.e. to move the COG forwards - counter trim by stern. For the case of a hydrofoil-supported boat, the hydrofoils act as natural dampeners for pitch and heave and so this instability may be avoided by careful balancing of the hydrofoils. Given the elevated position of the thrust force which will always tend to trim the bow down, it is unlikely that this will be a problem for sailing catamarans unless excessive lift is being produced on the hydrofoils which 'overpowers' the dampening of the free surface effects.

Since the dagger-boards (to which the main foil is attached) are found amidships the percentage of the lift force created by the aft foils is dependent on how far aft of^ainidships the COG is positioned. Ideally one would desire an even distribution of load on the foils fc jr. good pitch ¡stability. The COG however is fairly dependent on the construction of the boat - particularly wdeffi inboard engines and water and fuel tanks are mounted. Practically it is found to be close to 45%, which implies veiy poor distribution of load.

2.2.2 Yaw Stability

Yaw stability is defined as the ability for an object to remain 'pointing' in the direction in which it is travelling. In terms of boats, it is defined as the tendency to resist rotation about the vertical axis (z-axis, as defined in naval architecture). Another definition [Ber02] is - "the ability to move straight ahead in the absence of external disturbances at one rudder angle"

Yaw stability is achieved by one simple criterion- The COG remains in front of the CLR. This may be illustrated by a dart or arrow. When travelling through the air in the conventional direction, it is easily seen that it remains yaw stable and the reverse can be said about a dart or arrow travelling backwards.

Most sailing catamarans are designed with the daggerboards amidships and rudders near the stern and the COG not far aft of amidships. As a result the CLR is usually well aft of the COG. The only condition where yaw instability is likely is when the boat begins to pitchpole so that the CLR is moved forward significantly. This is a case of pith instability in any case. The effect of hydrofoils of yaw stability is not likely to be significant if the foils are attached to the rudders and daggerboards but a canard foil at the bow would shift the CLR forward, thus increasing the likelihood of yaw instability.

2.2.3 Sudden Loss in Foil Lift

The problem with foils as they near the surface is that of ventilation. This will be discussed in more detail in chapter 3 but in short, results in a sudden loss in lift. This would result in the section of boat supported by the ventilated foil dropping suddenly. This would increase the hull resistance at that point and may result in either yaw or pitch instability, depending on which foil becomes ventilated.

2.2.4 Other Aspects on Stability

Although these three aspects have been identified as the primary concerns with regard to the stability, there are many others. James Wharram [WB91] provides an interesting and insightful discussion on sailing catamaran stability, which stresses the importance of large displacement and that the CE (of the sails) is not too high above the waterline for maintaining good stability. The hydrofoils are expected to increase the displacement due to the added weight (and lower the COG) but also incre^e the height of the CE as the foils lift the hull out of the water. For a more complete evaluation of seawoi;.ihinig& reference should be made to Marchaj [Mar86] and Krushkov [Kru81] For the purpose of future research, guidelines for assessing the Seaworthiness of a similar vessel is laid out by the ITTC in [IT99(ii)]. A me;mis of quantifying stability (stability index or STIX calculations) for a small sailing monohull is given in appendix O. No equivalent was found for sailing catamarans.

2.3 Hullform Development.

Hullforms may be divided broadly into 3 categories; namely displacement, semi-displacement and planing hulls. These describe the range of speeds in which the hull is designed to operate and each will be discussed briefly.

2.3.1 Displacement hulls.

These are hulls designed to operate at relatively low Froude numbers, where the dynamic effects on the running conditions are very small. The hulls are typically 'canoe-like' in shape and they are characterised by large curvature in the aft section to allow for smooth flow that is split at the bow and rejoins at the stern. They also have round bilge station lines (the flow is too slow to separate cleanly off a hard chine hull) and thus vortex shedding at the chines is reduced at low speed. The boundary for this low speed is determined by the Froude number of the boat and these hulls generally operate at below Frv = 0.35 as above this the drag curve is dominated by the hump resistance. [Mig97] Below in figure 2.3 is a typical displacement hull.

Since the Froude number range for displacement hulls is for low Froude numbers, hydrofoils are typically not used for these hull shapes as very large foils (large WSA) are required to generate significant lift. These in turn have large viscous drag components and thus an overall reduction in drag is practically not achievable unless the hull operates at speeds above conventional Froude number ranges for this hull; in which case a semi-displacement hull would have been a more appropriate hull shape.

2.3.2 Planing hulls

Unlike displacement hulls these hulls are designed, to operate at much higher Froude numbers (above the hump speed of displacement hulls) where the dynamic, effects play a major roll in the running conditions of the boat. As a result, the large amount of curvature such as on the affection of displacement hulls would result in large drag due to flow separation and suction. In order to avoid this, planing hulls have less curvature and a 'cut-off' or transom stern. In general, curvature of the buttock lines (a.k.a. rocker) and of the station lines create dynamic suction that induces dynamic sinkage and encourages vortex shedding, thus adding to the drag. Curvature is thus minimised on planing hulls but this will be discussed a bit more in 2.3.4. The WSA and dynamic effects are reduced with the incorporation of hard chines and spray rails. These allow for relatively flat sections in the station lines and the flow to be separated off the hull and create additional lift. The same separating effect is desired from the transom stern. Below in figure 2.4 is a typical planing hull.

Since these hull shapes are associated with operation at high Froude numbers, the use of Hydrofoils is very applicable to these hulls. Most of the lift force on these hulls is derived from dynamic forces and since hydrofoils are approximately twice as efficient as planing surfaces (as will be explained in section 3.3.1), their introduction is likely to reduce drag on the boat significantly.

2.3.3 Semi-displacement hulls

These hulls are a compromise between planing and displacement hulls. They typically have transom sterns, bows similar to displacement hulls and may or may not include hard chines, but seldom include spray rails. (Insufficient speed for spray generation) Sailing boats are almost exclusively in this domain of design, particularly since they operate over a large speed range and are required to perform well in all conditions. All the sailing catamarans produced in South Africa that were found during the internet survey (Chapter 1.9) were round bilge and transom stern in nature and typically had surprisingly large amount of rocker.

Since these hulls are designed to operate at a Froude number range spanning from where buoyancy forces dominate at low speeds to where dynamic forces dominatfe at upper speeds, hydrofoils are applicable to these hull shapes where the performance is red; ed at i<% speed slightly due to increase WSA. As the speed increases the hull is raised out of the water and so the WSA and wave drag will be reduced, thus improving performance.

2.3.4 Rocker (Curvature of the buttock lines)

Rocker or curvature of the buttock lines is an important aspect of hull design and influences the wave-making resistance and seakeeping characteristics of the hull. Typically displacement hulls have lots of rocker while planing hulls have little or no rocker in the stern for planing effects (see following sections). This is because the curvature results in dynamic suction on the wetted surface of the hull, which in turn results in sinkage, increasing the drag on the hull. This effect is a function of free-stream velocity pressure (boat speed) and is therefore small at low speed. Displacement hulls have significant rocker since it reduces the cross-sectional area curve near the bow and stern and therefore reducing the wave making drag at low speed. At higher speed, the resulting sinkage is higher as the dynamic effect becomes significant.

Another aspect of rocker is the effects on seakeeping. Including some rocker in a hull design improves the performance of the hull in waves as it lowers the Centre of Buoyancy (COB) in relation to where the bow and stern enter and leave the water respectively. This allows the boat to respond more smoothly to waves by rocking so as to reduce the slamming and dragging of transom effects that would otherwise be experienced and improve stability. On the other hand, too much rocker can result in hobby-horsing effect in waves which is obviously not desirable in terms of seakeeping. [Shut05 (i)]

Despite dynamic suction effects, rocker may be considered advantageous when a hull supported by hydrofoils is lightly loaded. This combination results in the boat lifting out of the water substantially at relatively low speeds and because of the rocker, the WSA decreases rapidly. If the hulls were compared with foils, they would have very low aspect ratios (since they are slender) and as a result do not produce large dynamic sinkage despite their large area. (Refer to Chapter 3.3.4)

2.3.5 Aspects of bow and stern design

Very fine bows, having deep V shape and narrow angles of entrance are associated with low wave making. This low wave making results in less disturbed flow in the tunnel which is where the foils are found, thus less of an effect on flow over foils. Another advantage is that the wave piercing characteristics of fine bows, results in reduced slamming and heaving of the bows in waves.

The disadvantage of fine bows (as discussed in 2.2.1) is their inability to resist pitchpole and the sharp bows mean the CLR moves forward rapidly as the boa* pi':clies forward, reducing yaw stability. Fortunately for sailing cats, most of the lateral resistance comes from the rudders and daggerboards, diluting this effect.

Bulbous bows have been incorporated into some power cat designs, to reduce wave making resistance. The reduction in wave making drag is small on cats as it's already fairly small due to the narrow demihulls. [Mig97] They also tend to move the CLR forward, thus reducing yaw stability while also dampening pitching motion. Bulbous bows are almost never evident on sailing catamarans, and with therefore not be considered as part of the hull design.

The aft sections of power cats are usually designed in terms of stability and high speed performance. Stability is controlled more by the large rudders and daggerboards on sailing cats, and the high speeds achieved on power boats are only achieved in strong wind conditions. These aspects are therefore of less concern in the design of aft sections on sailing boats.

Since the sterns are generally broader and flatter, the effect of rocker is more significant on the mid to aft section. As a result rocker tends to result in natural trimming by stern which increases dynamically. This is to an extent desirable as it tends to counter the pitching moment but excessive rocker results in excessive induced and separation drag and sinkage as mentioned in 2.3.4.

2.3.6 Planing Effects

This dynamic effect acting on the hull is as a result of trim angle. It is not to be confused with planing speed but both are associated with relatively high speed (Froude number). As a result of trim angle, the hull rises out of the water. An analogy may be drawn between the hull and a flat plate at an angle of attack (See figure 2.5) where the effective downwash (as in a hydrofoil) results in a lift force on the hull. To ensure this trim angle, the LCG should be aft of zero trim position but this generally results in the transom dragging at low. (See 2.1.8)

Planing effects and dynamic suction resulting from hull shape (rocker and soft chines) will combine and result in the running conditions (trim and rise) which will vary with speed.

2.4 Hull Selection

After the size of the yacht was determined in Chapter 1.5, two possible means of hull selection were determined. An existing hull shape of a sailing catamaran currently being produced in South Africa could be used, or a standard hull shape of suitable characteristics could be used. In either case, a representative hull shape would need to be found in order to determine these characteristics.

2.4.1 Finding a Hull

The first thing noted about sailing catamarans produced in South Africa is that the demihulls are symmetric. This is owed to the large separation between demihulls resulting in relatively small interference.

After consulting with local sailing yacht manufacturers, a hull was found but some concerns were expressed regarding the large amount of rocker this boat displayed. The associated dynamic suction on the hull would need to be overcome by a lifting foil system if employed on this hull shape. Nonetheless, the characteristics of the hull were determined so that they could be compared to standard hull shapes. The hull characteristics were compared to the NPL [Bai76] and Series 64 [Yeh65] however no suitable match was found.

It was noted by the student that through his experience as a sailor, a fair amount of rocker is common in many sailing catamaran designs. As a result it was decided that the original hull shape would be used and if the problem of excessive rocker was overcome, this could be treated as a worst case scenario and even better results could be expected for applications to hull shapes with less rocker. Since the boat is fairly lightly loaded, this rocker may prove to be advantageous as discussed in 2.3.4.

Since no lines drawings were provided by the manufacturer, only 2D drawings and photos were provided, it was decided that demihulls based on the representative cat would be designed as part of this study. For the purposes of modelling the thrust force on the sails, details of the rig from the original hull are used. Since the hull generated by the student is aimed at being a representative hull of sailing catamarans produced in South Africa it was named Representative Hull 1 or RH1. (See figure 2.6)

2.4.2 Discussing hull shape

According to Tom Speer [BD99], an expert in the field of sailing boat design, "Since minimum wetted area and not form stability is the driving influence on multihull shapes, you can pretty well determine the lines yourself by taking the product of the profile and typical section shape." As a result it was decided that this method of determining the representative hull, although not accurate, should yield a suitably representative hull.

After referring to a discussion by Shuttleworth [Shut05(ii)] which explained how the buoyancy can be placed as a function of heave, so as to effect the trim of the boat, it was concluded that the bows were a little fine and should have been flared slightly above the waterline. This would however only affect upper speed conditions when the boat tended to nose-dive as only flat water model testing would be conducted.

Since the main parameters of RH1 were based on the original boat, these are expected to be accurate and suitable. Since all other aspect were fairly well defined by the 2D drawings, the hull was deemed suitable.

As can be seen in figure 2.8, there is significant rocker all along the keel line of RH1. From the discussion in 2.3.4 and 2.3.5, we therefore expect a reasonable amount of dynamic sinkage at high speed and due to the large rocker (giving low effective trim angles on the aft section of the boat - where the hull is broad and flat) and narrow demi-hulls, we expect very little planing effects.

2.4.3 Evaluating performance

Larsson et al. [LE02] provides a few ratios for the evaluation of monohull sailing boat performance. Although we expect the performance ratios to be a bit higher for catamarans given their reduced displacement for a given sail area, these are nonetheless used as a yardstick.

The sail area to wetted area (Where the viscous 'force's are being compared to thrust force on sail) should be above 2 for reasonable performance and 2.5 is considered good. The waterline length- displacement ratio needs to be above 5.7 to ensure that the boat will r< ch semi-planing conditions. The sail area - displacement ratio (Where the wave drag is being compared to the Hirust ftwce on sail) should be between 20 and 22 for good performance. As can be seen in the table below, RH1 should have a veiy good performance based on these.

Ratio	Good performance range	RH1
Sa / Sw	2 - 2.5	2.86
LWL / V1/3	> 5.7	7.4
Sa / V2/3	20 - 22	32

Table 2.1 - Evaluating performance ratios of RH1

Continue reading here: Horiuchi Twin Ducks

Was this article helpful?

Explain The Hydrodynamics Of A Daggerboard
Hull Construction - Ship Design
Catamaran Design Guide - Catamarans Guide
Geometry - Ship Design
Design Dynamics - Catamarans Guide
Catamaran Sailboat Wide Bodied Hulls

Readers' Questions

Are sailing boats hydrodynamic?

Yes, sailing boats are hydrodynamic because they rely on the water's resistance and flow around their hulls to move.

Catamaran Hull Speed Calculator For Beginners (Table and Free Spreadsheet)

As an Amazon Associate, we earn from qualifying purchases. We may also earn commissions if you purchase products from other retailers after clicking on a link from our site.

Speed is important, it can get you out of harm’s way, and it makes sailing much more fun, but figuring out how fast a catamaran will be able to sail can be tricky. One important aspect is to understand maximum hull speed.

In this article, I have calculated different hull speeds for different lengths of boats; this includes both monohulls and catamarans but focuses on the latter. Here is the catamaran maximum hull speed table:

Table of Contents

Catamaran Max Hull Speed Calculator Table

Length on Water		Displacement Max Hull Speed	Semi Planing Beyond Max Hull Speed
		0%	10%	20%	30%
Feet	Meter	Knots	Knots	Knots	Knots
26	8	6.9	7.5	9.1	11.8
30	9	7.3	8.0	9.6	12.5
33	10	7.7	8.4	10.1	13.2
36	11	8.0	8.8	10.6	13.8
39	12	8.4	9.2	11.1	14.4
43	13	8.7	9.6	11.5	15.0
46	14	9.1	10.0	12.0	15.6
49	15	9.4	10.3	12.4	16.1
52	16	9.7	10.7	12.8	16.6
56	17	10.0	11.0	13.2	17.2
59	18	10.3	11.3	13.6	17.7
62	19	10.6	11.6	14.0	18.1
66	20	10.8	11.9	14.3	18.6
69	21	11.1	12.2	14.7	19.1
72	22	11.4	12.5	15.0	19.5
75	23	11.6	12.8	15.3	20.0
79	24	11.9	13.1	15.7	20.4
82	25	12.1	13.3	16.0	20.8
85	26	12.4	13.6	16.3	21.2

Table Explanation

Length on the waterline (L.W.L.): Length of the boat when measured on the waterline, not to be confused with length overall (L.O.A.), which is the boat’s total length (above the waterline) including bowsprit, etc.
Displacement max hull speed: The max speed of a boat whose L.W.L doesn’t change when underway and where the vessel’s bow wave is the limiting speed factor.
Semi/Light Displacement or Semi planing hulls speed: A boat where the bow waves speed limiting factors can be partially overcome and therefore exceed the displacement hull speed. These hulls usually overcome hull speed by 10-30% .

How to use the Catamaran Hulls Speed Table

Choose your length on waterline in the left-most column, either in feet or meter.
Continue reading to your right and stop either at “Displacement hulls speed” or continue to “10,20, or 30%”, depending on your estimated hull efficiency. This will be your calculated maximum hull speed for a semi-displacement catamaran.

The Formula

First of all, we need to know the maximum hull speed for a displacement hull, and from that number, we will be able to calculate how much faster the semi-planing (or semi-displacement) hull will be. This is the formula for Maximum Hull Speed on a displacement boat:

Now we need to add the increased efficiency (loss of drag) of a semi-displacement hull, usually, this is somewhere between a 10-30% increase.

Note: “1.3” is the increase in efficiency, if you believe you are on the lower end of the scale this would be 1.2 or 1.1.

How to Exceed Hull Speed

This calculator offers a theoretical perspective, but many other factors such as sail plan, weight, and sailor skill, of course, have a profound impact on speed. As we have seen, a semi-displacement hull can exceed maximum hull speed, but we can also see that it isn’t by much. The next step is to reduce drag even further by utilizing a planning hull.

Catamaran Hull Speed Spreadsheet

If you want more info, calculate other lengths, or see the speeds in Km/h or Mph then I suggest you check out this free spreadsheet.

Catamaran Freedom Hull Speed Calculator

Note: If you want your own copy just click, File->make a copy.

Common Questions About Catamaran Hull Design

Below I will answer some of the questions I receive concerning catamaran hull design. The list will be updated as relevant questions come in.

Is a Catamaran a Planing hull?

As we have discussed above, a catamaran can definitely have a semi-planing hull, but can it be designed in a fully planing configuration as well?

Catamarans can be configured as planing hulls, although most sailing catamarans are set up as either semi-planing or hydrofoil. Due to the high speeds needed to get a boat to planing speed, this is only possible on racing sailboats or motor-powered catamarans such as high-speed ferries.

Owner of CatamaranFreedom.com. A minimalist that has lived in a caravan in Sweden, 35ft Monohull in the Bahamas, and right now in his self-built Van. He just started the next adventure, to circumnavigate the world on a Catamaran!

Catamaran Design Formulas

Post author By Rick
Post date June 29, 2010
10 Comments on Catamaran Design Formulas

Part 2: W ith permission from Terho Halme – Naval Architect

While Part 1 showcased design comments from Richard Woods , this second webpage on catamaran design is from a paper on “How to dimension a sailing catamaran”, written by the Finnish boat designer, Terho Halme. I found his paper easy to follow and all the Catamaran hull design equations were in one place. Terho was kind enough to grant permission to reproduce his work here.

Below are basic equations and parameters of catamaran design, courtesy of Terho Halme. There are also a few references from ISO boat standards. The first step of catamaran design is to decide the length of the boat and her purpose. Then we’ll try to optimize other dimensions, to give her decent performance. All dimensions on this page are metric, linear dimensions are in meters (m), areas are in square meters (m2), displacement volumes in cubic meters (m3), masses (displacement, weight) are in kilograms (kg), forces in Newton’s (N), powers in kilowatts (kW) and speeds in knots.

Please see our catamarans for sale by owner page if you are looking for great deals on affordable catamarans sold directly by their owners.

Length, Draft and Beam

There are two major dimensions of a boat hull: The length of the hull L H and length of waterline L WL . The following consist of arbitrary values to illustrate a calculated example.

L H = 12.20 L WL = 12.00

After deciding how big a boat we want we next enter the length/beam ratio of each hull, L BR . Heavy boats have low value and light racers high value. L BR below “8” leads to increased wave making and this should be avoided. Lower values increase loading capacity. Normal L BR for a cruiser is somewhere between 9 and 12. L BR has a definitive effect on boat displacement estimate.

B L / L	In this example L = 11.0 and beam waterline B will be:
Figure 2
B = 1.09	A narrow beam, of under 1 meter, will be impractical in designing accommodations in a hull.
B = B / T	A value near 2 minimizes friction resistance and slightly lower values minimize wave making. Reasonable values are from 1.5 to 2.8. Higher values increase load capacity. The deep-V bottomed boats have typically B between 1.1 and 1.4. B has also effect on boat displacement estimation.

T = B / B T = 0.57	Here we put B = 1.9 to minimize boat resistance (for her size) and get the draft calculation for a canoe body T (Figure 1).
	Midship coefficient – C
C = A / T (x) B	We need to estimate a few coefficients of the canoe body. where A is the maximum cross section area of the hull (Figure 3). C depends on the shape of the midship section: a deep-V-section has C = 0.5 while an ellipse section has C = 0.785. Midship coefficient has a linear relation to displacement. In this example we use ellipse hull shape to minimize wetted surface, so C = 0.785
Figure 3


C =D / A × L	where D is the displacement volume (m ) of the boat. Prismatic coefficient has an influence on boat resistance. C is typically between 0.55 and 0.64. Lower values (< 0.57) are optimized to displacement speeds, and higher values (>0.60) to speeds over the hull speed (hull speed ). In this example we are seeking for an all round performance cat and set C := 0.59


C = A / B × L	where A is water plane (horizontal) area. Typical value for water plane coefficient is C = 0.69 – 0.72. In our example C = 0.71


m = 2 × B x L × T × C × C × 1025 m = 7136	At last we can do our displacement estimation. In the next formula, 2 is for two hulls and 1025 is the density of sea water (kg/m3). Loaded displacement mass in kg’s


L = 6.3	L near five, the catamaran is a heavy one and made from solid laminate. Near six, the catamaran has a modern sandwich construction. In a performance cruiser L is usually between 6.0 and 7.0. Higher values than seven are reserved for big racers and super high tech beasts. Use 6.0 to 6.5 as a target for L in a glass-sandwich built cruising catamaran. To adjust L and fully loaded displacement m , change the length/beam ratio of hull, L .


m = 0.7 × m m = 4995	We can now estimate our empty boat displacement (kg): This value must be checked after weight calculation or prototype building of the boat.


m = 0.8 × m m = 5709	The light loaded displacement mass (kg); this is the mass we will use in stability and performance prediction:


	The beam of a sailing catamaran is a fundamental thing. Make it too narrow, and she can’t carry sails enough to be a decent sailboat. Make it too wide and you end up pitch-poling with too much sails on. The commonly accepted way is to design longitudinal and transversal metacenter heights equal. Here we use the height from buoyancy to metacenter (commonly named B ). The beam between hull centers is named B (Figure 4) and remember that the overall length of the hull is L .

Figure 4


	Length/beam ratio of the catamaran – L
L = L / B	If we set L = 2.2 , the longitudinal and transversal stability will come very near to the same value. You can design a sailing catamaran wider or narrower, if you like. Wider construction makes her heavier, narrower means that she carries less sail.

B = L / L B = 5.55	Beam between hull centers (m) – B

BM = 2[(B × L x C / 12) +( L × B × C x (0.5B ) )] × (1025 / m ) BM = 20.7	Transversal height from the center of buoyancy to metacenter, BM can be estimated

BM = (2 × 0.92 x L × B x C ) / 12 x (1025 / m ) BM = 20.9	Longitudinal height from the center of buoyancy to metacenter, BM can be estimated. Too low value of BM (well under 10) will make her sensitive to hobby-horsing

B = 1.4 × B	We still need to determine the beam of one hull B (Figure 4). If the hulls are asymmetric above waterline this is a sum of outer hull halves. B must be bigger than B of the hull. We’ll put here in our example:

B = B B B = 7.07	Now we can calculate the beam of our catamaran B (Figure 4):

Z = 0.06 × L Z = 0.72	Minimum wet deck clearance at fully loaded condition is defined here to be 6 % of L :

	EU Size factor
SF=1.75 x m SF = 82 x 10	While the length/beam ratio of catamaran, L is between 2.2 and 3.2, a catamaran can be certified to A category if SF > 40 000 and to B category if SF > 15 000.

	Engine Power Requirements
P = 4 x (m /1025)P = 28	The engine power needed for the catamaran is typically 4 kW/tonne and the motoring speed is near the hull speed. Installed power total in Kw
V = 2.44 V = 8.5	Motoring speed (knots)
Vol = 1.2(R / V )(con x P ) Vol = 356	motoring range in nautical miles R = 600, A diesel engine consume on half throttle approximately: con := 0.15 kg/kWh. The fuel tank of diesel with 20% of reserve is then

Tags Buying Advice , Catamaran Designers

Owner of a Catalac 8M and Catamaransite webmaster.

10 replies on “Catamaran Design Formulas”

Im working though these formuals to help in the conversion of a cat from diesel to electric. Range, Speed, effect of extra weight on the boat….. Im having a bit of trouble with the B_TR. First off what is it? You don’t call it out as to what it is anywhere that i could find. Second its listed as B TR = B WL / T c but then directly after that you have T c = B WL / B TR. these two equasion are circular….

Yes, I noted the same thing. I guess that TR means resistance.

I am new here and very intetested to continue the discussion! I believe that TR had to be looked at as in Btr (small letter = underscore). B = beam, t= draft and r (I believe) = ratio! As in Lbr, here it is Btr = Beam to draft ratio! This goes along with the further elaboration on the subject! Let me know if I am wrong! Regards PETER

I posted the author’s contact info. You have to contact him as he’s not going to answer here. – Rick

Thank you these formulas as I am planning a catamaran hull/ house boat. The planned length will be about thirty six ft. In length. This will help me in this new venture.

You have to ask the author. His link was above. https://www.facebook.com/terho.halme

I understood everything, accept nothing makes sense from Cm=Am/Tc*Bwl. Almost all equations from here on after is basically the answer to the dividend being divided into itself, which gives a constant answer of “1”. What am I missing? I contacted the original author on Facebook, but due to Facebook regulations, he’s bound never to receive it.

Hi Brian, B WL is the maximum hull breadth at the waterline and Tc is the maximum draft.

The equation B TW = B WL/Tc can be rearranged by multiplying both sides of the equation by Tc:

B TW * Tc = Tc * B WL / Tc

On the right hand side the Tc on the top is divided by the Tc on the bottom so the equal 1 and can both be crossed out.

Then divide both sides by B TW:

Cross out that B TW when it is on the top and the bottom and you get the new equation:

Tc = B WL/ B TW

Thank you all for this very useful article

Parfait j aimerais participer à une formation en ligne (perfect I would like to participate in an online training)

Semi-Displacement Hulls Explained (Illustrated Guide)

If you want to know exactly what a semi-displacement hull is, this article is right for you. Here, I explain simply what it is, how it works, and why it's different from other hulls.

What is a semi-displacement hull? A semi-displacement hull is a hull design that combines features of the displacement and planing hull. It displaces water at low speeds but is able to generate lift at cruising speed. It is more stable than planing hulls, and faster than displacement hulls. It planes at lower speeds than regular planing hulls.

Generally, you'll find this type of hull on motor cruisers, trawlers, and the likes. Sailboats don't really come in this flavor, and I'll explain why below.

A displacement hull lies inside the water, and displaces it as it moves. It holds up the boat using buoyancy.
A planing hull lies on top of the water as it moves and holds up the boat using lift. This is called planing .
A semi-displacement hull displaces water at low speeds, but is able to semi-plane at cruising speeds.

All three of these hull types come with their own unique advantages and disadvantages:

The displacement hull is efficient and very dependable in rough waters.
The planing hull is extremely fast and agile.

The semi-displacement hull is a bit of both. It's seaworthy and can be relied upon in rough waters, but at the same time, it's a lot faster than displacement hulls. Semi-displacement hulls are perfect for boats that need to be steady and seaworthy but fast at the same time.

Design Features

The most important thing to understand is that generally, planing hulls are fine and flat aft (in the back), and displacement hulls are bulky and round. The semi-displacement hull combines these two design features. It is flat aft but gets bulkier towards the front. The bow (the front), has more of a wedge shape to it, like a Deep-V. The bow has to be able to displace water and lift at the same time. That's why the quality of the design is really important with these boats.

From the front, it looks like a sailboat, from the back, it looks like a powerboat.

The hull shape allows it to semi-plane. Planing simply means it's riding its own bow wave, lifting it out of the water. Planing is a great way to add speed, but it decreases stability.

The semi-displacement hull is heavier than a planing hull but lighter than a displacement hull. Because of its weight, it can't generate enough lift to fully plane. But because of its weight, it's also a lot more stable in rough waters.

How Does a Semi-Displacement Hull Work?

So now we understand the different features of this hull type, let's see how it comes together. At low speeds , the hull acts as a displacement hull, cutting through the water instead of riding on top of it. This makes it stable and reliable in waves. Its weight and keel make sure it handles well in choppy waves and rough weather, which is why it is a great design for coastal cruisers and trawlers and the likes.

At high speeds , the hull acts as a planing hull, riding on top of the water. This makes it a lot faster. Its flat back allows it to plane, as this will lift the front out of the water (but only partially). At the same time, the design of the bow helps it to climb out of the water. Between 12 - 16 knots (cruising speed), the semi-displacement hull will start climbing its own wave, generating lift. This reduces drag (water resistance). This adds a lot to its top speed. Roughly anywhere between 5 - 10 knots. That's a lot.

The semi planes at lower speeds than planing hulls though. This is great because most semis aren't that fast. The reason? The center of gravity is farther forward than planing boats 1 .

The reason the semi-displacement hull is so much faster is that it's able to climb its own bow wave. Regular displacement hulls can't do this. This means they have an upper-speed limit, called the maximum hull speed. The maximum hull speed is a direct correlation between the length of the boat's waterline and the maximum speed.

To learn more about calculating maximum hull speed, and to view examples of different boat lengths and their upper limit , please check out my previous article here .

faster at cruising speed
can outrun storms thanks to speed
larger range
excellent rough-water boats
are able to cross oceans
shallower draft
less efficient at low speeds than a displacement hull
uses a lot of fuel at lower speeds
a bit less comfortable than a displacement hull
less storage due to flat aft and fine bow
not as fast as planing hulls

Difference between semi-displacement hulls and displacement hulls

Semi-displacement hulls:

have finer bow entry
are flatter aft
can generate lift, making it faster
can outrun storms because of their speeds
have less storage due to flat aft and fine bow

To learn more about displacement hulls , I recommend reading my full guide on it. It explains the pros and cons of displacement hulls .

Difference between semi-displacement hulls and planing hulls

are narrower than planing hulls
have a deeper and narrower hull forward, more like a deep V
are not as fast as planing hulls
are a lot steadier than planing hulls
vessels weight is still supported by buoyancy

Yes, that's right. You can cross oceans with a semi-displacement hull. With a planing hull, that's not recommended. However, for those types of journeys, I'd still pick a full displacement hull over a semi every day of the week.

For examples of the most common hull types , please read my Illustrated Guide to Boat Hull Types here .

If you want to have a comfortable ride, even in moderate to heavy chop, and want to have a reliable boat that also has some speed up its sleeve, this is a great hull design for you. With pretty decent speeds, often anywhere up to 20 - 30 knots, this hull can has plenty of thrill to it, while at the same time being comfortable and reliable. It's perhaps one of the most versatile hull designs out there. In my opinion, it offers the perfect fast-paced family cruiser for people who live near the coast and want to take her out for a bluewater spin.

I haven't heard of any sailboats - monohulls - that have semi-displacement hulls. (If you have, please let me know in the comments below.) I think the reason is simply that sailboats can't deliver the power necessary to generate any meaningful lift. Cruising speed for most semi-displacement boats is roughly 15 - 20 knots - that's when lift is generated and it starts to semi-plane. Monohulls can't get up to that speed.

Catamarans can. There are cats out there with wide and flat aft hull sections, enabling them to get into a semi-plane. This is one of the reasons why cats are so much faster than monohulls.

The most famous semi-displacement boat type is the Maine Lobster boat. Other boats that use semi-displacement are trawlers and motor cruisers.

https://www.soundingsonline.com/boats/how-different-hull-types-react-in-rough-water ↩

Thank you for a well formed explanation—assuming you’re correct. My wife and I are looking at boating as a retirement plan. She asked what the difference is between hull types. I can send her your link.

Stay safe and healthy!

Thank You for an excellent education, I learned much from your article. Is there a chance ou may provide your comments/comparison of the Majesty 140 and theh Benetti Diamond 145? I would so much appreciate your commentary. Most Appreciated !

MacGregor 26M is a semi displacement sail boat :-)

The MacGregor 26X sailboat is also a semi-displacement hull. Factory specs suggest 24 knot top speed under sail or with a 40 hp outboard motor. I have not reached those speeds under sail … more like 14 to 16 knots.

Thank you so much I understand the different types of hulls so much better!

robert salverda

Hi Shawn, I once owned a Lancer 39.5 foot sailboat. It had two Perkins 4-254 85 h.p engines in it. At full throttle it could actually plane, defying all the rules of a displacement hull, so I am assuming it must have had a semi-displacement hull.

You may also like, the illustrated guide to boat hull types (11 examples).

I didn't understand anything about boat hull types. So I've researched what hulls I need for different conditions. Here's a complete list of the most common hulls.

A Complete Guide to Displacement Hulls (Illustrated)

2024 BOAT BUYERS GUIDE
SHALLOW WATER FISHING
Email Newsletters
Boating Tips
Boating Safety
Electronics
Baits & Lures
Fishing Tackle
Fishing Travel
Conservation
Fishing Knots
Women in Fishing

Multihull, Catamaran Boats Invade the Offshore-Fishing Market

By Chris Woodward
Updated: August 23, 2019

I wouldn’t describe my father as a hardcore boat guy. He owned a cabin cruiser and a few sailboats during my childhood, but he spent much more time on the golf course.

That’s why it came as a big surprise, in the mid-1960s, when Dad—ever the gadget enthusiast—built himself a boat: a catamaran, in fact.

Now, I don’t recall spending much time aboard his homemade multihull, but I do recall sailing with him aboard the Hobie Cat he bought shortly thereafter. Cat boats evoke fond memories for me, however I’ve been a monohull powerboat owner ever since.

In the offshore-fishing world, I represent one side of the historic divide between V-hull owners and cat-boat enthusiasts. Make no mistake: V-hulls still dominate, but more American boat buyers have started gravitating to cats during the past decade.

Invincible catamarans running rough seas

New Designs “I’ve been impressed to see how [the two groups] have stayed segregated, if you will,” explains Bill Cordes, director of sales for Invincible Boats, which has introduced 40- and 37-foot catamaran offshore-fishing boats over the past two years, and yet remains successful with its popular stepped-V-hull vessels. “The guy coming in who’s already on a V-bottom boat is coming in for another V. But we’re getting new looks from guys who are cat guys, who are looking for a bigger alternative.”

Invincible and companies such as Freeman, Seahunter, Insetta and Calcutta are driving a resurgence in multihull-boat building, particularly in the offshore-fishing world. At press time, Calcutta was poised to splash its new 480, an express boat that will be available in inboard and outboard versions; Invincible is expected to add a 35-footer to its multihull stable; and TAG Boats, a new South Carolina company, should introduce its 36-footer. In the next six months, Barker Boatworks plans to debut a new 40-foot cat, and HHH BoatWorks will launch a 35-footer.

Depending on how they’re built, cat designs can offer some general advantages over V-hulls, including a better ride in rough head seas, a broader, more stable platform with little roll at rest, shallower draft, better fuel efficiency and greater deck space.

“On my first World Cat sea trial, I remember getting ready to stabilize myself as I saw that first big wave approaching,” recalls Capt. Shane Jarvis, of Sport Fish Panama Island Lodge ( fishpanamatoday.com ), which operates two World Cat 33TEs. “But the boat never jumped. It just cut through the wave like butter and didn’t buck like I was accustomed to.”

In Panama, captains do a lot of run-and-gun-style fishing and trolling, but they also drift live baits and jig. Offshore runs can total 20 to 50 miles. “The wide 10-foot-6-inch beam of our cats allows plenty of room to move around, and for multiple anglers to cast and fish with minimal to no roll when drifting,” he says.

In fact, outside the United States, catamarans have been popular for a long time.

“Certain parts of the world embraced cats a long time ago—places like South Africa, Australia and New Zealand, where the distances are great and the seas can be pretty rough,” says yacht designer Michael Peters, well-known in the fishing world for creating everything from large inboard sport-fishing vessels to stepped-hull center-consoles. Peters is also currently designing Barker’s new catamaran.

“Yet, the American market never did [embrace cats]. It seems that nobody really got the attention until Freeman. Why that was, I couldn’t completely explain, but suddenly there was a boat that seemed to get good balance and aesthetics, and the fishing world took notice.”

Billy Freeman started Freeman Boatworks in 2006, after years of working in boatyards and welding shops, and cutting up old hulls to rebuild them in his off time. “I bought an old cat, and wanted something bigger and better,” Freeman says. “I called up [naval design engineer] Jesse Rhodes, who was an unknown back then, and I said I wanted to design a cat. I said, ‘If it doesn’t work, it’s on me.’”

Quite simply: It worked.

Freeman says he started calling fishing guides in Venice, Louisiana, who make their living running well offshore into sometimes gnarly seas. “I didn’t have to talk them into a cat,” he says.

Freeman looked to resolve what are considered classic catamaran performance and handling issues, including bow steering and pulling and jerking in certain sea conditions. Traditional cats rode very well into head seas, and remained more stable at rest because of their wide beam. But in a following sea, “they could be a complete handful,” says Peters, who also spent 25 years building offshore racing catamarans. “In high-speed turns, they do what we call a ‘destroyer turn,’ as in a Navy vessel. They bank outboard.”

Cordes says Invincible sought the same handling innovations from its designers Morrelli & Melvin, a California firm. In fact, the Invincible boats bank inboard like traditional V-hulls.

Peters says the resurgence of the planing catamaran, and its subsequent redesign, has resulted from a perfect merge of technology and timing. “Part of it is the new, large outboards that put these boats in the 60-plus mph range,” Peters says.

Curb Appeal Freeman’s boats also looked better than traditional boxy cat boats, which feature near-vertical sides. Longtime multihull specialist World Cat—which evolved out of the original handful of Carolina cat builders from the 1980s—saw the new design trend and created its X series, beginning with the 280CC-X in 2017.

However, World Cat’s customers want greater amenities and comforts than do 100 percent hardcore fishermen. “This buyer [of the 280CC‑X] is 80 percent fish, 20 percent cruise,” says Wyatt Lane, World Cat’s national distribution manager. “Our design process started with a sales scope.” Criteria for that include the various attributes the boat needs for salability such as its speed, comfort factors and price point.

“With the 28, one of these requirements was aesthetically we had to work on changing the traditional cat look and raising that bar,” he says. “The deck cap and hullsides are a challenge. You can’t have a lot of flare because of how skinny the hull is. So you’ve got to be creative.”

The cats of the ’80s came from a similar surge of creativity, but they fell short for a variety of reasons. “Early designs were flawed. They were somebody’s backyard dream,” says Steve Ellis, who founded Calcutta Boats in 1996. “They didn’t have enough reserve buoyancy, and they all bow-steered really badly and sneezed really badly.” (Sneezing occurs when spray shoots out the front of the tunnel and blows back toward the helm.)

Those boats featured knife-edge entry angles, which could easily bury the bow into oncoming waves. “Very few of the cats were really designed” by naval architects, Ellis continues. Calcuttas and many of today’s modern cat builds come from the desks of designers such as Jim Ryan, of Locus International, who drew up the Calcutta 480.

Invincible’s Cordes says the evolving industry piqued his company’s interest. “I think the overall versatility of the boat has got the attention of a lot of folks,” he says. “The sheer volume of storage, and then you get into the fishing platform that it is. It really is something new and exciting.”

Venice, Louisiana, captain John Cole, with Southern Catch Outfitters, has run Invincible’s 40 Cat for the past two fishing seasons. “Invincible did a lot of homework and spent some money,” Cole says. “There are two aspects I like best: how it runs and the internal layout.”

He says it took a little time to learn how best to trim out the boat and engines, but he says he now has high confidence and that the handling is very predictable.

The layout is a mix between hardcore fishing boat and family boat, which serves all of his clients. “The 12-foot beam carries all the way to the bow. There’s so much deck space and square footage,” he says. “I can have a customer fighting a fish next to the console, and I can walk around him if I need to get ahead of him. Plus, the in‑deck storage on the cat boats is just ridiculous.”

Insetta Boatworks 45 cruising through rough seas

Hull Variations The majority of today’s catamarans feature planing hulls—designed to rise up on top of the water—rather than displacement hulls, which plow through the water. They can have asymmetrical or symmetrical sponsons. The former focuses more on forcing air down through the tunnel to create lift, says Rob Coffey, director of sales and marketing for Insetta Boatworks, which builds a 45-foot semicustom asymmetrical cat.

Looking at the bow of a boat with asymmetrical sponsons, what you’d see would look like a V-hull split down the middle: The inside sponson edges would be straight from hull bottom to the water, and the outside edges would angle. The symmetrical hull would look like two small V-hulls side by side.

Coffey says the asymmetrical sponsons float higher and help the boat turn with less outboard force.

Insettas also feature steps, as do Invincibles and several others. Steps do for cats what they do for V’s: They break the surface tension, and create better speed and efficiency.

With asymmetrical sponsons, “you’ve helped correct the turning, but you’ve not made the head sea ride quite as good,” Peters says. “We’re really at the beginning of the curve in perfecting the designs.”

To help the new Barker boat perform better in a head sea, Peters says he made the bow fuller, not as sharp and pointed. A too-sharp angle of entry means the bow can act like a rudder. “What we opted for was saying, if you put this much horsepower in the boat to go 60-plus mph, we want a boat that can absorb all that horsepower and not become difficult to drive,” he says. “We opted for a boat that we feel will bank more appropriately in a turn.”

Calcutta Boats 480 in its finishing stages

Creature Comforts Center-console catamaran builders also face a challenge when creating interior space. A boat that features two hulls with a tunnel between means no step-down beneath the console. To compensate, the builder can increase the interior-console height, but “how do you do that and not have a telephone booth in front of you?” World Cat’s Lane asks.

Cole says the V-hulls he owned prior to his Invincible cat always featured a spacious cabin with a V-berth. However, he says, that just became a black hole for storage anyway.

Companies such as Insetta, Calcutta and World Cat, whose customers often want overnighting capability, have come up with creative solutions. Insetta’s 45, for instance, features an interior-console full-size berth, a plumbed head, and a kitchenette with a microwave, freshwater sink and shower wand. At the bow, Insetta designed twin single V-berths.

Cat builders have also addressed the problem of bow sneezing. For instance, World Cat formed what it calls a Vector Pod in the tunnel near the bow. It helps break up incoming water, creating aeration and reducing friction. As air and water pass through the narrowing tunnel aft, the mixture increases in velocity with a subsequent decrease in forward pressure.

Read Next: Boat Review: Invincible 40 Catamaran

In the next decade, designs will continue to evolve, Peters says. Fishing-boat cat hulls are now at a point in their evolution that deep-V hulls experienced in the 1960s and ’70s. “We’ve got a perfect confluence of things happening. Development happens when things overlap at the same time,” he says.

Billy Freeman says that 35 years ago when the Carolina cats emerged, some builders predicted that monohulls would become obsolete. They didn’t. “I think they didn’t because [the early cat builders] screwed it up. They built on a budget. They had to cheap it out to get to the same price as a monohull.”

Clearly today’s crop of resurgent cat builders has planned a better outcome.

Crazy Cat Traits Out of necessity, catamarans employ a number of design characteristics most anglers would not expect. For instance:

1. Most cats feature a higher deck level than V-hulls, and in fact, the whole boat is generally taller, Calcutta Boats founder Steve Ellis says. That can result in higher gunwale heights and freeboard, making it tougher for anglers to reach the water to release a fish aft—without the assist of a side dive door or an aft tuna door.

2. An offshore cat boat’s height and its tunnel also necessitate at least twin outboards or quads—one or two per side—though Calcutta and Insetta have designed vessels for inboard power.

3. Trim tabs can create problems as well, competing with the outboards for space along the width of the sponson. However, yacht designer Michael Peters says interceptors—blades that drop down vertically—prove less problematic to mount.

4. The wake behind a cat is generally smaller than that created by a deep-V. That’s due to the weight of the boat over two hulls. The narrower hulls push less water out of the way, Peters says.

More: Catamarans , fishing boats , Offshore Fishing Boats

Best Kayaks for the Marsh

Small Boats for Offshore Fishing

What’s Good Fuel Economy for a Fishing Boat?

Yamaha Releases New 350 Horsepower Outboard

Nearshore Topwater Shark Fishing

Fishing the Mid-Atlantic Cobia Run

Make Mine a Jumbo

Sport Fishing’s Deals of the Week

Privacy Policy
Terms of Use
Cruising World
Sailing World
Salt Water Sportsman
Sport Fishing
Wakeboarding

Many products featured on this site were editorially chosen. Sport Fishing may receive financial compensation for products purchased through this site.

Build Your World Cat
Find A Dealer

Cats Vs Mono
Manufacturing Excellence

A Better Way To Boat

Testimonials
Cat'n Around (Blog)
2025 Flip Book - View/Download
2024 Full Line Brochure - View/Download
25th Anniversary Commemorative Catalog - View/Download
Owner’s Gallery
World Cat Premiere Event
Experience World Cat and A Better Way to Boat
Event Calendar
Cat’n Around Owners’ Events
Current Promotions
Monthly Newsletter
Media Coverage
Privacy Policy
Take The Ultimate Sea Trial
Request A Brochure

Register Your World Cat
Owner’s Manuals
Parts Ordering
Contact Your Dealer
How-To Videos

Stay in the Know with World Cat

" * " indicates required fields

Where You Do Your Boating

The advantages of a world cat catamaran. a smoother ride is only the beginning.

Take a sea trial on a World Cat catamaran and you’ll understand just how smooth, just how exhilarating, and just how much fun boating can be. But, a smoother ride is only one of the many advantages a power catamaran offers over a monohull. In fact, a World Cat offers you…

Before You Buy Your Next Boat, Think Twice. You’ll Discover That Two Hulls Are Better Than One.

Compared to a monohull, a World Cat brings you greater stability and a smoother ride in virtually any type of sea.

Chances are, you own – or owned – a monohull boat. And that means you’ve endured pounding, slamming, and slapping. Monohull manufacturers employ deep V designs to try to lessen the abuse, but the result is often just more rocking and rolling whenever you stop or troll your boat. But a World Cat power catamaran offers you a better boating experience without all the exhausting physical abuse.

Why is a World Cat power catamaran superior to a monohull?

Smoothness. stability. speed. world cat hulls are designed to deliver all of them..

When it comes to choosing the ideal boat, World Cat gives you more and better choices. It all begins with the two key hull designs that we offer. While each design shares the same major advantages of a power catamaran – a smoother ride, better stability, and shallower draft – our different hull choices can help you “fine tune” a better boating experience.

Planing Hull

(used in all World Cat boats) – if you’re looking for speed, you’ll find it with this design. By employing a flatter V-bottom surface, you get on plane quicker for maximum speed. This design delivers racecar like performance. And, while traditional monohull boats slam you hard when coming down, this action is greatly reduced in a World Cat power catamaran. Which means less abuse and more control for you.

Semi-Displacement Hull

(used in our Glacier Bay Edition boats) – this design offers a softer ride, and has unique handling characteristics, which its thousands of satisfied owners swear by. Its unique design provides outstanding seakeeping and a soft ride by keeping larger sections of the hull in the water for maximum stability while underway. Its balance of hull buoyancy and dynamic lift reduces drag and increases efficiency.

World Cat Intelligent Hull Design – The Science Behind The Smoother Ride.

World Cat boats deliver a smoother, softer, faster and drier catamaran ride in even the roughest, toughest conditions. The secret – and science – behind this better boating experience is intelligent engineering – which incorporates our VectorFlo® hull design.

This unique hull innovation combines air and water at the scoop-shaped bow and channels the mixture into the tunnel between the twin hulls. As part of the design, a Vector Pod, forward in the hull, helps to break up incoming water, creating aeration and reducing friction. As the mixture passes through the narrowing tunnel channel, it increases velocity, while decreasing forward pressure and drawing the aerated water and spray aft through the tunnel.

The result?

The rapidly moving aerated mixture forms a water-cushioned suspension between the hulls and beneath the boat’s tunnel
You get a softer ride, improved stability, and enhanced fuel efficiency
Because the aeration increases in rough seas, the cushioning effect becomes more evident the worse conditions get

So, you get a better boating experience in all kinds of seas. But, don’t take our word for it, take a sea trial and experience the VectorFlo® advantage yourself.

How We Build The Perfect Hull.

The process of designing a catamaran hull is equal parts physics, experience and technology, and as it is a significant investment in time and resources, it needs to be done extremely well. Here we describe what goes into World Cat’s ‘secret ingredient’ – the hull:

For example, a boat designed for fishing will have more livewell capacity, fish boxes and rod holders, and accessibility around the deck. A cruising boat will put a bigger priority on seating space, relaxing/refreshment areas and other creature comforts.
Then, a general layout of weight and balance factors begins – what are the biggest weight contributors on deck, and how can these be optimally balanced around a center of gravity? How does this shift as people are added or as fuel is depleted? What engines are appropriate and how does their weight and power impact performance? How can stability be ensured if, for example, several fishermen are fishing from the same side of the boat?
Once these basic priorities are identified, the hull design begins.

Below The Waterline Considerations

Amount of planing at certain speeds.
Speed to plane.
Angle at which hull reenters water after a wave (smooth reentry rather than pounding).

Catamarans generally have significantly greater planing characteristics than comparably sized monohulls.

Distribution of deadrise (angle at which hull departs from centerline on hull): shifts from 35° in bow to 12° amidships, with the planing surface at transom.
Shape of chines (deflects water in front; lifting surface aft).
Required lift – defined by four core coefficients of form, boats have to have enough dynamic lift upward to overcome the weight of the boat and ultimately result in planing. This is accomplished by the number, location and shape of lifting chines and strakes.
Planing surface (“delta pad”) – the flat surface that the boat rides on while planing. Catamaran planing surfaces are much more efficient than those on a monohull, which relies on lifting strakes to lift the boat. Instead, catamarans’ planing pads act like a lifting strake, which allows planing at lower speeds. The key to a great planing surface is the perfect balance of size, proportions, distribution, and how far forward on the hull it extends from the transom.

Tunnel Considerations

The defining aspect of a World Cat catamaran and the key to its advantages is the tunnel – as a unique situation where air is available underneath the boat , the resulting aerated water ‘cushion’ softens the ride.

Key balancing act: a high tunnel accommodates more water but forces higher gunwale heights.
Tunnel shape is designed to eliminate vibration and avoid ‘tunnel slap’ created by water movement.
Critical to incorporate elements to aerate incoming water to reduce water friction. The Vector Pod, situated in the tunnel directly between the forward bows, is a key factor here.
Also important to minimize ‘sneeze’ (the occasional forward puffing of water due to too much incoming water) – this is mitigated by downward deflection at the top of the tunnel in the bow.
Importantly, need to design so that it can be built with sufficient durability to provide years of trouble-free enjoyment.

Above The Waterline Considerations

At WorldCat, we work to create the perfect balance between function and style:

All design work includes the incorporation of essential features like anchors, through-hulls, doors, and motors.
Functional factors, such as hull stiffness in critical areas and water deflection at the bow, are explored.
Aesthetics are always a top priority, including everything from rubrail location to the way hull lines accommodate different colors.

At World Cat, we believe we have the industry’s best-performing hulls. But as we design new hulls, we continue to strive to seek out new shapes and materials to further improve the catamaran experience.

ArrowCat Power Catamarans Unmatched Craftsmanship Meets Unyielding Spirit

Experience the thrill of the open water with unparalleled comfort and tailored craftsmanship. Whether you are cruising the coastlines or anchored in serene bays, your ArrowCat power catamaran is your gateway to a life of adventure and tranquility.

Introducing Our Newest Addition, The 20' Power Cat Center Console Model

We intentionally designed this model to be simple and plain – simple to take the boat out, simple to use and to maintain, plain by allowing you the flexibility to customize it according to your preferences.

ArrowCat 20cc

This 20-foot center console is built with strength and purpose, designed for ease and simplicity. Effortlessly take it out on the water, and enjoy straightforward maintenance and operation. Its clean, adaptable design allows for extensive customization, whether on your own or through a dealer.

Fully Planing Hull Design LOA: 19′ 4″ / 5.89 meters Beam: 8′ 4″ / 2.54 meters Draft: 10″ / 0.254 meters

Explore Our 32' and 42' Signature Cabin Models

Perfect for inshore and offshore cruising, long distance and overnight trips, cold off seasons and hot boating seasons, and much more. The ArrowCat 32-foot and 42-foot models provide an exciting and versatile experience on the water. Explore to see which one could best suit your boating lifestyle.

ArrowCat 320

Merging express cruiser elegance with catamaran stability, the ArrowCat 320 Coupe features a planing hull design for smooth, swift rides. Powered by twin outboard motors, it promises dynamic performance and exhilarating adventures on every voyage.

Standard Layout: 2 Cabins/ 1 Wet Head Optional Tower Option LOA: 31′ 2″ / 9.5 meters Beam: 10’ / 3.05 meters Draft: 20″ / 0.508 meters

ArrowCat 420 Coupe

The ArrowCat 420 Coupe combines the luxury of an express cruiser yacht with the stability of a catamaran hull. Designed with a planing hull for agile, efficient navigation and powered by twin outboard motors, it delivers unmatched performance for your ocean adventures.

Standard Layout: 2 Cabins/ 1 Full Head LOA: 41′ 9″ / 12.73 meters Beam: 14′ 9″ / 4.50 meters Draft: 18″ / 0.46 meters

ArrowCat 420 Flybridge

This luxury express cruiser yacht boasts a catamaran hull for superior stability and a planing hull design that ensures efficient, agile handling. Powered by twin outboard motors, it offers robust performance. The addition of a flybridge enhances your view and enjoyment, making every journey unforgettable.

Standard Layout: 2 Cabins/ 1 Full Head LOA: 41′ 9″ / 12.73 meters Beam: 14′ 9″ / 4.50 meters Draft: 20″ / 0.51 meters

Our Unique Design

Enjoy your boat year-round and stay warm and dry during cooler weather or overnight trips, while also having a comfortable and private space to retreat for whenever you need a break. Here are a few reasons why an ArrowCat power cat is an excellent boat to consider.

Catamarans have two hulls, which provide more stability in the water. They are less likely to roll or pitch, which means they offer a more comfortable ride, especially in rough conditions and for people who are prone to seasickness.

Power catamarans are typically more fuel-efficient than monohull boats of the same size. It requires less energy and yields more performance per HP. The two hulls create little to no drag or resistance to get on plane, resulting in greater fuel economy. Allowing for longer journeys with fewer refueling stops.

Power catamarans have a shallow draft which means they can navigate diverse cruising grounds – beaches, islands, rivers, channels, and coastal areas with limited water depth.

Express Cruiser

Cabin boats are designed with comfortable sleeping quarters and living spaces. They feature a sleeping space with a bed, a galley with a stove, sink, and refrigerator, and a head with a shower and toilet.

Cabin boats provide protection from the elements, such as wind, sun, and rain. This allows for comfortable cruising in a variety of weather conditions, as well as providing a haven during storms

Express cruisers are designed for efficient and fast navigation, offering higher speeds compared to traditional cruising boats. They usually have powerful engines that enable them to cover long distances quickly, making them ideal for day trips or weekend getaways.

Powered By Outboard Motors

Outboard motors can provide excellent performance and speed. They can often reach higher speeds than inboard motors of the same horsepower.

Outboard motors have a simple and standard design and are relatively easy to install, they do not require additional components such as a transmission, propeller shaft, couplings, and struts, that inboard engines do. They are easily assessable and cost less to maintain than inboard motors because they are mounted outside at the rear of the boat.

Outboard motors are often designed with features that make them easy to maneuver. For example, they can be tilted or rotated to provide precise control and handling in tight spaces and shallower waters.

About ArrowCat

Crafting Versatility and Adventure on the Water Since 2008

At ArrowCat, we believe in creating boats that are ready for anything. Our American-owned company builds versatile, multi-functional catamarans designed not just for a specific activity, but for a lifestyle that demands freedom and reliability. Whether you’re cruising, fishing, or exploring, our boats deliver superior performance in all conditions.

Trusted by seasoned mariners across coastal and saltwater environments, ArrowCat power catamarans embody the spirit of adventure and the promise of quality.

Learn More About Us

Experience A Smooth, Fast, And Stable Ride

Discover the unparalleled stability and speed of an ArrowCat power catamaran. Connect with our experts to schedule your sea trial today and experience the thrill firsthand.

Log in or Sign up

You are using an out of date browser. It may not display this or other websites correctly. You should upgrade or use an alternative browser .

Planing catamaran vs planing mono hull vs displacement cat fuel 8kts

Discussion in ' Powerboats ' started by johnnythefish , Feb 23, 2024 .

johnnythefish Junior Member

For a given boat length - let us say 26 foot - and all else the same in terms of the boats payload, will a planning cat be more or less fuel efficient than an “average” planing mono hull at non-planning speed? Let us say 8kts. In the same vein how much more efficient is a displacement or semi-displacement catamaran (basically a catamaran not designed for very high speeds) compared to its planning catamaran counterpart at these speeds (8kts). I know that it is very hard to compare apple with apples here but I want to understand whether “the equivalent” planning catamaran is better than a mono hull in terms of fuel efficiency at these speeds; or if real efficiency gains at these speeds only happen with a displacement cat hull.

calevi Junior Member

johnnythefish said: ↑ For a given boat length - let us say 26 foot - and all else the same in terms of the boats payload, will a planning cat be more or less fuel efficient than an “average” planing mono hull at non-planning speed? Let us say 8kts. Click to expand...

philSweet Senior Member

What load? You need to be very specific. The relative performance is very sensitive to weight. Catamarans always have a lot more wetted surface area, which is bad 100 percent of the time. But they make less waves at some speeds, and can be tuned to have small wave losses at a narrow band of speeds. So they can be better at least some of the time for some displacements. Note that cats have a considerably lower payload percentage. If you are trying to build a boat around a specific load, the monohull is at a significant advantage because the craft will be lighter overall. But 8 knots is a really awkward speed for any 26'er with any weight to her.

fallguy Senior Member

I started a reply and lost it. The problem with the question is the premise is false. All else cannot be equal. The planing cat, offers more stability with wider beam, for a theoretical similar displacement and waterplane. The problem for you is you have left too much detail out of the query. If you want a high speed trolling platform; running at 6-8 kts is not efficient in many monohulls. The flipside is the monohulls allow for lotsa passengers and lotsa gear, but they are going to be doing poorly at that speed. The best way to fight this issue is a cat that is bigger than 26'. Of course, this is a classic OPM by me. @DogCavalry would tell you to build a 26' Seasled, but he still can't get into 15' of beam. DM me please as I have some private issues to mention. Let the group know a bit more about the plan and remember, in boats, ceteris parabis is mythical.

DogCavalry Senior Member

@johnnythefish , I'm going to echo the other fellows. Not enough information here to answer your question. The biggest thing missing is how much weight you want to carry. Then what sort of conditions you'd like to be safe in. @fallguy has obliquely hinted that I might like sea sleds. This is my 26' sea sled named Serenity. To save fuel I typically cruise at 5 knots, which requires 12 hp. But that's with 800# of people and beer and snax on board. I've done 15 knots with 3500# of cargo aboard, at 150hp. Or 32 knots with #700 at 250hp. I expect to seriously improve that last number, now that I have the prop @baeckmo recommended. As far as lumpy water goes, I've been out drinking a coffee from a regular cup, and been interrupted by a phone call from a watcher on shore to let me know that if I sank they'd have to watch, because their boat couldn't leave the dock under those conditions. I'm not sure where @fallguy got 15' beam from. I stopped at 10' but she's already stable enough. @Ad Hoc was kind enough to run the hydrostatics for me. Posted on my thread back in 21/08/07. More stable than a flat bottom barge. This is almost certainly TMI, but the question was raised. And it's an opportunity to praise the fine gentlemen who generously help folks on here who are willing to be helped.

Firstly - thank you for all the responses. @DogCavalry nice looking sled - where can I find more info on this concept. How does the sled do at 8kts? Let me try and define my parameters more specifically and let’s do aside with the comparison between a mono hull and a catamaran altogether and ask this instead…. At 8 Kts, how much more efficient will a “displacement” catamaran of 26 foot in length be than a planing catamaran if the beam are the same (can be more than trailerable width and no limit on how wide). The boat must carry 6 adult men, fishing tackle and enough food, fuel and water for an overnight; accommodation can be sparse and minimalistic; basically boat does not need the kitchen sink. More space obviously is preferable but not paramount. Weather conditions are warm and balmy. No chance of rain. Sleeping accommodation can be a mattress on deck that rolls up. Shade is important. The vessel will need to cover 80 miles as fast as possible given it’s design limitations. Faster is better but not to the detriment of the overall fuel economy so long as a minimum speed between of 16 mph can be maintained. The vessel must then travel 160 miles at 8kts. Sea conditions can range from flat calm to large swell and/ and or wind driven chop. Water is tropical.

Your issue with any boat, is most planing boats at 26' are not "out of the hole" at 6-8 kts. What you are really asking is what designs are more efficient at speeds of 6-8 kts. There are semi-displacement monohulls and cats. The cat can offer a more stable fishing platform, but make no mistake, building a cat is more difficult. Two hulls, beams and a bdeck, engines on each. It all depends on how much wirk you are in for for a more stable platform. A 30' cat can probably get into 15' beam which would be an advantage over the Sea Sled. Maybe 13.5 for a 26'er. But everything gets spendy, too. More cables, two start batteries, long cables to the helm. So, I recommend you find some semi-displacement monohulls that will do well at 7 kts (not in a hole), and wide enough for a nice fishing platform. The reason for semi-planing vs displacement is the speed desired and ability to take a 6 pak a farther distance than displacement.

I noticed you had the same type of thread going awhile back. I picked a boat for you. The Bluejacket 28. But make the cabin smaller and the cockpit bigger. This will shift lcg for'd some, so you'll need to accomodate the change somehow. Bluejacket 28 – Bluejacket Boats https://bluejacketboats.com/bluejacket-28/ The Bluejacket 27 is a better design for fishing; not sure if it has a head onboard. On further, probably not big enough in available displacement for a 6 pack, but close. Gonna be hard to find a 26' cat to accomodate a 6 pack as well...

johnnythefish said: ↑ Firstly - thank you for all the responses. @DogCavalry nice looking sled - where can I find more info on this concept. How does the sled do at 8kt? Click to expand...

The one issue with the Sea Sled, is a massive fishing cockpit might mean lcg is too far forward. I'll let John remark more.

Like yours? My LCG is too far forward because I built the bow super massive to carry an actual ton of material in the forward cockpit, or to push a ten ton barge with the bow. Both of those things are job related, so I'm fine with them. With the throttle at 15%, the bow pops up, and she's planing at about 3° with respect to the twin keels, or 6° with respect to the apex of the tunnel. So that's fine. Her drag is too high from huge wetted surface, if I was always running light. It's not the most efficient form. But I carry heavy stuff, and for that I think it's as good as there is.

DogCavalry said: ↑ Like yours? My LCG is too far forward because I built the bow super massive to carry an actual ton of material in the forward cockpit, or to push a ten ton barge with the bow. Both of those things are job related, so I'm fine with them. With the throttle at 15%, the bow pops up, and she's planing at about 3° with respect to the twin keels, or 6° with respect to the apex of the tunnel. So that's fine. Her drag is too high from huge wetted surface, if I was always running light. It's not the most efficient form. But I carry heavy stuff, and for that I think it's as good as there is. Click to expand...

I built the sides of the forward cockpit much too high for convenient fishing. I honestly never thought of it. I was thinking of keeping kids inside and water out, but folks regularly ask me about fishing, so I thinking I pooched that one. I just prioritized wrong.

Hollow Sections aft On Planing Hull

Planing hull boat bow.

Running planing hull at displacement speeds

Ballpark speed reduction amount of small or full (lobsterboat) keel on planing hull

How to Design Planing Hulls

newbie - planing?

Planing hull analysis using Savitsky

New Power Catamarans

37’ Power Catamaran

Outboards on small power catamaran

No, create an account now.
Yes, my password is:
Forgot your password?

Paddle Board

A Guide to Power Catamaran Boats

If you’re into offshore fishing or water sports, the Power Catamaran or “multi-hull powerboat” offers you a great option for your first vessel. These powerboats provide you an excellent combination of performance, stability, and maneuverability.

These boats have a catamaran design, relying on two hulls to float the vessel instead of the typical deep-V hull found on other powerboat models. The multi-hull powerboat is ideal for cruising, and you can set it up for fishing or watersports as well.

With the multi-hull powerboat, you get options for multiple fishing stations over each hull without disrupting the boat’s balance on the water. They are ideal for use in lakes and estuaries, and they excel on the open ocean.

These boats come in lengths ranging from 16 to 30-feet, with plenty of customizable options and accessories. Typically, you get a stern-drive or outboard motor configuration, with center consoles for the driver and loads of storage space onboard.

These boats can carry from six to eight passengers easily, and most models will fit on trailers. This post gives you all the information you need on selecting the right multi-hull powerboat to suit your aquatic needs.

What Is a Multi-Hull Powerboat?

The multi-hull powerboat features a catamaran design, with two hulls running down the boat’s length, featuring a gap between the two. This configuration makes the boat exceptionally stable at higher speeds, allowing fast movement through choppy water inshore or offshore.

The catamaran might seem like a niche boat design. However, it offers you several advantages on the water, such as a smooth ride, stability, and economy. These boats come in a wide range of designs and lengths, with the smallest versions measuring around 12-feet, and the largest extending up to 70-feet or longer.

The longer vessels come with liveaboard facilities and all the amenities you need to spend days out on the water. We like to think of the multi-hull powerboat as the catamaran design of the cabin cruiser or cuddy cabin boat. You get all the same advantages as these models but with an added performance on the water.

You get plenty of options for live wells, rod holders, gear storage, and integrated coolers for drinks and fish. Whether you’re planning a weekend trip or just going out for the day, the multi-hull powerboat is a great choice for your ocean-going excursion.

While the catamaran model is the most popular choice in this category, there are models featuring a tri-hull design. Typically, these vessels cater more towards fishing than performance or watersports, offering slightly less steering maneuverability than the dual hull setup. However, the addition of the third hull brings superior stability to the boat, making them ideal for fishing in choppy water or cruising from island to island on rougher seas.

The ripple hull models typically feature more liveaboard space, with some models having multiple separate living areas beneath the deck.

Benefits of Multi-Hull Powerboats

The Multi-hull powerboat offers you plenty of advantages for fishing, cruising, and watersports. Here are our top reasons for adding this boat to your shortlist of considerations.

Speed and Handling

The multi-hull boat relies on two separate hulls contacting the water. As a result, there is less drag from the hull when cutting through the water. You get faster speeds than you do with a mono-hull design and excellent handling with tight turning circles. These boats do well on open water, allowing for superior stability in rough waters when fishing offshore.

Dynamic Cruising

The multi-hull powerboat features dynamic cruising capability. These boats are most popular with recreational users that want to cruise down the coastline on the weekend or take a few days out on the water for a fishing trip. The built-in accommodations in many designs make it suitable for staying out on the water overnight.

Stability and Performance

Multi-hull powerboats can come with several engine configurations. The motors on these boats offer excellent performance, propelling the watercraft up to speeds of 50 to 80-mph, depending on the model. They also make suitable watersports boats, allowing for skiing and wakeboarding.

Plenty of Storage

The multi-hull boat offers you more storage capability than mono-hull models. You get loads of storage room above and below deck for your dive gear or fishing equipment. There is under-seat storage, and the v-berths in the bow of these models can include plenty of amenities.

Center Console Design

The center console driver configuration is common with the multi-hull performance boat. This driver position gives you more control over the vessel when turning. Some consoles may position closer to the bow or aft of the boat, depending on the length and design features of the boat.

Hardtop Designs

Most multi-hull powerboats come equipped for long ocean-going trips. As a result, they may have a covered driver cockpit leading to below deck accommodations or storage facilities. Some models have wraparound cockpits with doors sealing the cabin, allowing for air conditioning inside the boat on hot days. Other models come with an open plan design and a hard roof.

Trailerable

Most models of multi-hull power bats range from 16 to 24-feet, but there are plenty of longer models. The shorter lengths are easy to trailer, allowing for easy removal for the water and transportation. However, some models may be wider than 10-feet, requiring a special license to operate the loaded trailer. Check with your local authorities for trailer regulations and laws.

Fishing and Watersports Capability

These boats are excellent fishing vessels, offering you plenty of stability for casting on any side of the boat. The center console design means you have walkways on either side of the console, allowing the angler to chase the fish around the boat if it decides to drag the line. Most models also feature setups for watersports like wakeboarding, with T-tower bars or Bimini tops for higher tow points.

Outboard or Stern Motors

The multi-hull powerboat comes with a design for performance out on the water. As a result, these boats usually feature outboard motors with capacities ranging from 150-HP to 450-HP. Some models may use dual-motor setups or stern-mounted motors that hide out of sight.

Multiple Sizing Options

As mentioned, the multi-hull boat comes in a variety of lengths to suit your requirements. Whether you need a large boat for spending days out on the water or a simple day fishing vessel, there’s a multi-hull design to suit your requirements.

Disadvantages of Multi-Hull Powerboats

While the multi-hull powerboat is a flexible design suited for cruising, fishing, or water sports, it does come with a few drawbacks.

Large Engines and More Fuel

These boats feature design and construction for speed, with large outboard motors. As a result, they are somewhat heavy on fuel, especially with a large-capacity dual-motor setup.

Top Multi-Hull Powerboat Models

You have plenty of choices when selecting your multi-hull powerboat. Here are some of our top picks for the best models available.

Calcutta 480

This multi-hull powerboat has a 51-foot length, and it’s ideal for offshore use, providing exceptional stability thanks to the size and the 17-foot beam. It’s one of the largest models available, featuring world-class multi-hull design.

You get a spacious deck with a center console configuration and enough room to walk down either side of the boat when fishing. The dual hull provides exceptional stability combined with the long length, and you get options for diesel-powered or gasoline engines in outboard or in-stern setup to suit your requirements.

The Calcutta brand custom-builds boats for its clients. You get options for fully enclosed bow areas and fishing-style cabins with a roomy helm deck and a sleeping berth included in the bow. You also have an enclosed head for ablutions, but there is no option for a shower.

This model comes with an enclosed cockpit and air conditioning to keep you cool when cruising. The motors on this boat are monsters, featuring a twin setup of 550-HP Cummins diesel inboards available on the sports version for superior power and speed on the water while maintaining the boat’s maneuverability.

There’s a 600-gallon fuel capacity for the thirsty engines, allowing you to spend days out on the water without running out of fuel.

Insetta 35 IFC Hydrofoil

The Insetta 35 IFC hydrofoil offers you the smooth-sailing benefit of hydrofoils, with premium multi-hull designs. The hydrofoil system generates the lift under the hull, allowing for superior, stable sailing in rough water conditions.

The hydrofoil reduces friction and dragging on the hulls, reducing your fuel consumption by as much as 40% compared to other models with a similar dual hull design. The foil fits between the sponsons, featuring design and construction with stainless steel.

Another interesting design feature with this model is the way the inboard motors have positioning towards each other. This configuration allows for maximum thrust for the propellors on the asymmetrical multi-hull.

The foil and motor setup design also allow for much tighter turns than you get with other multi-hull models, giving you similar performance to what you expect in a mono-hull design.

The boat comes with a large coffin box with 156-gallons of space available and an insulated finish. You get eight rod-holders positioned in the bow and aft of the boat. You also get dual 30-gallon transom live wells and an option for a third below the mezzanine seat.

The Insetta 35 IFC hydrofoil comes with a three-pump sea chest, a folding bait station, and plenty of tackle storage. The boat gets its power and performance from dual Mercury 400 Verados, with the vessel topping out at speeds of 58-mph on open, calm waters.

Invincible 46 Cat

This model is the largest in the Invincible range, and it’s a great choice for offshore fishing. This flagship model comes with a 42-foot length and a center console design for easy driver operation. This multi-hull powerboat relies on a hybrid semi-asymmetrical multi-hull giving it great turning capability and maneuverability out on the open water.

The Invincible 46 Cat features a stepped hull with fast acceleration and plenty of lift. You get a quad engine setup with Mercury 450 Racing outboard motors, and the craft can reach a top-end speed of 78-mph. Other notable features of this boat include a vacuum-infused hull and grid-stringer system for an “invincible” boat that’s virtually unsinkable.

Bali Catspace

If you’re looking for a luxury powercat model, the Bali Catspace Motoryacht is a fantastic – but expensive choice. This model features a design from legendary boat maker Olivier Poncin. This model is a natural cruiser and ideal for the longest ocean-going trips.

The dual hull and high ride height from the water provide exceptional stability for the boat, even in the roughest offshore and coastal waters. The boat comes with a lounge on the deck, and there’s plenty of room around the center console cabin to walk the length of the boat on either side of the vessel. The top level of the boat features the captain’s station and wheelhouse, with luxury living quarters underneath.

You get a huge lounge and a v-berth with sleeping quarters for spending the night out on the water. The cockpit presents the captain with a 360-degree view of the water, and the high riding position gives you a view of the ocean that extends for miles.

The boat comes with all the amenities you need, including tables, a full kitchenette, and luxury sleeping accommodations. There are plenty of entertainment options for TVs and stereo systems down below, with an optional hardtop Bimini.

The Bali Catspace Motoryacht receives its power from a single or dual engine setup featuring 150-HP or 250-HP Yamaha motors.

Wrapping Up

With so much variety available in multi-hull powerboats, you have options for any activity out on the water. These boats are more common in coastal waters, and they make excellent fishing vessels.

Decide on the model that suits your activity, as most have a purpose-built design for fishing, watersports, or cruising. There are plenty of customization options, so make sure you keep a budget in mind as the additions can cost more than 20% of the boat’s initial sticker price, increasing your costs.

John is an experienced journalist and veteran boater. He heads up the content team at BoatingBeast and aims to share his many years experience of the marine world with our readers.

A Complete Guide to Micro Skiffs: All You Need to Know!

A complete guide to narrow boats: all you need to know, a guide to aluminum fishing boats.

Comments are closed.

Type above and press Enter to search. Press Esc to cancel.

Advantages of Catamarans and Catamaran Hull Speeds Calculation

Categories : Marine history
Tags : Marine engineering

What is a Catamaran?

Before going into the advantages of catamarans, I’d just like to define the term. Catamarans are a class of boat that has two hulls, generally equal in size. The hulls are connected by a structure of some sort. Catamaran hull speeds are very high, making them a good choice for recreational uses. Catamaran sailboats are common, but catamaran ferry designs are quickly growing in popularity due to their unique advantages.

The catamaran is a ship design with an interesting history. The twin-hull design was first observed by Europeans in India in the late 17th century, but it was actually invented by South Pacific islanders. The capabilities of catamarans were more or less ignored in the West until 1877, when an American named Nathanael Herreshoff began developing and racing catamarans. His designs were so much faster than traditional monohull designs that sailing authorities banned them from competition for nearly a century. Today, catamarans are a popular design for recreational and racing sailboats, and are being used in efficient, low-wake ferries .

Image: Flickr , Catamaran in Cozumel.

Advantages of Catamarans

Catamarans are, in general, faster than single hull boats. This is due to several factors, the most important of which is the hull shape. Displacement hulls are the most common shapes for single hull craft. A displacement hull is supported on the water entirely by buoyancy effects. This creates a hydrodynamic drag barrier, which slows the craft. Catamaran hulls can be designed as planing or displacement hulls, but they are not slowed nearly as much by the drag barrier. Catamarans reduce drag by making use of a very thin and pointed hull design.

Catamarans are also very light, which further reduces drag and displacement. Catamarans save on weight because the multihull design eliminates the need for a keel counterweight, as the same purpose (righting the ship) is served by the hull spacing. This can make them difficult to turn, however.

Catamarans are also more stable than monohull ships, enabling designers to use more sail per foot of the boat. Stability comes from the wide beam legth; beam length is the distance from one side of the boat to the other. This wide beam and stability also allows catamarans to gain more power from heavy gusts, because they do not tend to heel over like monohulls do.

Catamaran Hull Speed

More efficient single hull boats are designed to travel with a low Froude number, which reduces but does not eliminate the drag barrier. This drag limits hull speed based on a relation involving length at waterline, given by the formula V=(gL/(2pi))1/2, where g is the gravitational constant and L is the waterline length. This is the theoretical limit for single hull displacement craft. Note that ships can sometimes exceed this theoretical limit, but to do so requires a very large power source.

The calculations for a catamaran are more complicated. The formula for catamaran hull speed is 1.34*(wetted length)1/2; however, this drag formula is generally not the limiting factor for catamaran hull speed. This is because boats with waterline length to beam ratios greater than 8:1 are not limited by hydrodynamic drag factors, whereas smaller boats need to plane to do so (planing requires enormous amounts of power for displacement hulls). A more important factor to consider is the prismatic coefficient, Cp. Cp = V/(LBP*Am), where V is the volume of water displaced by the hull, LBP is the length between perpendiculars, and Am is the area at midship.

Very fast boats actually require a high prismatic coefficient, which in turn requires a less-narrow boat. However, narrower hulls can get away with a lower prismatic coefficient. The ideal range of Cp for a catamaran is between 0.61 and 0.65. There are a few ways of increasing the prismatic coefficients: sailors can use bulb bows, a wide planing aft segment, or a flat hull rocker in conjunction with a bustle aft. Though high prismatic coefficients increase drag at low speed, at high speeds they can reduce drag by as much as ten percent.

Catamaran Hulls and Low-Wake craft.

As explained here , catamarans can be used to create low-wake ferries. Because of their wide bows and low weights, catamarans can be made to travel quickly with minimal wake. Catamaran ferries are also more fuel-efficient, because of the previously-mentioned factors.

Image Credit: Wikimedia Commons , Catamaran Ferry

Catamaran Boat Design

Hull Speed Calculations

Catamaran Hull Comparisons

IMAGES

Planing hull catamaran for high speed operation in a seaway Patent
Optimising Hull Lines for Performance
Mystic catamaran plans ~ Free tunnel hull boat plans
yues Parliers planing hull catamaran
Catamaran ferry plans
Planing hull catamaran for high speed operation in a seaway Patent

COMMENTS

Catamaran Hull Design
A catamaran hull is much like a frigate hull (similar SLR, L/B ratios and Froude numbers) and there is plenty of data available for those. There is also a lot of data for the round bilge narrow non planing motorboats popular in the 1930'-50's which again are similar to a single multihull hull.
The Planing Power Catamaran: A Different Kind Of Cat
Mike Myers, vice president of product development for World Cat explains: "Catamaran hulls experience little to no drag or resistance to get on plane, resulting in greater fuel economy. They have a steady rise in speed and fuel burn with little to no spikes in fuel consumption."Planing powercats have a unique trait — which many cat lovers ...
How We Build the Perfect Hull
Instead, catamaran planing pads act like a lifting strake, which allows planing at lower speeds. The key to a great planing surface is the perfect balance of size, proportions, distribution, and how far forward on the hull it extends from the transom. Tunnel Considerations
Planing Boat Hull Designs: Pros & Cons of Basic Types
Planing boats are designed to rise up and glide on the water when enough power is applied. Whether you're in the market to buy a planing boat or you want to ...
Planing hulls for sailing cat
The planing hull catamarans that have been designed, built and sailed have usually had stepped hull shapes based on the floats of water planes. I guess Yves Parliers boat is the most famous of these. They claim it worked very well under the right conditions until it crashed and burned. Gareth. grob,May 22, 2006.
Optimising Hull Lines for Performance
A discussion about catamaran hull shapes and how they relate to performance and sea keeping with specific reference to the 33' catamaran Mad Max. ... possibly promoting some planing effect. If we had a more even rocker line we would slightly reduce the wetted area, but we would increase the pitching and the water would exit the hull aft at a ...
Hull Hydrodynamics and Design
Figure 2.4 - Typical Planing hull - a) Side view b) Mid-ship station c) Top View. ... An existing hull shape of a sailing catamaran currently being produced in South Africa could be used, or a standard hull shape of suitable characteristics could be used. In either case, a representative hull shape would need to be found in order to determine ...
Catamaran Hull Speed Calculator For Beginners (Table and Free
This will be your calculated maximum hull speed for a semi-displacement catamaran. The Formula. First of all, we need to know the maximum hull speed for a displacement hull, and from that number, we will be able to calculate how much faster the semi-planing (or semi-displacement) hull will be. This is the formula for Maximum Hull Speed on a ...
The Illustrated Guide To Boat Hull Types (11 Examples)
Catamaran Hull. The catamaran is similar to the pontoon hull (read on to learn more on that one), but it is a displacement multihull instead of a planing one. So it has two hulls, that lie inside the water and displace it. Like the pontoon, you will have to try really hard to capsize this design (and it won't work). Examples: well, catamaran ...
Experimental and numerical investigation on the resistance
A 1:4 scale model of planing catamaran hull has been built and the geometry of the planing catamaran in this study is shown in Fig. 1.And the main parameters of this catamaran are presented in Table 1.The hull is divided into two demi-hulls arranged aside by a tunnel, so this kind of planing catamaran is also named tunneled planing hull (Roshan et al., 2020).
Catamaran Design Formulas
T c = 0.57. Here we put B TR = 1.9 to minimize boat resistance (for her size) and get the draft calculation for a canoe body T c (Figure 1). Midship coefficient - C m. C m = A m / T c (x) B WL. We need to estimate a few coefficients of the canoe body. where A m is the maximum cross section area of the hull (Figure 3).
Semi-Displacement Hulls Explained (Illustrated Guide)
A semi-displacement hull is a hull design that combines features of the displacement and planing hull. It displaces water at low speeds but is able to generate lift at cruising speed. It is more stable than planing hulls, and faster than displacement hulls. ... Catamarans can. There are cats out there with wide and flat aft hull sections ...
Hydrodynamic modeling of planing catamarans with symmetric hulls
Boats moving at high speeds operate in the planing mode when the hydrodynamic lift on their hulls overcomes the hydrostatic lift. To improve lateral stability of such boats and to increase available deck space, twin-hull arrangements are often implemented (Fig. 1).The hydrodynamic interaction between hulls operating in a proximity to each other can be significant and needs to be accounted for ...
Multihull, Catamaran Boats Invade the Offshore-Fishing Market
The majority of today's catamarans feature planing hulls—designed to rise up on top of the water—rather than displacement hulls, which plow through the water. They can have asymmetrical or symmetrical sponsons. The former focuses more on forcing air down through the tunnel to create lift, says Rob Coffey, director of sales and marketing ...
A Better Way To Boat
Catamarans generally have significantly greater planing characteristics than comparably sized monohulls. Factors in design: Distribution of deadrise (angle at which hull departs from centerline on hull): shifts from 35° in bow to 12° amidships, with the planing surface at transom. Shape of chines (deflects water in front; lifting surface aft).
A review on the hydrodynamics of planing hulls
There are automatic generation codes for the stepped planing hulls or catamarans and this is an area where emerging knowledge from computer science can be employed (see Section 10.1.2). Table 3 . Different methods developed for parametric generation of semi-planing and planing hulls (This table is originally presented in Khan et al. (2017) ).
PDF DISPLACEMENT POWER CATAMARANS
Planing Catamaran The planing cat's ride is still rough compared to dis-placement cats, but better than on a planing single hull boat. The top speed of these craft again are only dependant on the amount of power applied. Oddly, power needed, and efficiency are not much better than single hull planing craft, at planing speeds. Below
ArrowCat Power Catamarans
ArrowCat 320. Merging express cruiser elegance with catamaran stability, the ArrowCat 320 Coupe features a planing hull design for smooth, swift rides. Powered by twin outboard motors, it promises dynamic performance and exhilarating adventures on every voyage. Standard Layout: 2 Cabins/ 1 Wet Head. Optional Tower Option. LOA: 31′ 2″ / 9.5 ...
Are squared hull/transoms essential for a planing catamaran hull
A short, fat hull (like a Formula board which is 3' wide and about 7' long) is very inefficient at low speeds but if you can summon the power to drive it fast enough to reach its planing threshold, it abruptly starts to plane at lower speeds and more efficiently than the long skinny hull. The jump from "displacement" to planing speeds and vice ...
Planing catamaran vs planing mono hull vs displacement cat fuel 8kts
At 8 Kts, how much more efficient will a "displacement" catamaran of 26 foot in length be than a planing catamaran if the beam are the same (can be more than trailerable width and no limit on how wide). The boat must carry 6 adult men, fishing tackle and enough food, fuel and water for an overnight; accommodation can be sparse and ...
(Pdf) Effect of Hull Design Variations on The Resistance Profile and
In catamaran planing, we found that the hull resistance decreased by 7.8% at the 30° deadrise angle compared to the most significant resistance at other angles. Geometric scheme of the deadrise ...
A Complete Guide to Power Catamarans: All You Need to Know!
The multi-hull powerboat features a catamaran design, with two hulls running down the boat's length, featuring a gap between the two. ... Whether you're planning a weekend trip or just going out for the day, the multi-hull powerboat is a great choice for your ocean-going excursion. While the catamaran model is the most popular choice in ...
Advantages of Catamarans and Catamaran Hull Speeds Calculation
Catamaran hulls can be designed as planing or displacement hulls, but they are not slowed nearly as much by the drag barrier. Catamarans reduce drag by making use of a very thin and pointed hull design. Catamarans are also very light, which further reduces drag and displacement. Catamarans save on weight because the multihull design eliminates ...
Watch: Twin-hull hydrofoiling catamaran smashes big-wave storm tests
The red twin-hull catamaran in the images and video above is Safehaven's new T-2000 Voyager, a 20-m (66-ft), long-range, high-performance, semi-wave piercing explorer boat capable of speeds over ...