Daggerboards or Mini Keels for Catamarans?

Comparing fixed keels and daggerboards. 

Fin keels, fixed keels, minikeels, skegs. Call them what you like, we know what you’re talking about. Choosing between fixed keels and daggerboards is probably one of the most debated options for a cruising boat and a difficult decision for a first time boat builder or purchaser. There’s no doubt high aspect ratio daggers give the optimum lift to windward and quicker tacking, but how significant is the downside of going to fixed keels, and are there significant disadvantages to living with daggerboards?

Fixed keels often get a bad rap. But it’s largely because there’s a lot of fixed keels out there that aren’t well designed. Poor section shapes and too shallow are the main problems.

Not all fixed keels are created equal, and because fixed keels by nature have a lower aspect ratio than a daggerboard, the foil section and planform (side view) selection are critical to minimise the tip loss - or the flow that runs under the keel and reduces the lift which provides windward ability. Obviously a relatively deep keel of moderate length will develop more lift than a long shallow keel, and this can be further improved by fitting an end plate to the base of the keel.

Construction Time and Cost

The dagger and dagger case combination probably take a little longer to manufacture and install and you will need to use carbon in the spar caps, but otherwise there is no significant difference in materials costs. 

However for daggerboards you do need to pay particular attention to the hull to dagger case bonding areas which need to be de-cored and strongly reinforced. Also there is the additional hardware involved in the controls required for daggerboards, and a little extra deck clutter.

Asymmetric boards? These are totally inappropriate on a cruising cat in my view. With asymmetric boards the windward boards should ideally be lifted each time you tack or it will be causing drag. This means each board needs to be larger to get the same lift. An unnecessary complication for a minimal difference in performance on a cruising boat.

Building Implications

The keels are at a disadvantage here in that the building space will require 500mm or more of roof height if they are attached to the hulls at the start of the project. This also means more climbing and more scaffolding to work on the boat. One option here is to fit the keels and glass tape them to the hulls after the boat is extracted from the building space and lifted to the required height.

Performance Comparison

The difference in windward performance between fixed keels and daggerboards on cruising cats varies depending on the conditions. In very light airs the daggerboards have a clear advantage, providing reduced wetted area and greater lift at angles of attack where a large percentage of the fixed keel’s area is likely to be suffering from flow separation. The boat with a fixed keel is likely to point nearly as high as one with dagger boards, but will lose ground though increased leeway.

As the wind strength increases the fixed keels will typically operate much more efficiently and from personal experience the difference in performance between two similar cats -one with efficient fixed keels, the other with daggers - is minimal in 10 knots or more of breeze, and negligible in 15 knots and above.

The boat with the fixed keel might be expected to suffer more friction drag downwind due to increased wetted area, but once again from personal experience this disadvantage is only clearly evident in light airs.

Other Factors

Most of the cruising boats I have designed have been fitted with fixed keels - even though an option for dagger boards is provided in the plans. This is mainly because of the advantage of being able to dry the boat out on the beach with the hull bottom easily accessible for cleaning, but also for the protection the keel provides to the rudder and sail drive leg.

These factors don’t rule out dagger boards - most boats will sit on the hull bottom and the rudder without suffering damage to the rudder stock - however I do recommend that the dagger board, if fitted has a restraining strap or line which can be released to prevent the board being raised above the height of the rudder tip or sail drive leg when sailing.

Keep in mind that with fixed keels you will have less draft than a boat sailing with the boards fully down so the chance of a grounding is increased with daggers, and because the fixed keel has an angled entry profile it is less subject to damage from a sudden collision. Dagger boards take very high loadings and cedar and glass construction is generally more forgiving than amateur composite construction.

Daggerboards can be noisy inside the case, both when raised or lowered. This problem can be minimised with soft bearings at the top and bottom inside the case (synthetic grass or carpet) but is not always easy to completely avoid.

On a boat any bigger than about ten or eleven metres the daggerboards are a big chunk of stuff and can be difficult to raise and remove from the boat for maintenance.

We sometimes hear the argument that fixed keels could cause the boat to trip and capsize sideways in a sea. In steep seas you're most likely to be running (possibly with a drogue) or laying to a sea anchor.

I've never heard of a cat tripping sideways over its' keels but please enlighten me if this has been the case.

Making the Decision

I generally recommend that daggerboards are a worthwhile investment if the owner has a preference for sailing to windward rather than motoring or waiting for a favourable breeze, and if the boat is going to be kept light enough to have a power to weight ratio that will do justice to the dagger board configuration. 

As a rough guide I would suggest that a 40’ cat which had a sailing weight of six tonnes or less would benefit from dagger boards, while a relatively heavy cruising cat would gain very little  benefit from the more efficient foils. On a 50’ cat the cut off point is probably about 10.5 to 11 tonnes.

Dagger boards should be at least partially raised when reaching at high speed, and the need to raise and lower the board at various times is an added complication that a lot of cruising people would prefer not to have.

Daggerboards do break from time to time. Until now I have not heard of a cat having a structural problem with fixed keels, except for one that was washed sideways across a coral reef and then mercilessly pounded by waves. In that case the keels may have provided some additional safety.

 Generally speaking cruising cats with fixed keels are probably easier to resell than cats with daggerboards. If you’re finding it difficult to make the decision and you're not interested in racing then it’s likely that fixed keels are right for you.

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Sirius Yachts - twin keels are the most popular

What are the pros and cons of different keels?

We all sail for different reasons, in different cruising grounds and use our yachts differently, so it makes sense that there is no one-size-fits-all keel design. At Sirius, however, we like to make the perfect yacht for each individual owner. One of the ways we serve our customers is our choice of keels – at least six different options for each model. It’s one of the ways we stand out – or should that be stand up?

We offer three styles of keel: fin, twin and lifting swing keel. All of our keels excel in many ways, but every design does have drawbacks – this is not unique to Sirius, but the keel affects the way you use the boat, so it’s important to choose the right one for you.

These are the keels we currently offer:

Standard Fin (310 DS, 35 DS, 40 DS) Performance Fin (310 DS, 35 DS, 40 DS) Medium Fin (310 DS, 35 DS, 40 DS) Shallow Fin (310 DS) Shallow Twin (310 DS, 35 DS, 40 DS) Performance Twin (35 DS, 40 DS) Lifting Swing Keel (310 DS, 35 DS, 40 DS)

Sirius Yachts - Whichever keel you choose they all have the same stability

Does the choice of keel compromise ocean capability?

For Sirius yachts, absolutely not. It’s important to realise that choosing one keel style over the other does not affect the yacht’s righting moment or compromise its ocean-going capabilities at all!

Whichever keel you choose, deep or shallow, twin or fin, they all have the same stability. This is achieved by putting more weight in the bulbs of the shallower keels as the shorter lever can be balanced with higher weight. Most of the blue water cruising and circumnavigations in Sirius Yachts have been made with twin-keel or reduced/shallow fin keel yachts.

Sirius Yachts - Most circumnavigations in Sirius Yachts have been made with twin-keel or shallow fin keel

Does keel choice affect performance?

As our shallow keels are heavier the weight dampens the yachts’ motion at sea, but as a downside, you have more weight to move with sails or engine. Once you’re moving there isn’t a difference but when tacking or gybing, or when not steered well, you will lose a bit in sailing performance. The shallower draught yachts also lose a few degrees to windward compared to their deeper keeled sisters, but they are still good all-round performers. Our customers with racing backgrounds always try to go for a keel as deep and light as their sailing area permits, either with a single or twin keel.

Sirius Yachts - performance fin keel

Pros and cons of fin keels

The standard keel on our yachts is a fin keel. Most sailing boats today use a fin keel because it gives a good all-round performance on all points of sail. By keeping the ballast lower it gives the most comfortable motion. The main downsides are that the draught (the depth of water required to stay afloat) is the greatest, and it’s very important to avoid running aground on a falling tide. Fin keel boats cannot dry out without additional support, either from a harbour wall or by fitting a pair of beaching legs. Some fin keel yachts are not built strongly enough to stand on their keels when out of the water, so they can’t dry out alongside a harbour wall and they need to be kept in a special cradle when stored ashore to avoid the risk of the hull deforming under its own weight. By contrast, all Sirius yachts can stand on their keels for any length of time with no problem at all.

We offer four types of fin keel. The standard fin is available on the 310 DS, 35 DS and 40 DS and is fully cast-iron. It offers the best value, good performance, and excellent responsiveness. It is the deepest of our fixed-keel options, so if you want less draught you may want to look at our other fin keels.

We also offer a performance fin keel for all our models. This uses a cast iron fin with a lead bulb at the tip (bottom). The structural strength of cast iron means the fin is the slimmest profile, but lead is denser than iron so the same volume of lead will weigh around 1.4 times more than cast iron, giving more righting moment. The heavier, softer lead down low has less volume in the bulb so achieves a slimmer profile with less drag and therefore better performance.

A lead bulb is also safer if it hits something. Lead can absorb 60% of the energy in flexing and deformation so that only 40% of the force will be transferred to the laminated structure of the keel reinforcement. A lead bulb is very forgiving and easy to reshape and will not start to rust where the coating is damaged. We can use less volume of lead than iron, and achieve better stability than a wholly cast-iron keel. We can also reduce the depth of the keel and retain excellent stability. However, lead is more expensive than cast iron and the bulb must be attached very securely to the iron fin, so this option does cost more.

If you want less draught, we also offer a medium fin. This reduces the draught of the 310 DS and 35 DS by around 40cm/1ft 4in and 55cm/1ft 9in on the 40 DS. Like the performance fin, it uses a cast iron fin with a lead bulb. To retain the keel’s grip in the water it has to have a longer chord (the distance from fore to aft). While this gives the boat better directional stability, it does make her a little less responsive and a little slower to manoeuvre.

On our 310 DS, we offer a shallow fin option – a special version for very shallow cruising grounds. This fin keel offers the least draught of any of our fixed keel options at 1.15m/3ft 9in and draws 10cm/4in less than the twin keel version. The keel has a significantly longer chord (2.24m/7ft 4in compared to 0.7m/2ft 3in of the standard keel) so she has the reassuring directional stability of a long-keeled yacht but with better manoeuvrability.

Sirius Yachts - twin keel

Pros and cons of twin keels

Our twin keels are the most popular option. About 70-80% of all Sirius Yachts are delivered with them – and on the 40 DS it’s 90%. Some folk still believe there is a big performance penalty with twin keels. In the past this used to be true but it’s no longer the case with modern twin keel designs, from Sirius at least. We have conducted many two-boat comparison tests, often battling for hours, by ourselves, with owners, and for sailing magazines and we have found that there may only be one or two boat lengths of difference at the end of a long windward leg, if at all. At the end of many of these comparison tests, the crews could not point out which of the boats had the twin keel.

If you cruise tidal areas, twin keels will reward you time and time again. Not only do they give you a shallower draught than the typical fin keel, they also give you the ability to dry the yacht out, whether that’s for a motion-free night’s sleep, to explore cruising grounds others cannot reach, or just for cheaper mooring and maintenance costs.

Siriius Yachts - performance keels have a deeper draught and thinner chord

We offer two styles of twin keels; performance and shallow draught. Both options have a cast iron fin with a lead bulb. The performance keels have a deeper draught and a thinner chord so they act and feel a bit livelier when sailing and manoeuvring. The shorter keels have a longer chord, but give you the ability to navigate shallower areas. Like all keel designs, twin keels do have some downsides. They are more expensive than fin keels, and when you’re sailing fast in choppy seas at a steep angle of heel, you can occasionally get a slapping sound when an air pocket is caught and pressed out under the windward fin. Lastly, we’ve yet to meet an owner who enjoys antifouling between the keels. Thankfully it only has to be done once a year and with twin keels you might get away with doing it less frequently. A twin keel yacht can be kept on a drying mooring, where fouling is reduced because the hull spends more time out of the water. And when you’re off cruising it’s easy to give the bottom a quick scrub while the yacht is dried out.

Our yachts will happily sit on their keels on a hard surface, like a drying grid, or for winter storage but on softer surfaces we use the rudder for additional support. The rudders on our twin keel yachts are specially reinforced for this: we use a Delrin sheave to take the weight of the hull and the tip of the rudder has a wide, foil-like foot to spread the weight.

Sirius Yachts - we don’t use a grounding plate to take the weight of the yacht

A lifting swing keel

We are one of a few manufacturers to offer a lifting swing keel. There’s a lot of confusion with the term ‘lifting keel’, it seems to encompass all yachts that have centreboards, variable draught, lift-keels or swing keels. To us, a lifting keel boat should have all the ballasted weight of the boat in the keel, and that keel needs to be retracted into the hull.

Sirius Yachts - swing keel has a ballasted fin with a single pivot point

Technically, a lifting keel is a keel that can be lifted or lowered and gives the boat the ability to dry out when the tide goes out. A lift-keel is a ballasted keel that raises and lowers vertically. A swing keel has a ballasted fin that has a single pivot point and the keel swings up into the boat. There are other variants of design, for example some have a lifting keel to reduce the draught of the vessel but they cannot dry out on it, others have a ballasted keel and ballasted grounding plate. All these examples have a keel that does two things: keep the boat upright and stop her sliding sideways. Our swing keel is designed with a NACA profile to give the most efficient performance.

Centreboard yachts have a centreplate to provide grip in the water and reduce leeway. The plate may carry only 15-20% of the ballast but the rest of the yacht’s ballast is within the hull and/or in the grounding plate. This is called an “integral keel” and is more common as it’s less complicated to build. The lower a yacht’s ballast is located, the better her stability, the more comfortable her motion and the better she stands up to her sail area. The most efficient place for the ballast is as low down on the deepest keel possible – this is why race boats have deep skinny keels with large torpedo-shaped bulbs on the bottom, but they don’t make practical cruising sailboats.

Our keel designs have more weight in the tip (bottom) – using a bulb on the fin and twin keel design and flaring the lower sections on our lifting swing keel yachts. You don’t have this with centreboard and integral keel yachts.

It might be surprising, but a lot of owners come to us thinking that a lifting swing keel is the best option for them. Sometimes it is, but about 98% of customers who approach us because we offer swing keels end up sailing away on a twin-keel Sirius.

Sirius Yachts - drying out

The downsides of a lifting keel

A lifting swing keel does give you more cruising options. It will lift should you run into something and, of course, it gives you the shallowest draught. But that difference is only 40-50cm (1ft 4in to 1ft 8in) less draught than our shallow twin keel option. The lifting keel increases the complexity of the build and the final cost of the yacht; it also sometimes limits the internal layout and engine drive options, and you need to have twin rudders too. Twin rudders make the boat less manoeuvrable in a marina – you can opt for a third central rudder which does improve the handling, but again comes at an extra cost.

On the lifting swing keel, 40 and 310 owners are restricted to the use of a shaft drive, which is less efficient and you have to accept a bit more noise and vibration. When drying out, the drive is more vulnerable to damage, whereas it’s totally clear when taking the ground on twin keels. With twin keels, you also do not have to worry about something sticking out of the beach or stones lying around because the hull is high above the ground. With the hull up high, you do not have to dig a hole in the sand and slide down on your stomach to check or change your anodes as you would on a swing keel.

Sailors who are attracted to the idea of a lifting swing keel should carefully consider the pros and cons to compromise the least. When owners understand the repercussions of choosing a lifting keel yacht, many of them feel it restricts their options too much. They could have a lifting keel or they can sail with twin keels, dry out, have better close-quarters handling and save money in the process. Unless you need the shallowest possible draught – 0.75m (2ft 5in) on the 310 DS, 0.9m (2ft 11in) on the 35 DS or 0.95m (3ft 1in) on the 40 DS – a twin keel might well be a better option.

Sirius Yachts - keel attachment

How are the keels attached?

The design of the keel is important but the way they are attached is just as important, if not more so. All of our fixed keels are through-bolted. Every keel has a wide flange at the root (top) of the keel and the flange sits into a reinforced recess in the hull. The flange and the recess work together to spread the loads of the keel/s into the yacht’s hull. The keels are bonded and bolted to the hull. We use up to twelve 20mm and 24mm bolts (per keel) and these go through rolled stainless steel backing plates inside the hull to spread the bolt loads evenly into the fully laminated keel grid which goes all the way up to the chainplates and also carries the mast support.

For our lifting swing keel, we laminate a substantial keel box as part of the hull to accept the keel and the hydraulic mechanism needed to retract the keel into the hull. Unlike most other boatbuilders we don’t use a grounding plate to take the weight of the yacht, our yachts sit on the length of the leading edge of the keel. Integral keels with the majority of the ballast in the grounding plates move the ballast (weight) from low down in the keel to inside the hull. This negatively affects the stability as the more weight you have lower down, the better.

We also don’t like grounding plates because they bring the hull in contact with the ground. By leaving 10-15 cm (4-6in) of the keel out of the hull when it’s retracted, most of the time the hull is kept clear of the beach and anything that could damage it.

The problem with too much form stability

With only 15-12% of their ballast in the centreboard, most lifting-keel yachts cannot rely on keel weight for stability so their hulls need to be designed with extra form stability instead. This means the hull sections have to be much wider and flatter. A flat-bottomed hull is not what you want for a comfortable ocean cruising yacht; it isn’t sea-kindly or easy to steer in waves and gusty winds conditions. We don’t make that compromise at Sirius. With all the ballast in the swinging part of our swing keel design, we can use the same seaworthy, ocean-capable hull shape designed for our yachts with fixed keels.

If you don’t know which keel would be best for your Sirius, contact us to discuss the type of sailing you intend to do, where you want to sail and what your cruising aspirations are.

General Manager – Torsten Schmidt SIRIUS-WERFT GmbH Ascheberger Straße 68 24306 Plön/Holstein

Fax: 0049 – 4522 – 744 61-29

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Multihull Keels and Daggerboards

Aeroyacht Multihull Specialists Catamarans for Sale

Fountaine Pajot BELIZE 43 resting on her keels, showing good bridge deck height

Uncovering the compromises of both underwater appendages and analyzing their disadvantages and merits.

Man has learned much from nature, and sailboats and their underwater appendages are one of the areas that we have applied what works and what doesn’t. As we all know, boats – as most things in life- are compromises and often we are prepared to make concessions in one area in order to gain an advantage in another. Not only should this brief discussion illustrate the basic virtues and drawbacks of daggerboards and mini keels on multihulls, but also point out their active and passive safety aspects.

Most multihulls in todays marketplace come in two varieties. By far the vast majority of the production cruising catamarans (about 90%) are keelboats and have low aspect ratio, unballasted keels. These well known French, Australian or S.African boat manufacturers market their boats for private ownership and the charter industry and make a great product. Without sounding too general, these boats have very spacious interiors and are perfectly adaptable for live aboard families or the charter trade. This multi million dollar charter industry, has an obvious influence on the requirements and design of their charter fleet, which further has a trickle down effect to other builders who also market their cats for private ownership. Design parameters for these charter cats are often centered around 2 week multiple family vacations in the steady Trade Winds and day sails between closely spaced islands of the Caribbean. The features of these keel catamarans obviously put less demand on pure sailing performance or extreme upwind sailing characteristics, but rather try to please by offering solid construction, live aboard comfort and simple and reliable handling characteristics.

The other type of multihull is the catamaran with articulating daggerboards or centerboards. Centerboard and daggerboard multihulls both share the same basic concept, except their deployment and storage relies on different principles. The daggerboard lives in a scabbard, or daggerboard trunk. It moves up and down, vs. a pivoting centerboard, which is raised and lowered around a massive pin. High aspect ratio daggerboards are by far the most efficient foils. They have none of the problems associated with centerboards, which when fully lowered are only braced by a small area on top. When going upwind in a hurry, loads on the trunk act as a giant lever, which constantly work to spread the trunk apart. This is the reason why it is recommended to operate foils as pairs in heavier conditions to divide the loads. Also the large area of the remaining centerboard slot creates a lot of unwanted turbulence making them unpopular for performance minded sailors. Lastly declining popularity of the centerboarders can also be lead back to the often large intrusion on the interior space the trunks create. Although the Gemini catamaran is a good example of a very popular and well thought out production catamaran, there are few other mass produced centerboarders around. It should be noted that one great advantage centerboards have vs. any other underbody configuration is that they theoretically retract when colliding with an object. In spite of this, most offshore or performance orientated sailors clearly favor daggerboards or low aspect ration keels.

Aeroyacht Multihull Specialists Catamarans for Sale

Illustration A

Daggerboard catamarans have several advantages over their keel counterparts, some of which are well known and others that are more subtle and sometimes only recognized by people who have used them. Active safety aspects are advantages created by speed and the ability to retract underwater appendages.

I am a firm believer of “faster” rather than “slower” on ocean going performance multihulls. Many people might say, going fast is only for racers. But lets think about this. The ability to reduce exposure time through speed is invaluable for cruisers. If say on a transatlantic passage you can shave off 5 days you have already increased your safety factor, in some cases, by 25%. Not being a “sitting duck” is a nice thing indeed. By being able to have the choice, bad weather can be avoided, which can sometimes lead to a negative spiral of incidences. In general, especially on long passages, a daggerboard cat will have the edge on speed over her keel counterpart.

Lets face it, we are all in it for the fun of sailing as famed designer Francis L. Herrshoff said: “The fun of sailing is directly proportional to the speed of sailing”. Maybe this is the reason he designed Amaryllis, his revolutionary catamaran, which was later banned from racing. Generally speaking daggerboard catamarans will always be slightly faster than their keel equivalents. The speed advantage of most daggerboard catamarans vs. keel catamarans though is often exaggerated. On a typical day sail a well trimmed and tuned keel cat will only be slightly slower than a daggerboard cat.

Multihulls lack the feeling of being in the “groove”, which monohull sailors enjoy. Effortless high average speeds, acceleration and sustained high velocity surfs is something fast multihulls compensate with. It should be noted that anything (even a barn door) surfs in the right conditions. Even keel catamarans can surf at speeds up to 30 knots down large seas.

Upwind Advantages: Depending on sea state going upwind, daggerboard catamarans vs. their keel relatives will point up to 5 degrees higher and also experience 2-5 degrees less leeway, which isn’t much one would think. But in an uncomfortable 100 mile beat this ads up to being more than 17 miles closer to your destination! (Illustration B).

Aeroyacht Multihull Specialists Catamarans for Sale

Illustration B

sin A=a/c (sin A) c =a (0.1736) * 100 = a a = 17.36 miles

Lets take a 45’ catamaran: the lift (to windward) generated by a daggerboard is almost twice that of a low aspect ratio keel and the drag with the board all the way down would be almost 20% less. The most recent generation cats with large beams and stately bridge deck houses benefit especially from high lift foils, since the windage of their projected area can ad up quickly. The same cats with keels usually suffer from excessive leeway and sideslip. Keel cat’s however, especially in a calm sea state, lessen this disadvantage as boat speed and flow over their keels increases.

Reaching and Running Deep: When running deep in fresh conditions the fixed could act as a brake, that one cannot disengage. Since they cannot be retracted, their volume and resistance slows the boat’s progress and in combination with the forward pressure of the sails forces the bows down. This is especially the case with catamarans that have long bridgedecks, heavy extremities and low volume- fine bows. The bigger the friction in the water, the bigger the pressure on the mast and the more the boat is burdened. Another neat trick is lifting the daggerboards, one can actually increase apparent wind by pointing up and induce leeway, crabbing faster to ones destination.

Tacking and Helm Feedback: Cruising catamarans are often mistaken to tack slowly and behave sluggishly to movements of the helm. This is certainly true for some heavy keel catamarans and much less so for ones equipped with boards. In complex seas, some of the heavier keel cat’s only option for a safe tack is back winding the jib. Monohulls with only one fin will always tack quicker than multihulls, just as foil equipped catamarans will be more responsive than keel multihulls. Modern hydraulic steering systems are easy to build and with most forward helm stations behind the main coachroof, pose sometimes the only alternative for the builder. Mechanical steering and daggerboards will give the ultimate feedback and fun at the helm…if one is driving at all, since usually the autopilot is engaged for longer legs.

Aeroyacht Multihull Specialists Catamarans for Sale

Inclined foils on the Aeroyacht H42 performance catamaran built by Edel

Another advantage of daggerboards is the better maneuverability under one engine. If you loose one engine and retract the board on the hull which has no engine power, but leave the powered hull’s foil down, the boat will turn easier. Because of the retracted foil on the un-powered side there will be much less drag induced turning moment, the boat will be more balanced and the other sides deployed board will provide sufficient bite for “survival” steerage. Especially in high crosswind situations a catamaran with both boards down is much easier to maneuver than one with shallower keels. Usually harbor maneuvers under power are low speed operations, and this is where the high lift capacity of twin boards excel yet again and facilitate handling and precision steerage. Daggerboard cats also can motor a bit faster, since by retracting the foils they have less resistance.

Aeroyacht Multihull Specialists Catamarans for Sale

Keel and daggerboard cats usually have less draft than keel monohulls, opening access to shallow anchorages. An advantage daggerboards have, are their gauging characteristics when entering shallow harbors or unknown territory. By lowering them, one actually creates a “safety depth”. When they touch bottom one still has the possibility of raising them, performing a U-turn and getting into deeper water. With fixed- non retractable keels, especially with a monohull, groundings or unplanned bottom encounters could end tragic. At least a monhull can attempt in heeling the boat to reduce draft and re float. This obviously is not an option on a keel cat. If you get stuck in the mud you are dedicated to await the next high tide to get you off. Crashing into a coral reef could be a different story altogether and only a haul out can asses and repair damage. Depending on their design, boards could be rotated or flipped, and even repaired underway. In general daggerboard cats also have less draft than keel multihulls allowing access to even more harbors and anchorages. They can be beached high up for repairs or inspections, increasing the window of exposure time between tides. I know, I once performed a 5 hour long emergency repair on our 43’ catamaran this way, saving 1000’s in yard bills and days of headaches.

It should be notes that keel catamarans however, can be beached just as easy as daggerboard cats. They can be left sitting, high and dry, completely safe on their keels, without having to worry about damaging the hulls or getting debris or sea life stuck into the vulnerable daggerboard trunks.

Bruno Nicoletti is an old friend and one of the most low key-expert sailors I know. He has logged more than 130,000 miles (geriatric miles as he calls them) on his 44’ daggerboard catamaran.. At a recent meeting with him in France we talked about his experiences of his record, double handed – one stop- Southern Ocean circumnavigation at age 63. The French Press compares Bruno to sailing legend Vito Dumas and has published his accounts in an article called: “The Impossible Route”. He explained, that in the Southern Ocean, in the most convoluted conditions he would simply raise both boards, lock the helm to windward and lay a-hull with no sails. “Brumas Patagonia” would safely slide down the steep faces of waves and minimally drift to leeward at about 1 mile per hour while he was either resting or reading. The water spoil of his side wards drift would help keep the edge off cresting waves and often prevent them from breaking (Illustration D) While it was blowing 70 knots and higher he felt very safe and in fact the world around him turned peaceful and quiet. His confidence in this system is impressive: I am currently helping him with sea trials on his new 47 footer for a yet another go at a High Latitude – geriatric- circumnavigation. This time he is planning to take his 78 year old brother and do it non stop !

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Illustration D

It is not only in the Southern Ocean that one encounters steep, breaking waves. Major capes or the Gulf Stream are notorious for rough conditions where the ability to navigate safely becomes imperative. In these environments any proven and strongly built daggerboard cat would have a slight advantage by lifting her boards, although a well designed keel catamaran could get through unscathed. In extreme weather, and I am talking beyond Force 10, it is very important to enable a catamaran to side-slip rather than encouraging the possibility to trip, and maybe flip. A catamarans behavior in towering side waves is decisive and the possibility to lift underwater appendages is essential, especially if one has lost the ability to steer. The disadvantage of a keel catamaran in huge beam seas is more psychological than real, as these types of vessels typically also slip sideways. In survival conditions or emergencies, the use of parachute anchors, which force the boat into a certain attitude is often thought to be the only answer for most boats. In my mind this tactic is questionable since it puts enormous strains on the boat, is accident prone and renders one helpless when the odd rogue wave from a different direction smacks into the boat. It is better to manage survival conditions, by controlling and slowing the vessel with the use of drogues.

We have learned a great deal from aerospace industry and the trickle down effect to monohull keels. Multihull designers and builders greatly profit from the advanced research, that has been done in the field of NACA sections (National Advisory Committee for Aeronautics) and foil performance. Keels and daggerboards come in a variety of aspect ratios but most are based on low speed foils, which drag/lift characteristics have been optimized. To increase lift even further, some performance catamarans even utilize asymmetrical shaped daggerboards, shaped flat on the outside (leeward) and cambered on the (windward) inside. As they can only be used one at a time asymmetrical boards are somewhat limited in their adaptation for cruisers. (Illustration E)

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Illustration E

Since multihulls do not heel, their underwater appendages are more effective in retaining positive flow than their monohull relatives.(this is also the reason why autopilots burn out less often and can be used in heavier conditions on catamarans) As the monohull heels, not only is the upper part partly blanketed by the underbody of the vessel but also flow is lost as the water slips past the angled keel to leeward. (Illustration F) This was especially prevalent on the early IOR monos, which had extremely beamy and shallow bilges and high prismatic coefficient center sections. To compensate for this loss of heel induced flow, monhulls need deep draft keels to make good progress to windward. The keel or daggerboard catamaran on the other hand can more efficiently create lift for the same keel plan view area, not only because it has two vs. one keel, but also through its minimal heel is able to keep its underwater appendages perpendicular in the water.

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Illustration F

Usually catamaran daggerboards have a higher aspect ratio and are deeper than the equivalent length monohull, since they have the ability to retract them and therefore have no draft considerations. It is therefore not surprising to see that in wind speeds starting at around 10 knots a well designed and sailed daggerboard cat, will often outpoint and outpace a performance monohull. Even well sailed keel catamarans can often arrive earlier at an upwind destination compared to heavy monohulls. The couple of degrees they sacrifice in their ability to point to windward is often made up by their higher speed and VMG (Velocity Made Good). This performance difference increases proportionally to the wind speed and is very noticeable in F.4 conditions and beyond.

A well known fact and possibly the single biggest psychological deterrent of daggerboard cats is the vulnerability of the boards and trunk in collisions. The true Achilles heel are actually ill constructed and designed trunks, which cause flooding in an impact. This obviously is not the case with keels, which would deflect a minor obstacle, or in case of hitting a container or whale, simply break off. In the case of sacrificial keels, they would sheer leaving the hull completely intact.

Obviously the most critical area in daggerboard design is the construction of the daggerboard trunk. It is usually heavily reinforced with massive gussets, especially at its aft bottom end and extends from the bilge to the overhead. Typical forces on the trunk easily exceed the pressure of the wind on the sails. Dynamic forces of wave action and the shock loads of slamming into seas or solid objects must make this area one of the strongest and best engineered of the entire vessel. Usually the foils are located just aft of the main- mast bearing crossbeam and are somehow tied into this unit to profit from its stiffness. The more “left over” daggerboard remains in the scabbard in the fully down position, the better it is braced, so it is not surprising to see foils that are 18’ long for a 60’ boat. Builders who take their job seriously go through great lengths to make this key area as strong as possible. In a recent conversation with the manager of a reputable French production yard it was pointed out that the daggerboard trunk is engineered and constructed 7 times stronger than the composite board. In case of a violent impact, the foil, which has weak spots designed into it, will snap and leave the daggerboard trunk unscathed. It is a type of sacrificial impact philosophy or a safety fuse, just as it is used on sacrificial fixed keels. The careful engineering and experience necessary in building reliable daggerboard or keel cats stresses the importance of a production yard, which has consistently built them. This aspect should not be taken lightly if one ventures out into the open sea, even if it is only 20 miles offshore.

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Often builders of keel cats ad daggerboards to their designs, with the objective to market increased performance and safety. This usually ends up in a compromise, since the hydrodynamic hull requirements of both types could differ substantially, let alone the experience of proper integration, engineering and construction might be lacking altogether. Building with keels does not require the experience, careful construction and engineering which are necessary with retractable foils.

Keels offer advantages, as they do not need to be operated at all. They do their work silently and its usually one less thing to go wrong. On the other hand people who have never sailed with daggerboards think their operation is complicated. In fact they are as easy to use as outhauls or travelers. In normal conditions with 2 people – one pumping the daggerbaords up haul line directly at the board- the other taking up the slack of the up haul, it literally takes 3 seconds to raise a 15’ daggerboard. Single-handed it is a 10 second affair involving 2-3 wraps of the up haul around a winch and 10 cranks. Most boards are slightly heavier than the water they displace and often only weigh 80 lbs. Dropping takes half a second, by simply opening the up haul sheet stopper and easing the foil down. Loads on the boards increase as the speed and pressure builds, so if one has the choice, these maneuvers are usually performed just before tacking. It is a misconception that the operation of foils will depend on the wind speed. It is rather the boat speed, in regards to sea state – which in turn limits progress – which dictates the proper positioning of the daggerboards.

Nothing is perfect and this analysis would be worthless without mentioning the pro’s and con’s of either underwater appendage. Daggerboards, their surrounding structures and systems are more expensive to build, so builders prefer to stay away from them given the choice. In some catamarans, especially smaller ones or ones that have the trunk in the center of the hull, the interior passage in the hulls can be crammed. Lastly it should be mentioned that incorrectly designed and constructed, daggerboards multihulls can be more of a detriment than virtue and in some cases can be extremely dangerous. If the trunks are not massively reinforced and in case of a violent collision one could flood one hull and cause a capsize. In this case one is better off with a well designed and constructed keel multihull than a mediocre or untested one with daggerboards.

Yet daggerboards give you choices, that keels deny you. It is like the new generation of cars with Tiptronic gearboxes, which offer you an automatic transmission with manual override. By physically selecting the proper gear, torque can be adjusted to suit the conditions. It’s the same with the daggerboard equipped catamaran. The boat can be fine tuned to optimize the level of efficiency of the vessels motion through the water. By being able adjust the foils, superior sailing characteristics result in speed and generally more fun on the water. Active safety aspects of reduced exposure time, better maneuverability and shallow draft provide significant benefits.

For safety reasons, most catamarans builders opt for fixed keels and completely separate the keels from the hulls, a feature which preserves and protects the water tightness of the boat in the event of violent impact. Furthermore, if such a situation arises, it makes them easier to repair or replace. Fixed keels require no manipulation, such as daggerboards and give perfect protection to drive shafts, propellers, rudder blades and hull bottoms in the event of grounding. Lastly the absence of a centreboard case means saving of space in the interior of the vessel and usually results in a larger living space.

Back to the birds. We all know the giant Albatross as an extreme example of a sorer that can glide for days in varying conditions, without having to move its wings. This Southern Ocean beast, who calls his home the most inhospitable area on our planet, is better adapted to handle extreme circumstances than any other flying animal. The Albatrosses’ wings are sophisticated in shape, but basically are articulating high aspect ratio foils very similar to daggerboards. Similar to the Falcons “high speed” wings, the Abatross can modify its wings aspect ratio to adjust to the breeze. The designers of the Polynesian multihulls, legendary Viking long ships and American Cargo Schooners understood this too. By adding movable leeway devices, center- or daggerboards, they made their boats more seaworthy and even sailors in the past preferred them over fixed keels.

Today we continue to strive to find the perfect compromise in our vessels to satisfy our most important requirements. We should be extremely thankful to the 1000’s of keel multihulls that have revolutionized the charter industry and many good boats have evolved from this trend. Reputable dagger board catamarans and well designed keel multihulls will continue to be the choice of future generations of serious offshore voyagers. Both offer their advantages and disadvantages and it is important to understand both in order to make the proper choice.

Gregor Tarjan, a trained naval architect and longtime multihull enthusiast is writer of numerous articles for Multihulls Magazine and various other trade publications. He has been involved in Dennis Conner’s “Stars and Sripes” 1984 Americas Cup Campaign, is the founder of Aeroyacht.He is also the co-editor and contributing author of the “Sailors Multihull Guide”, the book by Kevin Jeffrey.

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Catamaran Structure – Bridge Decks and Cross Beams

  • Post author By BJ Porter
  • Post date April 30, 2021
  • 7 Comments on Catamaran Structure – Bridge Decks and Cross Beams

catamaran keel design

Editor’s Note: Many thanks to Ted Clements of Antares Catamarans and Shane Grover of Seawind Catamarans for patiently answering our questions.

There are a lot of reasons why catamarans are more expensive than monohulls. It’s not just the two hulls. There are many more complicated calculations and structures needed to build the complex shapes.

Building a bridge deck and the structures around a pair of hulls is a lot more difficult to design and build than a single hull, and we’ll explore a little about why.

Part 1: Forces on the Hulls

Load and force calculations on a boat hull isn’t a simple calculation, and even monohulls take a lot of designing to build a shape which performs well and has the strength to hold together at sea. Land vehicles have fairly predictable forces and motion on them, but boats can take forces and stresses in any direction.

catamaran keel design

Waves slam from all directions, boats plunge off waves and get smacked around in chop. Wind forces stress masts and rigging, which applies bending moments and forces to chain plates and the hull. Hulls flex and bend with this motion, and even from tensions applied to the rig.

Elements of drag, hull shape, keel shape and rig design all factor in, whether it’s a heavy, stiff cruising boat or a light, high performance racing machine. Those forces have to be figured, and materials and constructions are made to suit the conditions and situations where the boat will sail.

And that’s just a single hull. When you add a second hull to mix, you add in a whole new set of loads.

catamaran keel design

Bananas and Pencils

To illustrate these additional loads, we’ll do a thought experiment with a couple of household items. You can try it for real if you want to – but you’ll need two bananas and a few pencils.

Start with the bananas laying parallel to each other, then run a single pencil through the midpoint of each banana (the hulls) to connect them. They’re connected, but when you pick it up, what happens? They don’t stay parallel, of course. We need a second pencil or even a third one, to keep them in place.

catamaran keel design

The pencils are the crossbeams you’ll hear about in catamaran construction. If you put two of them through the bananas to connect them about 1/3 of the way from the end of each banana, you’ll get a fairly stable platform (for something made from soft fruit and pencils).

Imagine picking up this banamaran with two crossbeams with one hull in each hand. How can we still move the hulls?

catamaran keel design

First, we can twist one half back and one hand forward, putting lots of force on the crossbeams. To stop this, we could use much stronger beams, and we could put more beams at the ends of the bananas.

If you rotate your hands and the bananas, you demonstrate another type of force on the crossbeams. Pushing the bows or stern together also can move the hulls.

Building the Boat

Now imagine putting weight on the pencils – you’re adding the bridge deck. The mast sits on the bridge deck and creates additional loads and stresses on the crossbeams and hulls.

catamaran keel design

Finally, we add a sailing rig on top of the weight on the pencils. The rig needs support to stay up. On a monohull, stays run to the bow and stern to support the rig. But a catamaran the mast is centered between the hulls. The tension on the rig will provide upward pull on the hull shapes and usually attach to bow crossbeams. So we’re now pushing down on the middle of the bananas while pulling up on the tips.

The challenge to the catamaran builders and designers is to account for all these forces and build a pair of hulls capable of absorbing these loads without breaking or separating the hulls.

Part 2: Crossbeam Design

Catamarans are not new concepts; double hulled sailing canoes were used in Polynesia and Melanesia long before European explorers arrived. One of the first recreational catamarans was designed and built in 1876 by Nathanael Herreshoff and sailed well enough that the New York Yacht Club banned multihulls from racing.

catamaran keel design

Most beams are hollow to save weight in increase strength for the amount of material used. A hollow cylinder or rectangular tube gives more resistance to bending per pound of material than a solid rod of the same weight. There are mathematical explanations beyond both the scope of this article (and my ability to explain), but it’s important to know where the loads on beams are supported to understand this.

Greatly simplified – bending a beam creates compressive loads. The further from the center of the beam, the higher the resistance to compression. A rod will have a narrow diameter and resistance is lower. But if you make a cylinder or square tube from the same amount of metal (or fiberglass) you will have the same cross-sectional area, but the compressive force is applied further from the center of the cylinder.

Think of an I-beam from building construction. The compression is on the sides of the I-Beam, but the part in the middle is mostly to keep the sides in place, not bear the load on the beam in high stress applications.

Original Beams

A close look at these boats shows clear and obvious crossbeams connecting the hulls. Duplex , the early Herreshoff boat, had three clear crossbeams and a cockpit on the aft two between the hulls.

catamaran keel design

Smaller beach and racing cats will have obvious crossbeams, since the decking is usually a stretched piece of canvas or webbing. Other open bridgedeck catamarans, including many home builds, may have actual beams across them holding the hulls in place. And most cats will have some sort of beam across the bows as well.

But when you look at modern bridgedeck catamarans, you notice something strange about the beams. There aren’t any actual “beams” built into the boats.

catamaran keel design

Modern Cruising Cats – There Are No Beams

It’s more you notice something missing about the beams. Modern bridgedeck catamarans don’t generally have actual crossbeams built into them, as if you were glassing a beam or post into the boat. Instead, the construction of the boat is built around a design the provides the mathematical equivalent of a “beam.”

Bear with me. It took a while for me to get my head around this, too.

Picture a box – even a simple shoebox has rigidity to its sides. Yes, you can crush it, but the hollow sided box offers a lot more stiffness than a piece of cardboard on its own. The structure of the beams is in essence a box built between the hulls, with super strong modern laminate materials providing stiffness to take the loads and stresses.

When modern cats are designed, the “crossbeam” is a combination of internal structures built and connected to the bulkheads that create the load bearing capabilities of a hollow beam section. So the “beam” exists mathematically in the designer’s wireframe drawing of the boat, but when it is built, it is not an external beam added to the structure, but rather a set of structures that act like a beam because of their physical design.

catamaran keel design

Developments in materials technology over the last few decades allows for shapes and strengths that couldn’t be built with traditional materials like wood or metal.

Part 3: Building a Bridgedeck

Building the bridgedeck is the key piece of fiberglass catamaran strength. To be able to build a boat which can handle all these twisting and torsion forces, creating that “box” to add the strength, catamaran builders take one of several approaches. All can be effective and meet the design requirements, but there may be other reasons a builder chooses a particular approach.

“Tooling” refers to all the molds needed to shape a fiberglass hull. Tooling can be made from a number of different materials and represents a significant investment for any new production catamaran. Costs can run to many millions for tooling durable enough to build a hundred or more boats without changing design and build tolerances.

catamaran keel design

Molding and tooling to build hulls is a major expense, no matter which approach a builder takes. Building tools and molds can run into millions of dollars in expenses for materials and labor, and molds built for production runs of boats are considerably more expensive than tooling for a one-off or unique design. All of these factors into the decision making behind a build process.

Building a multihull also presents unique challenges compared to building monohulls. Building a single hull can be a fairly linear process – the hull is laid on the mold and built, the inside is fitted out, and decks are attached. While that’s a simplification of the process, it is relatively straight-forward because there is only one hull.

catamaran keel design

For catamarans, the integral bridgedeck structure doesn’t lend itself to a step-wise assembly. The crossbeams and bridgedeck need to be built integrally before the interior and decks are completely finished. For the builder, this means some parts of the boat have to be finished with reduced access to interior sections of the boat. For the designer, the challenge to is to make the boat so the builder can build it efficiently. For production vessels with build runs in the hundreds, cost effectiveness and production efficiency is crucial.

catamaran keel design

Not only does the bridgedeck hold two hulls together against all the twisting and torsion forces we’ve discussed, it also has to carry cargo. It holds the living space in the main saloon, as well as passengers and equipment.

Think back on the shoebox – it has compressive strength from the ends, but any individual side is fairly weak. You can deflect the sides easily. While this is fine on a shoebox, it would be disconcerting if the deck flexed and bounced when you walked across it. The bridgedeck also needs strength from the top and bottom to take this load in the central parts of the boat, away from the “box.” You can’t just make the decks and floors massive, that sacrifices headroom and internal volume. Instead, internal structures and stiff construction materials have to take up the load.

Two Piece Molding

Some builders build two hulls individually. One mold can be used if the hulls are identical, and the hulls joined later in the build process. Like any design and build decision, there are pros and cons for the builder which can affect the cost of the final product. If built correctly, there are no compromises in strength from a one piece mold.

catamaran keel design

For the builder, a one piece mold is much easier to handle. Any hull built on a mold will have to be removed from the mold once the hull is laid, and a single mold is smaller, lighter, and narrower. Breaking a hull out of a mold is a complex process, and may involve cranes and heavy equipment to support the hull as it comes of the mold and to protect the tooling from damage. To remove a hull, you need space to lift the mold and to get heavy equipment around the tooling. And you’ve got to put the molds some place when it’s done.

catamaran keel design

Connecting two hulls precisely once they’re molded is a more complex task. Unidirectional fibers bond key structures to bulkheads to build the support “box” making up the crossbeams. Although a one piece mold will give an inherently stronger single-material connection between the hulls, more than sufficient strength can be built in with advanced fiber and resin choices (such as carbon fiber and epoxy) when then build the deck connection.

One Piece Molding

Most production catamaran builders have moved to one piece molding. A single tool is built to lay up both hulls and the bottom of the bridgedeck connection in a single large piece. The fundamental strength of the build is higher, allowing for less expensive materials to get the same strength.

“One piece” is a slight misnomer, as the top of the hulls and decking is built in a second mold which is laid over the hulls and bonded along the joint near the top of the hull molds. The loads on a join between the bottom hull and top deck are considerably lower than those on the bridgedeck. There’s no real twisting and torsion on that part of the boat relative to the join between the hulls, so it can be laminated without the same concerns as building between the hulls. ( Editor’s Note: Some builders mold the bridgedeck and inboard half of the hulls together and then mold on the outboard halves of the hulls and deck on top. There is a seam running stem to stern centerline along the bottom of each hull with this technique. )

catamaran keel design

There are a number of production and build advantages to this approach. Though the tooling is larger, more expensive and awkward, the lay up process incorporates the core hull joins in the initial build. You don’t have to line anything up and glass it in place when your hulls and deck are built connected. There isn’t need for as much material buildup to shore up the “box” since it’s part of the integral build.

Fitting the interior joinery and finish is more challenging, since the crossbeams and related components must be built early in the process. Extra care and planning in the design process can make this more efficient, but access to internal areas of the hull can be difficult during the build.

catamaran keel design

Alternative Materials – Wood and Metal

Composite construction – fibers and resin – lets builders make nearly any shape. Fiberglass has allowed for the wide range of affordable to high end production catamarans available today. Stronger fibers and better resins have only expanded the possibilities for light, fast, and strong boats.

catamaran keel design

Wood is rarely used for structural elements in modern production catamarans. It can be heavy, and it doesn’t lend itself to the complex molded shapes designers demand for optimal strength and seaworthiness. Plywood may be used for stiffening in places, and balsa cored decks may still be found. But mostly in older boats, home builds, and kit boats.

catamaran keel design

Metal construction has its own problems, and very few catamarans are built from metals. Curved shapes are difficult with metals; bending a smooth radius into a flat sheet without bending it, then welding it into place requires time and skill. And weight will be a problem. On the whole, fiberglass is a better solution for a light, strong catamaran.

catamaran keel design

Conclusion: Tying it Together

This overview only touches on some of the challenges multihull builders face, which monohull builders do not. But what does it mean for you when you’re looking at catamarans to buy? All the fiberglass build techniques will result in a strong boat if built properly. There may be differences in the amount of materials used, and choices for resins and fibers, but the boat should be evaluated as a whole.

It’s good to understand how your boat is built, and to be aware of some of the strengths and limitations of each build technique. But no build technique is inherently better or worse than the other – no matter how your next boat is built, the designer and builder will ensure the build is up to the task.

  • Tags Buying Advice

BJ Porter

By BJ Porter

Owner of Hallberg Rassy 53; world explorer.

7 replies on “Catamaran Structure – Bridge Decks and Cross Beams”

Proof read much? But the no build technique is inherently better or worse than the other… Good article spoilt by miss spellings and errors.

I corrected the extra “the” you mention in that sentence. I do not see other errors. We do our best and thank you for reading and commenting.

Great article. Very informative for us non-technical types. Great use of graphics and photos to explain the various torsion forces and different build methods. What is the purpose of the wing-like, rear spoiler-looking feature that you sometimes see spanning across the back of the deck/bridge and sometimes connecting the two hulls?

It doesn’t actually connect the hulls, it rests on them. In the good old days this was named a “RADAR Arch” and was used to mount the RADAR antenna and dinghy davits.

Over the last 20 years this stern “ARCH” became a very convenient spot to mount RADAR, solar panels and it is in this role it serves very well while also handling various antennas and of course, the dinghy.

Thanks for the article, great stuff, very informative and useful info. Kudos

I’m actually researching and trying to figure out how to build my own catamaran. But information is very hard to find.

Within the past few weeks I saw an illustration (photo?), of a pair of new catamaran hulls, with almost a full-length, flat bridgedeck connecting the length of the upper-inner edge of each hull. (I’m assuming it’s relevant only for a party-or-barge-type, power cat, to be completed by the buyer. I believe they were saying they can offer this bare-bones, hulls-and-bridgedeck package for about 40′ to 70′ lengths.

Any idea who may be offering this? I only skimmed the advt, but I think (??) it was a U.S., east-coast company. Now I’m seriously interested.

Much Thanks R.S.G.

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Catamaran Design Guide

Spectacular sunsets in the Pacific turn the horizon into a brilliant spectrum of gold and orange colors.

Copyright © 2006, 2008 by Gregor Tarjan. Click here for terms of use.

performance, yet desire high daily averages and passage times, which should be as short as possible. When choosing a large multihull, sailors look, above all else, for safety and comfort, long before the consideration for flat-out speed comes into the discussion. Nevertheless, performance is a highly important design consideration. No catamaran sailor wants to sail slower than a same length ballasted keelboat. Below are some EVALUATION & COEFFICIENTS useful coefficients, which will help compare monohulls and multihulls objectively.

Bruce Number (BN)

below "Indigo," a magnificent Wormwood 70, sailing in sparkling Caribbean waters.

Wormwood Catamaran

Various multihull characteristics and design features can be expressed in mathematical formulas. Their results are crucial and will give prospective owners a basis of comparison between different types of catamarans. These numbers are important, as they eliminate ambiguity and clearly display various advantages or concessions of a design, which would be hard to quantify any other way. Mathematical coefficients not only will provide insight into a boat's performance in varying conditions, they also reflect concerns about loads to be carried safely, speed and stability.

We have already mentioned the Displacement/Length and Sail Area/ Displacement ratio in our chapter on Multihull Advantages, illustrating the point of a multihull's efficiency. Let's look at some other coefficients that give us an indication of a boat's performance.

What is performance and how do we really measure it? Most people who buy a cruising catamaran are not really interested in racing

The Bruce Number is very similar to the Sail Area to Displacement ratio although the formula is slightly different. It is the square root of the sail area in feet, divided by the cube root of the boat's displacement in pounds:

SA = upwind sail area (mainsail and 100% jib)

Displ = weight of the boat in pounds

Similar to the Sail Area to Displacement ratio, the higher the coefficient the faster the boat and better is its performance in light air. Typically a BN of 1.1 will be the threshold between fast and more sluggish multihulls. A heavy displacement monohull might have a BN of .7, whereas a modern cruising catamaran shows a BN of 1.3. Offshore multihull racers can have BNs of 2.0 and higher. The BN will also tell us about a catamaran's ability to withstand stronger winds before reefing. A boat with a higher BN is usually overcanvassed in strong conditions and will have to be reefed earlier than one with a lower coefficient.

On the other hand, they will be able to produce more "power" than their counterparts in lighter winds and perform better.

Sail Area to Wetted Surface (SAWS)

SA/WS = Sail Area Wetted Surface Coefficient

SA = upwind sail area

WS = total underwater surface area (hull and appendages)

This formula simply divides the upwind sail area of the boat (mainsail and 100% jib) by the wetted surface. This coefficient will give us a statistical indication of the multihull's lightair performance since in low wind conditions skin friction becomes an important factor. Monohulls can have coefficients of at least 7% more than multihulls.

Hull Fineness Ratio (HFR)

The Hull Fineness Ratio, known as the hull's beam-to-length ratio, is an interesting number. It is derived by simply dividing the waterline length of the hull by the waterline beam of the hull.

Max. WL/Max. Beam WL = Hull Fineness Ratio Max. WL = length of the hull at waterline in ft. Max. Beam WL = beam of the hull at the waterline in feet.

Monohulls, when compared to multihulls, have low hull/fineness ratios. In Part 1 of this

Catamaran Proportions

book, discussing "Efficiency," we saw that ballasted keelboats are limited to Archimedes' principle of hull speed (1.34 x VWL). Multihulls do not have these theoretical barriers, because their hulls are narrower.

The thinner the hull the faster it will be able to travel through the water. But, attention! It will also carry less unless you are on a mega cat. Typically, a 40' cruising catamaran's HFR will range from 8:1 to 10:1. Dennis Conner's above While sailing under spinnaker and experiencing virtually no roll at all, guests will always find a comfortable spot to relax on the foredeck, an impossibility on a monohull.

There are various methods of calculating the transverse stability of a catamaran. One of the simplest and most utilized techniques is establishing a relationship between the height of the Center of Effort (CE), displacement, beam and sail area. Multihull designer, James Wharram added safety factors of 20% to compensate for gusts and the dynamic environment of the ocean. Another method is described in the text below.

Multihull Stability & Capsizing Moment d - Displacement (kg) x half beam (m) max ~ Sail Area (sq m) x Height of Center of Effort (m)

P max = maximum pressure exerted onto sails

Multihull Stability & Capsizing Moment

Trimaran Center Effort

height of sailplan CE

half overall beam (half hull beam)

racing cat "Stars and Stripes" had a 16:1 HFR. Of course, the larger the boat, the narrower the hulls will become in comparison to its length. For example, the HFR of a 100' luxury catamaran may be 12:1, providing it with a high speed potential. However, monohulls can show HFRs of 3:1, though the comparison is complicated as their angle of heel affects the measurement.

One has to be very careful when analyzing the Hull Fineness Ratio of a cruising catamaran, because other factors such as the actual shape of the hull cross sections (Prismatic Coefficient, PC) can throw the analysis off balance. Go-fast sailors like to think that fine hulls are always fast. That is not necessarily true because a slim hull could have a large underwater volume, thus slowing it down. Consequently, a wide waterline-beam hull could have less drag than a narrower one. It could have a shallow underbody (low PC), which would be beneficial to load carrying (Pounds Per Inch Immersion Number, PPI) and early surfing characteristics at speed.

Stability Coefficient (SC)

This mathematical formula has been devised by the distinguished catamaran designer and sailor James Wharram and his team. This coefficient analyzes a multihull's ability (in a static environment) to resist capsizing due to wind.

( 0.682 VW x (.5 Boa) ) x .555 = CW .00178 x SA x h

W = Wind speed, apparent, in mph CW = Critical Wind Speed to capsize in mph SA = upwind sail area in sq ft. h = height of Center of Effort (CE) of total sail area

Boa = Beam overall

This formula will tell us how much wind it will take to overturn our multihull. By instinct we will know that a catamaran with a wide stance and a conservative sail plan will be very stable offshore. The SC formula will inevitably illustrate that a wider beamed catamaran with a tall sail plan will be as resistant to wind induced capsize as a short-rigged, narrower boat. This is not so if one considers the chaotic environment of waves and the real world of heavy weather sailing. It is interesting to note that a wide beamed boat (regardless of the SC) is more resistant to capsize in seas due to the effects of a higher moment of inertia. In an open-ocean environment, which is everything but static, the SC formula has little meaning. Nevertheless, it serves as a good basis to evaluate stability as a factor of wind force.

below When the wind suddenly comes up, all that is needed is a couple of turns on the jib furler to quickly reduce the headsail size. The catamaran will hardly sail any slower, but feel more comfortable.

Ship Hull Fineness

Wide hulls and a large overall beam will increase the overall righting moment of a catamaran. A word of caution: Excessive beam will reduce the fore and aft stability. Designers strive to compromise hull fineness ratios, place heavy weights towards the CG (Center of Gravity), and engineer hull and overall beam to achieve a seaworthy balance, which is safe, yet provides ample liveaboard accommodations.

Catamaran Stability Considerations

Seaworthy Catamaran

Diagonal Stability & Beam-to-Length Ratio (BLR)

Stability of a multihull, or the resistance to capsize, should be seen as three components. Athwartship Stability is one well-publicized type and the one often talked about. The other much more important types are Fore and Aft and Diagonal Stability. Fore and aft stability is established by the relationship between the boat's waterline length and the distance between the hull centerlines. It will reflect the catamaran's resistance to tripping. This relationship should be in the vicinity of 39% to 42%. For a seaworthy cruising multihull it is important maintain the proper ratio between length and beam, which, in turn, balances equal amounts of athwartship with diagonal stability. The goal should be to prevent the possibility of a sudden discrepancy of powers between fore and aft and sideways resistance. Most of today's multihulls keep these two component forces in equilibrium, making them extremely seakindly and safe.

Some early design multihulls were very narrow, partly due to the material limitations of that time. But things have changed. Contemporary composite construction allows designers to build wider boats without compromising stiffness. Production catamarans of today have a wide stance and have the benefit of greater safety margins in gusty wind conditions than their older cousins. Multihulls are sophisticated structures and true modern miracles. They provide a more comfortable ride and more interior room. Thanks to modern materials they weigh less and perform better than catamarans built only 10 years ago.

Some catamarans, especially production boats, which are very popular in the charter fleets, are growing wider by the year. The businesses who rent these beamy monsters adore them. Lots of room plus open decks are ideal for clients and the bigger (wider) the boat, the more paying guests can share the fees. But there certainly is a limit as to how wide is too wide. Extreme beam can be dangerous. It can lead to instability fore and aft and to excessive bridgedeck slamming, as the relative distance from the bridge deck to the water will decrease with an increase in width. A vessel with excessive beam might seem stable athwartships, but it will compromise overall stability.

We know that multihulls can, in extreme cases of seamanship error in wild storms, be thrown over from any side - front, back and beam-on. The best examples of this phenomenon are racing multihulls, especially Formula 1 trimarans, which have fine hulls for speed and huge sailplans to provide driving power. They are initially extremely stable athwartships (High Beam-to-Length Ratio), but have a tendency to become unstable fore and aft. They will surf down waves and reach a point where the power of the sails, and speed, will exceed the ability to keep the bows out of the water and the boat will pitchpole. This is the reason why catamaran designers usually draw their multihulls with a Beam-to-Length relationship of between 50% and 55%. The longer the vessel the lower that percentage becomes.

I am currently involved in the "Gemini" project, which presents an example. It very well might become the world's largest sailing catamaran. She will have an overall length of 145 feet, yet her beam will "only" be 54.4'.

Stable Catamaran Dingy

Please, don't worry. "Gemini" will not be tender and tip over in the slightest breeze. On the contrary, this monster will be one of the most stable craft afloat, although the beam-to-length relationship is only 37%. The relatively low beam-to-length ratio also involves the fact that the boat would be too heavy and building costs would be prohibitive if she were to have a standard 52% BL relationship. Most importantly, could you imagine turning a 75-foot-wide boat?

above Asymmetric spinnakers on furlers are great inventions. They add instant sail area, yet can be doused in a matter of seconds when the wind picks up strength.

Catamaran Underwing

above Although this Edel 35 was a good-looking and popular catamaran, it suffered from excessive bridgedeck pounding, which was caused by only several inches of clearance between the saloon's underwing and the sea.

Obviously there is a sweet spot in the beam vs. stability question. Designing too beamy a boat will also necessitate more freeboard to preserve bridgedeck clearance which, in turn, will increase windage and complicate maneuvering. Unless sophisticated aramid construction methods are utilized, more beam will also add more weight and stress to the structure. Adding more mass will, to a certain point, help make the boat more stable, but where do we stop? Is it better to add weight or width to make a boat stiffer? Of course, both characteristics are interrelated as a beamier boat normally is also heavier. Just adding weight to a catamaran simply to make her more stable will not pay off. Consequently, making a boat too wide might increase living space yet it will also burden the structure, require a beefier manufacture, and yield an even heavier boat. Needless to say, a boat which is too wide will also create practical restrictions such as maneuvering, the ability to haul the vessel and much higher building costs.

Beam has a great effect on bridgedeck clearance, which is one of the most vital characteristics of a good cruising catamaran. As standard practice, the well-known rule of 1" of bridgedeck clearance for each foot of beam was a safe way to prevent excessive wave slap. The wider the beam the more the relationship changes and the necessary height of 1" per foot of beam needs to be increased to 1.3" or more. In the extreme case of overly square boats, that number will have to be closer to 1.8" per foot of beam. This will have a negative effect on any seaworthy multihull that has a bridgedeck saloon. The wide beam will necessitate a high cabin sole to remain a safe distance from the waterline. In order to provide standing headroom, the coachroof might be higher than practical, which could result in a boxy, high-windage multihull. Not only will this be unattractive, but also raise the Center of Gravity (CG) which really should be kept as low as possible.

More overall beam on the other hand (given that there is still sufficient bridgedeck height) has a less known benefit, as it reduces the possibility of hull-wave interference, which is particularly important for fast designs. The wave interaction between the hulls can lead to additional resistance, and especially in an agitated sea state, the formation of wave crests can pound the bridge deck. Most early narrow-beamed catamarans suffered from this phenomenon,

Ultimately, a boat's design has a major influence on its ability to stand against the forces of nature, and to keep occupants safe. Manufacturing excessively wide catamarans is like trying to market monohulls with super deep-draft keels. Both are totally impractical. We designers have to make sensible compromises and learn from past experiences of what has worked at sea by balancing the benefits of a wide boat with its disadvantages.

below This narrow-hulled Outremer 64 Light has completed her third circumnavigation with the same owners. Note the smooth underwing clearance, lacking any protrusions or steps.

Outremer Standard

"A great cape, for us, can't be expressed in latitude and longitude alone. A great cape has a soul, with very soft, very violent shadows and colors. A soul as smooth as a child's, and as hard as a criminal's. And that is why we go!"

~ Bernard Moitessier

Catamaran Underwing

Dinghies, windsurfers and every imaginable type of water toy can be stored conveniently on large catamarans and easily launched from the wide transom steps for shore-side pleasures. Note the twin life rafts located in special compartments on the massive aft crossbeam.

Continue reading here: Hull

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Readers' Questions

What length should a stub keel be to waterline length on a catamaran?
There is no set rule for the length of a stub keel on a catamaran in relation to its waterline length. The length of the stub keel will depend on various factors, such as the size and design of the catamaran, intended use, and specific requirements of the boat builder. Generally, the stub keel on a catamaran is designed to provide stability and improve sailing performance, so it is important to consult with a naval architect or boat designer to determine the appropriate length for a specific catamaran.
What is a 16 passenger catarmarn like?
A 16-passenger catamaran is a type of boat or vessel specifically designed to carry 16 people comfortably. Catamarans are multihull boats with two parallel hulls, which are connected by a deck or a structure. They offer stability, speed, and efficiency in the water. A 16-passenger catamaran can vary in size and design, but generally, it will have enough seating or lounge areas for all passengers. It may have indoor cabins with beds or seating areas, as well as outdoor spaces for relaxation or socializing. These boats often come equipped with amenities such as bathrooms, kitchens or galleys for meals, and sometimes even entertainment systems. The catamaran's size can influence its specific features. Some catamarans are designed for day trips or shorter excursions, while others are built for longer journeys or overnight accommodations. Additionally, they can be used for various purposes, such as whale watching, diving trips, ferry services, or private charters. Overall, a 16-passenger catamaran provides a comfortable and stable platform for small groups or gatherings, allowing passengers to enjoy the beauty of the water while ensuring safety and comfort.
Is the catamaran hull floor always on the waterline?
No, the hull floor of a catamaran is not always on the waterline. The design of a catamaran allows for the hulls to be elevated above the waterline, reducing drag and increasing speed. The position of the hulls in relation to the waterline can vary depending on factors such as the weight distribution, load, and sailing conditions.
How close to a catamarans design reefing points should you go?
You should always be careful when approaching reefing points on a catamaran and stay as far away as possible. Generally, you should aim to stay at least 10 meters away.
What keel to length ratio for catamarans?
The keel-to-length ratio for catamarans typically ranges from 0.1 to 0.25.
Is 70% length to beam ok for a catAMARAN?
Yes, it is generally accepted that a catamaran should have a length to beam ratio of between approximately 6:1 and 8:1. Therefore, a 70% length to beam ratio would be within an acceptable range.
What is the waterline length to baem ratio of a typical cruising catamarans?
This ratio will vary depending on the type and size of the catamaran. Generally, the ratio should be between 1:1.5 and 1:2.5, with 1:2 being the most common.


Sailboat Keel Types: A Complete Guide

Sailboat Keel Types | Life of Sailing

Last Updated by

Daniel Wade

June 15, 2022

A keel is a very important part of a sailboat, crucial to its stability and ability to sail upwind. This article will discuss the fundamentals of all keels down to the details of the best keel for your boat. We will answer questions including what it does, what it's made of, and even some problems that a keel can cause. By the end of this article, you will be an expert on all things keels!

Table of contents

What Is a Keel?

A keel is the robust underwater centerline of a boat, and often extends into a long, hydrodynamically shaped blade.. It can vary in size and shape depending on the make and model of the boat. But, generally, it will look very much like a fin. If you imagine the fin on the underside of a surfboard you will have a pretty good idea of what it looks like under the boat.

It is crucial to helping with steering and control. The word keel itself comes from Norse and Dutch roots. The word simply means a structural component of the boat. You may have heard the term keel-haul, where pirates would drag someone underneath the boat across its keel. This is pretty barbaric, but it shows just how long a keel has been an important part of the boat. From Vikings to pirates to modern sailboats it has always remained an important part of a sailboat.

What is a keel made of?

A keel will typically be made of whatever the rest of the hull is made of. If the boat is wooden, it will most likely have a wooden keel. If the boat is metal, the keel will be metal, and so on. Sometimes, particularly on fiberglass boats, the core of a keel will be reinforced with lead or a similar ballast while the outside is covered with fiberglass.

The keel needs to be strong enough to withstand a lot of pressure and strain. Traditionally Viking sailing boats would have a wooden keel with some metal plating on it. Casting metal was tedious so putting it on your boat was seen as a waste. Traditional wooden sailing boats, like you would imagine pirates sailing, would also have a wooden keel. It would be made from thick hardwood like oak and could also be metal plated.

Since it is used to control the direction it must brunt the force of the change of direction. If you imagine a boat turning sharply, there will be a lot of pressure on the side that is on the outside of the turn. The hull of your boat will stand up to this pressure easily, as it is very large and very strong. The keel must be strong enough to withstand this too. The keel also can drag on the ocean floor or the boat ramp as the boat is lowered into the water. For this reason, it needs to be strong enough to hold up to the weight and pressure of the boat too. If it were made of plastic it would break every time you brought the boat in and out of the water.

What does a keel do?

The keel is there primarily for stability and guidance. The keel provides all sorts of benefits to the boat. It improves the righting moment and controls the boat’s sideways movement. The keel will also typically hold the boat’s ballast. The ballast keeps the boat weighed down and helps prevent capsizing. The ballast is typically made of lead, sand, or water.

Keels can be fixed or moveable. Some keels can be removed completely or may just retract slightly so they aren’t damaged when the boat enters or exits the water. When a boat leans to one side, because it is turning or there is strong wind/waves, the keel provides the righting moment that keeps the boat from flipping. On larger boats, it is designed to be so heavy that ig will be able to recover a boat from almost any angle of heel. Without a keel, your boat may tip too far or roll completely. On bigger ships, this can be disastrous. In its ability to prevent this alone, the keel is one of the most important parts of the ship.

Does my sailboat need a keel?

Yes, you do need a keel. Pretty much all sailboats have a keel, with exceptions for multi-hulled and/or smaller boats.  

In the case of catamarans, the very design principles that lead to the dual-hull design render the keel obsolete. The stability introduced by the outrigged, dual-hulls replaces the necessary righting moment from the keel. Because almost all the structure of a catamaran is between the two hulls, unlike a monohull which builds out from and around its centerline, there is not a lot of weight pressing out to flip the boat. Related to this, since most catamarans have the length of their hulls pressing into the water, the steering force normally placed on a keel is distributed across the length of the two hulls, which additionally will carry their own ballast.

On smaller dinghies, a small swinging centerboard or daggerboard will suffice to play the role of a keel. The centerboards can be big enough to keep the boat flowing nicely through the water without the need for a large keel. Additionally, it is not a disaster for a small sailing or racing dinghy to capsize, as they are designed to do so and recover rather easily, so the ballast from the keel is also not terribly necessary. 

If you have a motorboat you wouldn’t need a keel, unless it is a very large container ship or military vessel. The reason being that they are outboard propelled. The leg provides enough stability on its own. This is only the case with full plane powerboats. Displacement (even semi-displacement) craft will still need a keel of sorts for stability purposes.

What are some downsides to having a longer keel?

If you have a long keel that doesn’t retract or detach, you may have some problems coming in or out of the water. The keel extends far below the bottom of the boat, so if you are bringing your boat up or down a boat ramp you may find that it scrapes on the bottom. If you are not careful, you may damage the keel rather badly.

Since the keel is made of metal, wood, or fiberglass it can bear the brunt of the weight quite well. If you are putting your boat in the water and cannot retract the keel, it is a good idea to go as far into the water as possible before taking your boat off its trailer. The deeper you are in the water when the boat is released the better.

Are there any nautical traditions about the keel of the boat?

The keel is interestingly very important when it comes to boat or shipbuilding.

Traditionally, the keel is one of the first parts of the ship to be made, as the rest of the ship must sometimes be built around it. This tradition is called “laying the keel,” and is a momentous occasion. It is essentially the boat’s birthday. The boat’s age is dated from this moment, and there is also typically a celebration of sorts. This goes back to the days of seafaring exploration. The only day more important in a boat’s life is the day it is finally launched.

Can other types of boats have keels?

Yes! Many other types of boats have keels, not just sailboats. A good example would be a big shipping trawler. These trawlers are very large and need all the help they can get to stay balanced. Because of this, they often have what’s called a bar keel. This is a large rectangular piece of metal that runs along the bottom of the boat’s hull. It is very thick and heavy. The idea is that it gives the boat some more directional control when steering.

Furthermore, it helps keep the trawler balanced when out at sea in rough conditions. The extra weight keeps the boat’s center of gravity as low as possible. This makes tipping the boat almost impossible. It does slow it down a bit, but that is a small price to pay for increased safety.

Huge cargo ships also have a keel, though it is different from a bar. Their keel is known as a plate keel. It is essentially another layer of the boat under the hull. Its only purpose is added weight and protection. A plate keel runs along the centreline of the bottom plate of the ship so the weight is all concentrated in the lowest place possible. This kind of keel works similarly to how the spine of a person does. It keeps your back strong and as straight as possible.

How important is it to keep my keel clean?

It is very important to keep your keel clean, just as it is important to keep the rest of your hull clean.

For any boat kept on the water rather than hauled out every day, there is always the need to clean the hulls and keels of any barnacles and other sea growth. Barnacles not only affect your performance, but can, in the long run, greatly increase your maintenance costs if not regularly addressed. 

To do so, you have to do what is known as scraping. Scraping is the process of physically scraping off all the barnacles and other sea life that has attached itself to the underside of your boat. Many marinas offer this service, but you can do it on your own with a basic plastic paint scraper and a wetsuit. When you do this, it is key to get all the way down to the bottom of the keel and all across the hull. If you don’t scrape it off, it can start to erode your boat away over time. It can also slow you down.The barnacles and other marine life create a very rough bottom. This creates more friction and will reduce your speed more and more the worse it gets. 

It is important to check with your port authority before you start scraping. Scraping is not allowed in some places as you may introduce invasive species to the area. It depends where you have been more than where you are. If you sailed from New York to Chicago, you will be fine. If you sailed from Cuba to New York, probably not so much.

How to maintain a sailboat keel

As mentioned above, it is important to scrape your keel from time to time. While racing boats will actually do this before every day at an event, it is at least a good idea for you to do this a couple of times a season. A great time to do this is when you plan on applying that season’s bottom paint, though anytime you plan to go on your boat is a good excuse to maintain!

You may want to cut off any of the kelp and seaweed that wraps itself around the keel. This is more likely to happen if you have a fin keel. If you do find that there is a lot of kelp and seaweed wrapped around it, you will want to buy yourself a kelp cutter. Unfortunately, the only way to cut the kelp off without taking the boat out of the water is to dive in and do it yourself. It is a good idea to do this in shallow-ish water with the proper flags displayed to inform other boaters that there is someone in the water. Swimming around under your boat, even when it isn’t moving, can be dangerous.

What do I do if my keel breaks at sea?

It is very rare for keels to just break off. It is even rarer at sea. After all, what is going to break it off? The only way a keel will break off ordinarily is if you run aground.

If you should accidentally make your way into shallow waters and break your keel off it is a good idea to set sail for home. You will manage well enough in the short term but will struggle over time. You are far more likely to capsize without the keel keeping you balanced.

If you have a detachable keel it is a good idea to keep a replacement. If one breaks off, you can just install the spare one. This isn’t the easiest thing to do at sea in rough conditions, but it is possible. Make a judgment call using your common sense whether it is worth the risk or not.

Another reason your keel might break or come loose is if the keel bolts come out. These bolts are what holds the keel in place. If you happen to have a keel held on by bolts, then doing proper maintenance is even more important. If the bolts come loose, the keel can come loose.

Since the keel is typically welded on to the boat’s hull the chances of it coming off completely are slim to none. Most often, running aground on a sandbar or anything short of an incredibly rocky bottom in heavy weather will crack off a piece or severely bend the keel, which requires a major repair. If you do notice that the keel is loose, you are better off taking it back to the marina. The bolts may not come off without using some machinery, meaning you might have to take your boat out of the water. If your keel starts to rust, you may need to speak to a professional.

What are the different keel types?

Now you know what a keel is, what it does, why it is important, and how to care for one it is time to learn about the specific types of keels. Big trawlers and cargo ships have bar or plate keels, but sailboats do not. Here are the 6 different types of keels typically found on sailboats and their purposes:

The full keel is one of the most common types of the keel that you are likely to see on a sailboat. A full keel runs from end to end of the boat lengthways. A full keel, as the name implies, runs almost the entire length of the boat. At a minimum, it must run 50% of the length of the boat. A full keel is one of the most stable keel types, which is why it is so common. Full keels are also safer should you run aground. If a boat with a full keel should come ashore, it will cut its way through the sand and eventually land on its side. Whether you are grounding your boat intentionally or not, your boat will have far better odds of surviving the ordeal with a full keel.

A fin keel is similar to a full keel, just shorter. There may be one or two fin keels along the length of the boat hull. A fin keel is defined by being less than 50% the length of the boat. The fin keel works almost entirely the same way that a shark's fin does. When you wish to turn, the keel provides the resistive force that keeps you turning. This means that it essentially acts as your tires going into a turn. Whereas a full keel is essentially just a long fin, a fin keel has very different benefits. A full keel is more stable and safer overall. A fin keel is sleeker, smaller, and most importantly makes you faster. Most racing sailboats have fin keels.

A bulbed keel is very similar to a fin keel. In fact, it is possible to make a bulb keel by shaving off part of a fin keel and attaching a bulb. Once the keel has been made substantially shorter, the bulb is fitted. The bulb is shaped similarly to how a torpedo would be on a submarine. This keel works the same as a fin keel does, offering a slightly more stability without sacrificing speed. The biggest difference between a bulb and a fin keel (besides shape and length) is where they are used. Bulb keels are most commonly used in places with very shallow waters and lots of rock/shale/coral outcrops. Somewhere like the Caribbean would be the perfect place for a bulb keel. The rounded bulb bounces off the rocks and is less likely to break off. It just isn’t going to be as quick as if you used a fin keel.

The wing keel is another alternative to your standard fin keel. Just like the bulb keel, a wing keel is an extension to the standard fin keel with an extra fitting at the bottom. A wing keel is far more streamlined than a bulbed one, at the expense of being more susceptible to breaking. A wing keel looks very similar to the tail of an airplane. It works the same way, too. The water can pass by either side of the wings, allowing you to adjust your course easily. But, a wing keel does have one major problem. If you do run aground, digging out a wing keel can be very difficult. Whereas digging out a standard fin is as simple as scraping sand away from the sides of it, a wing keel must be dug out completely. The wings act like little shovels and wedge themselves into the sand. These are generally limited to higher performance racing classes.

Centerboard Keel

A centerboard keel works similarly to a fin keel but it can retract slightly. It works by having a dagger that folds out downwards. When you are sailing, the dagger protrudes outwards and offers you all the stability and balance of a fin keel. When you are in shallow water, the dagger can be retracted upwards, essentially shortening the keel temporarily. This should be done when you are sailing in shallow waters or removing the boat from the water entirely using a boat ramp. Some centerboards work on a loose hinge. When the boat is sailing along, the dagger is out and the fin works as normal. If you should bump into something though, like some shallow rocks, the hinge would push the daggerboard back inside. This stops the keel from breaking, instead, it just moves out the way. This only works if you are only just deep enough. If you are in very shallow water you would just break the centerboard off.

Canting Keel

A canting keel also works on a hinge. Instead of working end to end, it works port to starboard. When the boat turns a corner, the canting keel swings from side to side. This allows the boat to maximize its balance and speed. Eventually, this will become the norm in racing. But at the moment it is still quite experimental. The biggest downside is that the hinge works on hydraulics, and hydraulics can fail. If they should fail at sea there is very little you can do to repair them. Once they have perfected these canting keels, they will move first into the racing classes and high performance boats, then to all new cruising boats as boatbuilders improve the technology. .

Hopefully, you now have a good idea about what a sailboat keel is,how it works, why it is so important, and, of course, all the different types. Chances are, when you buy a sailboat , the keel it has is going to be at the bottom of your list of priorities. That being said, if you are planning on sailing somewhere in particular, it is a good idea to think about what keel type you are using. Replacing them doesn’t have to be expensive, but you can go a long way to saving yourself that money either way by being prepared for your home waters!

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Low aspect ratio keels are very popular on cruising catamarans as they offer a good compromise between easy sailing and cost while also protecting the hull from possible grounding damage and usually add to the load carrying potential.

Unfortunately low aspect ratio keels are also both very inefficient in preventing leeway and in increasing pitching. So it is tempting to improve them by making them deeper and shorter. However deeper keels tends to negate one of the great advantages of catamarans - shallow draft. Short keels result in even more problems. Imagine drying out and have the crew go forward only for the bow to drop as the boat overbalances! Think of the ensuring damage to the boat, never mind to the crew if this happened on hard ground!!

No wonder that - as this photo of a well known design shows - many catamarans need props under the bows and often also under the sterns (a bit hard to fit them while drying out on a beach I would have thought!)

That is why I draw relatively long keels and accept the compromise between performance and practicality. See my article on LAR keels versus daggerboards on my Articles pages for more on this subject

catamaran keel design

Top 6 Characteristics of a Good Catamaran

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What Makes a Good Catamaran?

As we (Stephen and Estelle) self-isolate during this pandemic aboard Zuri, our Bali 5.4, we thought we would share some insights about important features you should think about when buying a catamaran. We started out with 5 then at the end, Stephen threw in another 3. What are your thoughts? What are important features on a catamaran. Let us know below in the comment section.

Like all marine vessels, certain characteristics make some catamarans better than others. You will not find a perfect catamaran because no boat is perfect. Compromise is always required. But with forethought about how you will utilize your multihull, and matching your requirements to high quality design, you can get pretty close to your “dream” catamaran.

Crucial attributes to consider for a cruising catamaran are:

  • Weight-carrying ability
  • Bridge deck clearance
  • Structural integrity and seaworthiness
  • Windward ability
  • Deck surface design
  • Ease of handling

We will explore each of these catamaran characteristics, and how they affect performance, comfort, and, ultimately, safety.

Early catamaran designs were heavy by today’s standards. The multihulls were designed with a narrow beam, high freeboard, and inefficient underwater design. They were prone to hobby horsing and pitching. Today, with the technological advancements of lighter, stronger composite materials, catamarans performance has greatly improved.

In the last 20 years or so, boat builders have painstakingly studied and resolved the issues affecting catamarans, effectively increasing their seaworthiness by leaps and bounds. Constant reform and transformation of even the basic tenets of catamaran design continue today as is evident in the radical catamaran designs in the 2013 America’s Cup Challenge. (The foiling Gunboat G4 designs spring to mind). In fact, many of the problems of the older multihull designs have been eliminated altogether.

The new generation of cruising catamarans offers exciting, fun sailing vessels with great livability, space, comfort, and safety. This was made possible, in no small part, by the early multihull pioneers.

Catamaran Stability & Weight-Carrying Ability

catamaran keel design

WEIGHT: Unlike the monohull design that can carry weight without much loss of performance, an overloaded catamaran rapidly loses performance and, eventually, safety. In order to counter this, multihull manufacturers are continually looking for ways to reduce construction weight to increase the potential payload capacity while retaining optimal performance.To select a light catamaran, look for hulls with cored construction and interiors built with lightweight materials.  Lighter construction enables catamarans to carry more weight and perform faster, so this characteristic is very important when selecting a cruising catamaran.

Monohulls can heel and spill wind when the sails are overpowered. But a catamaran’s only available response to pressure of a wind gust is to accelerate. So the boat has to be very strong to hold together. Cored construction makes the catamaran strong and very stiff. Stiffness gives a catamaran good performance.

Consider that the catamaran’s hulls are actually two boats joined together by a bridge deck. These “boats” are constantly fighting each other and trying to go in their own direction. The boat structure must be strong enough to counter this and, at the same time, deal with the downforce of the mast in the center of the bridge deck. The catamaran performs an amazing feat contending with all the opposing forces inherent in the multihull design. For that reason, it is critical to ensure the design and manufacturer are reputable with a track record to demonstrate structural integrity.

STABILITY: Unlike monohulls that rely on a heavy lead keel to keep it upright, a catamaran relies on the beam of the boat and the buoyancy of the hulls for stability. Typically cruising catamarans will have a beam to length ratio of roughly 50%. So a 45-ft long catamaran will be about 22-ft wide, providing a very stable platform. Good stability in a catamaran is when the righting forces and healing forces are in balance. So if the righting moment is greater than the capsizing moment, the boat stays upright. When the capsizing moment becomes bigger than the righting moment due to an increase in wind, the boat starts to capsize. <

This diagram and article by James Wharram, king of “self-build” catamarans, offer valuable insight into catamaran stability. Also, read this article by Shuttleworth “ Design Considerations for Seaworthiness “.

Catamaran Bridge Deck Clearance

Catamaran Bridgedeck clearance comparisons of too low and well-proportioned catamarans

Bridge deck clearance is a key factor in predicting the slamming level of a catamaran design. A higher clearance produces less slamming. My rule of thumb is; 4% of the overall length is low, 5% is acceptable, and 6% is good.

When it comes to slamming, another important factor is weight. If the boat is heavy due to construction or payload, it will tend to go into a wave. This slams the chest of the boat into the wave. A lighter boat with more buoyancy will rise on the wave reducing slamming substantially. “ The Space Between ” by Sackville Currie, explains in detail the different options, problems, and compromises.

Windward Ability: Catamaran Keels vs. Dagger Boards

Catamaran daggerboards

Catamarans with dagger boards are able to point much better than catamarans with keels. The claim is that a catamaran with dagger boards is safer because if the boards are up and the boat gets sideways on a wave, it will skid down the wave sideways. Dagger board proponents believe a keel trips the boat as it tries to navigate down the side of a wave causing capsize.

I can only speak from personal experience. I was caught sideways on a 20-foot wave a few days out of Cape Town on a 40ft catamaran with keels. The boat was hit by successive three waves before we could get it back on track. The boat skidded down the waves and there was no capsize, but it took some nifty maneuvering at the helm. My personal belief is that both types are safe and acceptable provided they are operated correctly.

Catamaran Deck Surfaces

Deck surfaces should be safe and clear

Cockpit to Mast

Older designs often have decks with two levels from the coach roof windows to the gunnels which form a side deck with a trip hazard. Most modern catamaran deck designs are now one flat surface being wide enough to walk unhindered from the cockpit to the mast.

The cockpit and the saloon should be on one level with no step down into the saloon, if possible. Modern designs have achieved this and it really makes a big difference for ease of movement and safety while at sea.

The steps on the stern should be wide and easy to climb with a reasonable angle. If the steps are too steep or narrow, they become a hazard and lose space for recreation. The stern should be easily accessible from the dinghy.

Ease of Handling: Catamaran Deck Layout

Safe catamaran helm stations

  • Both jib sheets
  • The sheet from the opposite side of the boat should be run through a turn block and across the coach roof through a clutch to the helm
  • Jib furling line
  • Single line reefing lines
  • Main halyard
  • Traveler control lines.

Visibility from the helm is also very important. All round visibility while underway, maneuvering, or docking is key to safety of your boat and others’ property and life. When standing at the helm, you should be able to see both bows or, at the very least, the pulpits. The center of the crossbeam where the anchor is handled from should be visible as well as both sterns for when you dock “stern to”. If all these stations are not visible while standing at the helm where the engine controls are, you may encounter problems because of blind spots.

Catamaran Characteristics Conclusion

Modern catamaran designs are much more advanced than the early models that were slower, heavier, and underpowered. Problems like hobby horsing, burying the bows, and underpowered rigs have been largely eliminated.

Even though composite construction technology gives a huge advantage in lighter materials and sleeker designs, no one design element or piece of high tech gear should dominate the vessel to the detriment of others. With some compromise, a good naval architect can design a vessel pleasing to most people and the result can be very exciting, safe, and seaworthy.

Top Tip: Use a Broker at No Cost

Which compromises should you make when selecting your catamaran? It all depends on how you will use her. Catamaran Guru helps new and veteran multihull owners select the right boat for their dreams at no cost! The yacht seller pays broker commissions so our advice from gleaned from thousands of boat-buying transactions is free to you. We will help you find right boat, the right program, or the right situation. If you want to explore more on your own, visit our buying a catamaran section.

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Savvino-Storozhevsky Monastery and Museum

Zvenigorod's most famous sight is the Savvino-Storozhevsky Monastery, which was founded in 1398 by the monk Savva from the Troitse-Sergieva Lavra, at the invitation and with the support of Prince Yury Dmitrievich of Zvenigorod. Savva was later canonised as St Sabbas (Savva) of Storozhev. The monastery late flourished under the reign of Tsar Alexis, who chose the monastery as his family church and often went on pilgrimage there and made lots of donations to it. Most of the monastery’s buildings date from this time. The monastery is heavily fortified with thick walls and six towers, the most impressive of which is the Krasny Tower which also serves as the eastern entrance. The monastery was closed in 1918 and only reopened in 1995. In 1998 Patriarch Alexius II took part in a service to return the relics of St Sabbas to the monastery. Today the monastery has the status of a stauropegic monastery, which is second in status to a lavra. In addition to being a working monastery, it also holds the Zvenigorod Historical, Architectural and Art Museum.

Belfry and Neighbouring Churches

catamaran keel design

Located near the main entrance is the monastery's belfry which is perhaps the calling card of the monastery due to its uniqueness. It was built in the 1650s and the St Sergius of Radonezh’s Church was opened on the middle tier in the mid-17th century, although it was originally dedicated to the Trinity. The belfry's 35-tonne Great Bladgovestny Bell fell in 1941 and was only restored and returned in 2003. Attached to the belfry is a large refectory and the Transfiguration Church, both of which were built on the orders of Tsar Alexis in the 1650s.  

catamaran keel design

To the left of the belfry is another, smaller, refectory which is attached to the Trinity Gate-Church, which was also constructed in the 1650s on the orders of Tsar Alexis who made it his own family church. The church is elaborately decorated with colourful trims and underneath the archway is a beautiful 19th century fresco.

Nativity of Virgin Mary Cathedral

catamaran keel design

The Nativity of Virgin Mary Cathedral is the oldest building in the monastery and among the oldest buildings in the Moscow Region. It was built between 1404 and 1405 during the lifetime of St Sabbas and using the funds of Prince Yury of Zvenigorod. The white-stone cathedral is a standard four-pillar design with a single golden dome. After the death of St Sabbas he was interred in the cathedral and a new altar dedicated to him was added.

catamaran keel design

Under the reign of Tsar Alexis the cathedral was decorated with frescoes by Stepan Ryazanets, some of which remain today. Tsar Alexis also presented the cathedral with a five-tier iconostasis, the top row of icons have been preserved.

Tsaritsa's Chambers

catamaran keel design

The Nativity of Virgin Mary Cathedral is located between the Tsaritsa's Chambers of the left and the Palace of Tsar Alexis on the right. The Tsaritsa's Chambers were built in the mid-17th century for the wife of Tsar Alexey - Tsaritsa Maria Ilinichna Miloskavskaya. The design of the building is influenced by the ancient Russian architectural style. Is prettier than the Tsar's chambers opposite, being red in colour with elaborately decorated window frames and entrance.

catamaran keel design

At present the Tsaritsa's Chambers houses the Zvenigorod Historical, Architectural and Art Museum. Among its displays is an accurate recreation of the interior of a noble lady's chambers including furniture, decorations and a decorated tiled oven, and an exhibition on the history of Zvenigorod and the monastery.

Palace of Tsar Alexis

catamaran keel design

The Palace of Tsar Alexis was built in the 1650s and is now one of the best surviving examples of non-religious architecture of that era. It was built especially for Tsar Alexis who often visited the monastery on religious pilgrimages. Its most striking feature is its pretty row of nine chimney spouts which resemble towers.

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Website Monastery - http://savvastor.ru Museum - http://zvenmuseum.ru/

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new keel design

Discussion in ' All Things Boats & Boating ' started by rob8 , Dec 29, 2013 .


rob8 New Member

I have a new catamaran keel design based on my extensive experience in another field of water sports. I'm looking for an experienced sailor to help demonstrate it's potential in the Brisbane Australia area. It is a new and unique design with potential for high speed and VMG.  


upchurchmr Senior Member

Do you really mean a keel? Not a centerboard or a daggerboard?  
Yes, technically I think its a keel. This hull does not use a CB or daggerboard.  
This new catamaran hull/keel shape is unique below the waterline. Above the waterline, it can be any designer’s imagination. It can be adapted to cruising and racing cats, even power cats with design modification. The design objectives are high speed, excellent VMG and good sea manners. I’ve been working on this project for 5 years, mostly with scaled models, and the most difficult part has been to find someone to take me seriously enough to understand the concept and the way the principles have been applied to this particular design. I’m not a sailor and have applied principles gained through experience in another field of water sports and now need help with the prototype stage – a beach cat. I chose a beach cat to prove the hull/keel shape because of its obvious cost effectiveness. The round hull wave piercing shape is a very refined low power/speed efficient design and the designers have done well to make improvements to get it to this stage of its peak performance but which are now incremental at best and why they are turning to foils. My hull shapes is an alternative with good sea manners. Any experienced cat sailors willing to help in the Brisbane, Australia area?  
I'm along way from Brisbane, but I wouldn't touch it without more information. Boats are expensive. You are offering what we call here a "Pig in a poke"  


kapnD Senior Member

Beach cats are so named as they are designed to be run onto/off of the beach, so keels are not an option. They are equipped with retractable gear, or the hulls are designed in such a way as to operate without keels. Fins used on surfboards, sailboards, water skis, etc are best described as pivot points to improve turning abilities, not the best charachteristic to have on a sailboat where the goal is to hold a straight course into the wind. You would do well to get some practical sailing experience under your belt before you try to re-engineer something that has obviously been quite well hashed over already.  
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I wouldn't describe what I'm doing with my design as 're-hashing', to the contrary, I think I have something new and unique. The fact that I'm not a sailor has allowed me the insight to come up with this hull design. Is there nothing new under the sun? Wait and see.  


What kind of boat is this? 20-30 ft Keelboat


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The Unique Burial of a Child of Early Scythian Time at the Cemetery of Saryg-Bulun (Tuva)

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Pages:  379-406

In 1988, the Tuvan Archaeological Expedition (led by M. E. Kilunovskaya and V. A. Semenov) discovered a unique burial of the early Iron Age at Saryg-Bulun in Central Tuva. There are two burial mounds of the Aldy-Bel culture dated by 7th century BC. Within the barrows, which adjoined one another, forming a figure-of-eight, there were discovered 7 burials, from which a representative collection of artifacts was recovered. Burial 5 was the most unique, it was found in a coffin made of a larch trunk, with a tightly closed lid. Due to the preservative properties of larch and lack of air access, the coffin contained a well-preserved mummy of a child with an accompanying set of grave goods. The interred individual retained the skin on his face and had a leather headdress painted with red pigment and a coat, sewn from jerboa fur. The coat was belted with a leather belt with bronze ornaments and buckles. Besides that, a leather quiver with arrows with the shafts decorated with painted ornaments, fully preserved battle pick and a bow were buried in the coffin. Unexpectedly, the full-genomic analysis, showed that the individual was female. This fact opens a new aspect in the study of the social history of the Scythian society and perhaps brings us back to the myth of the Amazons, discussed by Herodotus. Of course, this discovery is unique in its preservation for the Scythian culture of Tuva and requires careful study and conservation.

Keywords: Tuva, Early Iron Age, early Scythian period, Aldy-Bel culture, barrow, burial in the coffin, mummy, full genome sequencing, aDNA

Information about authors: Marina Kilunovskaya (Saint Petersburg, Russian Federation). Candidate of Historical Sciences. Institute for the History of Material Culture of the Russian Academy of Sciences. Dvortsovaya Emb., 18, Saint Petersburg, 191186, Russian Federation E-mail: [email protected] Vladimir Semenov (Saint Petersburg, Russian Federation). Candidate of Historical Sciences. Institute for the History of Material Culture of the Russian Academy of Sciences. Dvortsovaya Emb., 18, Saint Petersburg, 191186, Russian Federation E-mail: [email protected] Varvara Busova  (Moscow, Russian Federation).  (Saint Petersburg, Russian Federation). Institute for the History of Material Culture of the Russian Academy of Sciences.  Dvortsovaya Emb., 18, Saint Petersburg, 191186, Russian Federation E-mail:  [email protected] Kharis Mustafin  (Moscow, Russian Federation). Candidate of Technical Sciences. Moscow Institute of Physics and Technology.  Institutsky Lane, 9, Dolgoprudny, 141701, Moscow Oblast, Russian Federation E-mail:  [email protected] Irina Alborova  (Moscow, Russian Federation). Candidate of Biological Sciences. Moscow Institute of Physics and Technology.  Institutsky Lane, 9, Dolgoprudny, 141701, Moscow Oblast, Russian Federation E-mail:  [email protected] Alina Matzvai  (Moscow, Russian Federation). Moscow Institute of Physics and Technology.  Institutsky Lane, 9, Dolgoprudny, 141701, Moscow Oblast, Russian Federation E-mail:  [email protected]

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