catamaran hull length to beam ratio

Choosing the right beam for a multihull

QUESTION:   When assessing or designing a trimaran or catamaran, what guidance can you give to guide the choice of beam ?             

                    Lech K:  Gdansk, PL

ANSWER: An interesting question as we do see quite a variation on existing boats.

First, let’s call the Overall Beam to Length ratio B/L and the individual hull length to hull beam, L/b .

Here are a few basics to consider as inputs to your overall beam choice.

*    More beam gives more transverse stability, permitting a powerful rig to drive a boat faster, but also,       excessive beam tends to lower diagonal stability so increasing pitch-poling.    More beam also tends to allow more  fore & aft pitching.

*    More beam requires stronger connecting beams (called akas on trimarans), aggravated by the two hulls potentially being now be in different waves

*    More beam can be a problem in a marina where space is increasingly limited

*    Folding trimarans can be limited in beam due to geometric space when folded, such as:     

Transverse folding system (Farrier etc) are limited by how far down the hulls can be managed when folded.  

Swing-arm folding is limited by the overall length increase when folded.

          Hinge & latch systems are limited by what height and weight can be lifted  

*    Less beam allows a boat to heel more, thereby reducing sail exposure to side wind.

*   Less beam brings the hulls closer together, reducing beam strength requirements and weight, but potentially adding to resistance from hull-flow interaction.

*   Hulls with a high L/b ratio can be closer together than hulls with a low L/b ratio if overall stability permits.

*   As smaller boats need proportionally more displacement due to crew and structural weight, they cannot have a very high L/b ratio as they then have insufficient displacement.

------------------------------------------------------------------------------------------------------------------------------

So what do all these points finally lead to ?      Well, let’s see.

For Catamarans , the sweet spot seems to be with a L/B of 2 to 2.1.

If the beam is excessively increased, pitching and reduced diagonal stability (see dwg) start to become an issue and when such boats are lengthened to make their L/B slightly above 2, they generally become faster and have less negative issues ... but over about 2.3, their relatively lower transverse stability then starts to kick back.  

If the beam is decreased, stability drops quickly and one may start to also add wave interference between the hulls unless the boat is very light with slim hulls.

Of course, this is a simplification of things as top weight, windage, wing clearance, center of gravity, sail plan, etc etc .. all have their effects, though individually less than the important L/B ratio.

Let me give you an example of how other design criteria can move things from what may initially seem the ideal.  

Beam also has a huge effect on stability.   But the designer Jan Gougeon (then of West Systems) was an inventive guy, so he approached this design in a non-conventional way.   To achieve his first criteria .. “a fast non demountable weekend catamaran” , he needed to address the obvious lack of stability in other ways.  A low rig could work with low weight, that would then allow very slim, fast hulls.   Then he added water ballast to help keep the windward side down …. and finally, a masthead float to prevent the boat from turning turtle, where she would stay like virtually all other multihulls do IF that happens.  In this case, it was rather often as unfortunately, most sailors were not ready to adapt to this new way of sailing and with capsizes happening too quickly for most, only a dozen or so were sold.  But I did get to try the boat and felt the concept did work in the sense that the boat IS fast and also comfortable & dry, as with such long, narrow hulls, there is very little disturbance of the surface water so spray is minimal and even if the hulls are pretty close, they are too slim to create any significant cross-hull wake- interference. 

To keep the rig low (mast is shorter than the boat), she uses 2 foresails that can be furled up fast.   Those that still own one have learned to understand them and can enjoy their merits … but this is not a boat with reserve stability for sudden gusts, so you need to sail this boat more like a race dinghy and also reduce sail early.  This further means that sailing at night when you cannot see squall warnings in the sky is best avoided unless the stars are truly out for you.

But it IS an example of thinking WAY outside the box .. even if the result is not for everyone.   So ‘sweetspot L/B ratios’ do not necessarily mean they give the only solution .. just that you, as a designer, also need to work differently around the rest of the design to solve the issues you might create if you are well outside the norm.    

The lesson here is:  If you choose to go outside the norm, fully understand the implications and work around them.  You cannot ignore them and still expect success. If the designer failed at all with this radical G32 design, it was in not sufficiently educating new owners of the different sailing nuances needed to keep the boat on its feet.

For Trimarans, my studies and observations show that the preferred B/L ratio changes with boat size. 

To some degree, the same effect on diagonal stability (as for Cats) will occur with excessive beam, but with a trimaran, the two hulls in the water will be closer so it’s also important to allow for good flow between them.  So as very large racing tris can have slimmer hulls due to great length and low weight, they can have proportionally lower B/L ratios than smaller boats that need proportionally fatter central hulls just to support the displacement they need.  After all, we cannot just change things in proportion, because the weight of things (such as crew, structure etc) will not automatically get smaller for a smaller boat .. in fact it proportionally and typically, gets greater!    So smaller multihulls can often be harder to design than larger ones, where you have more space and volume to work with.    The above observations led me to plot data from good boats and create this simple little formula that fits their B/L curve pretty well.

Here is what the curve gives as a recommended B/L ratio for a sailing trimaran

                            (Sailing Trimaran) B/L ratio  = 1.48 ÷ (L  ^ 0.21)        [ Length L in feet ].

While this may initially look complex to calculate for some, it’s very easy with the right help.  Download the Mobi Calculator on your phone or tablet.  You can then add the expression x n to your basic calculator by first hitting the 3 dots [ ... ] that brings you to the Scientific Options, and then   clicking on [ x n ] that will add this feature to your basic calculator.    You can now enter the formula exactly as written, typing 1.48 ÷ (  your L value , and then x n and finally 0.21 and the closing bracket ) and then ‘ = ‘.

If you enter say L = 17 , it will give you a B/L ratio of 0.816, closely matching a W17 , while for an L of 100 ft, it will give you a B/L of only 0.562, closely matching a big ocean racing tri like Sodeb’O.

While of course you can go outside the calculated ratio, IMHO you should have a very good reason and specific justification for a deviation of more than 15% either way.   Use the list at the beginning of this article to justify your change.

For both Tris and Cats, there may be other factors that will change your design, but this gives a good starting and target point that’s based on both practical and justified design needs.

Enjoy …. playing with figures is fun ;)

mike … march 2022

ADDED NOTE ... re MOTOR MULTIS

As noted, the above ratios refer to Sailing Craft.  Without the heeling force of a sail, pure motor-tris and cats are not bound by the same needs.

A motor catamaran can have less beam, with a clean flow between the hulls now taking prominence over high beam for sailing stability, so L/B ratios of 2.5 to 3 are now more appropriate.

Hulls may need to be asymmetrical with a straighter side on the inside to avoid unfavorable hull wave interaction between them.

For a powered trimaran , overall beam should also be reduced or the motion will become uncomfortable.  (With a sailing tri, the boat is heeled with one ama out, but with a motor tri, all three hulls are immersed so wave action on the boat would be too severe if the boat is too wide) .    Amas (pontoons) now need to be narrow but deep, as a slow gentle roll of slightly greater amplitude, is more comfortable than a short quick one.  These amas (now only 40-50% of the main hull length), seem best with their center about 60-70% aft of the main hull length and need to be of fine section and relatively deep with the connecting bridge arched high above any waves, so that neither ama or aka-bridge will slam when re-entering a wave.  L/b ratios of all 3 hulls can be at their most efficient, namely 13-16 at the waterline.   The amas are now more like permanent training wheels and with a much longer central hull and no heeling force from sails, diagonal stability is no longer something to consider.    Overall beam will depend on maintaining a clean flow between the main hull and the shorter slim amas, that need to extend well down into the water, so that motion is acceptable in waves.   Typical overall B/L ratios might now be down around 0.4, becoming even less as boat design gets bigger, provided the center of gravity is kept low.

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Catamaran Hull Design

• Post author By Rick
• Post date June 29, 2010
• 2 Comments on Catamaran Hull Design

Part 1: Notes from Richard Woods

Since the America’s Cup experimented with going multihull, there’s been a lot of interest in catamaran performance and the catamaran hull designs that define performance. Many guys are investigating whether to buy a catamaran or design and build their dream boat. Let it be said here that building a large catamaran is not for the faint of heart. People begin building 100s of boats a year, yet few are ever completed, as life always seems to have a way of interfering with a good boat build. 

Never the less, since the rest of this website is about selecting and buying a boat , it only seems fair to have at least one webpage that covers catamaran design. This page contains notes on boat hull design goals and an accompanying page from Terho Halme has mathematical formulas used in actual catamaran hull design. It has become a popular research stop and an important reference to the catamaran design community.

The content of this page was reproduced from the maestro of Catamaran designs, renown British naval architect, Richard Woods, who not only designs catamarans, he sails them across oceans…. repeatedly. He has a lot to say on the subject of catamaran hull design.

“…When it’ all said and done, the performance of a sailing catamaran is dependent on three primary specs: length, sail area and weight. If the boat is longer it generally means it’ a faster boat. If she has more sail area, it means she’ a faster boat and if she’ light it means she’ a faster boat.  Of course, there are limits: Too much sail area capsizes the boat in brisk winds. If the boat is designed too light, she will not take any kind of punishment. Too slim a hull design and the boat becomes a large Hobie Cat capable of only carrying your lunch. Of course, too long and large and you’d have to be Bill Gates to afford one. Then there are lot of additional and very important factors like underwater hull shape, aspect ratios of boards and sails, wet deck clearance, rotating or fixed rigging and so on….” Richard Woods

All Catamarans are not equal, but all sailboats have two things in common: They travel on water and they’re wind powered, so the Catamaran design equations in the 2nd part should apply to every catamaran from a heavy cruising Cat to a true ocean racer.

Richard Wood’s comments on catamaran design:

We all know that multihulls can be made faster by making them longer or lighter or by adding more sail. Those factors are the most important and why they are used as the basis of most rating rules. However using just those figures is a bit like determining a cars performance just by its hp and curbside weight. It would also imply that a Tornado would sail as fast forwards as backwards (OK, I know I just wrote that a Catalac went faster backwards than forwards)

So what next?? Weight and length can be combined into the Slenderness Ratio (SLR). But since most multihulls have similar Depth/WL beam ratios you can pretty much say the SLR equates to the LWL/BWL ratio. Typically this will be 8-10:1 for a slow cruising catamaran (or the main hull of most trimarans), 12-14:1 for a performance cruiser and 20:1 for an extreme racer.

So by and large faster boats have finer hulls. But the wetted surface area (WSA) increases proportionately as fineness increases (for a given displacement the half orange shape gives the least WSA) so fine hulls tend to be slower in low wind speeds.

The most important catamaran design hull shape factor, is the Prismatic Coefficient (Cp). This is a measure of the fullness of the ends of the hull. Instinctively you might think that fine ends would be faster as they would “cut through the water better”. But in fact you want a high Cp for high speeds. However everything is interrelated. If you have fine hulls you can use a lower Cp. Most monohulls have a Cp of 0.55- 0.57. And that is about right for displacement speeds.

However the key to Catamaran design is you need a higher Cp if you want to sail fast. So a multihull should be at least 0.61 and a heavy displacement multihull a bit higher still. It is difficult to get much over 0.67 without a very distorted hull shape or one with excessive WSA. So all multihulls should have a Cp between 0.61 and 0.65. None of this is very special or new. It has been well known by naval architects for at least 50 years.

There are various ways of achieving a high Cp. You could fit bulb bows (as Lock Crowther did). Note this bow is a bit different from those seen on ships (which work at very specific hull speeds – which are very low for their LOA). But one problem with them is that these tend to slam in a seaway. 

Another way is to have a very wide planing aft section. But that can increase WSA and leads to other problems I’ll mention in a minute. Finally you can flatten out the hull rocker (the keel shape seen from the side) and add a bustle aft. That is the approach I use, in part because that adds displacement aft, just where it is most needed.

I agree that a high Cp increases drag at low speeds. But at speeds over hull speed drag decreases dramatically on a high Cp boat relative to one with a low Cp. With the correct Cp drag can be reduced by over 10%. In other words you will go 10% faster (and that is a lot!) in the same wind and with the same sails as a boat with a unfavorable Cp. In light winds it is easy to overcome the extra drag because you have lots of stability and so can fly extra light weather sails.

The time you really need a high Cp boat is when beating to windward in a big sea. Then you don’t have the stability and really want to get to your destination fast. At least I do, I don’t mind slowly drifting along in a calm. But I hate “windward bashing”

But when you sail to windward the boat pitches. The sea isn’t like a test tank or a computer program. And here I agree with Evan. Immersed transoms will slow you down (that is why I use a narrower transom than most designers).

I also agree with Evan (and why not, he knows more about Volvo 60 design than nearly anyone else on the planet) in that I don’t think you should compare a catamaran hull to a monohull, even a racing one. Why chose a Volvo 60/Vendee boat with an immersed transom? Why not chose a 60ft Americas Cup boat with a narrow out of the water transom?? 

To be honest I haven’t use Michelet so cannot really comment. But I have tested model catamarans in a big test tank and I know how inaccurate tank test results can be. I cannot believe that a computer program will be better.

It would be easy to prove one way or the other though. A catamaran hull is much like a frigate hull (similar SLR, L/B ratios and Froude numbers) and there is plenty of data available for those. There is also a lot of data for the round bilge narrow non planing motorboats popular in the 1930’-50’s which again are similar to a single multihull hull.

One of the key findings I discovered with my tank test work was just how great the drag was due to wave interference between the hulls. Even a catamaran with a modern wide hull spacing had a drag increase of up to 20 % when compared to hulls at infinite spacing. One reason why just flying a hull is fast (the Cp increases when you do as well, which also helps). So you cannot just double the drag of a single hull and expect to get accurate results. And any speed prediction formula must include a windage factor if it is to give meaningful results.About 25 years ago we sailed two identical 24ft Striders next to each other. They were the same speed. Then we moved the crew of one boat to the bow. That boat IMMEDIATELY went ½ knot faster. That is why I now arrange the deck layout of my racing boats so that the crew can stay in front of the mast at all times, even when tacking or using the spinnaker.

I once raced against a bridge deck cabin catamaran whose skipper kept the 5 crew on the forward netting beam the whole race. He won.

Richard Woods of Woods Designs www.sailingcatamarans.com

• Tags Buying Advice , Catamaran Designers

Owner of a Catalac 8M and Catamaransite webmaster.

2 replies on “Catamaran Hull Design”

I totally agree with what you say. But Uli only talk sailing catamarans.

If only solar power. You need the very best. As limited watts. Hp.

The closer to 1-20 the better.

Closing the hulls to fit in cheaper marina berth. ?

You say not too close. But is that for sailing only.

Any comment is greatly appreciated

Kind regards Jeppe

Superb article

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Hull Resistance and Hull Shape Comparisons

Introduction

As I've said elsewhere, I only like to design boats that are fun to sail. I also know from personal experience just how much effort is involved in building even the smallest boat. I've found that it is the psychological effort that's particularly hard, especially if you are a home builder building alone in your spare time. I also know that there are other people, like myself, who's keeness to build is not matched by manual dexterity.

So I try to design boats that are straightforward to build. In simple terms, if I can build it then anyone can! To do this I try to keep to simple shapes and use flat panels where ever possible. Flat decks in particular have many advantages. For a start they are easier to walk on, while coming alongside and boarding from a marina pontoon or dinghy is a lot safer. Flat panel hulls may not offer ultimate speed, but to be honest, few cruising sailors need the fastest boat while I've found that most people don't have either the skill or daring to sail a racing boat to its full potential.

You have to remember that a cruising boat, especially, isn't just for sailing. It has to be a practical floating cottage as well. And the design of that often over rules otherwise desirable sailing features. And also remember that boats have to be usable in harbours and marinas. It's not like the "good old days" when Slocum and even the Pardey's first went to sea - with no engines and few marinas or even cruisers. So all cruising boats MUST maneuver reliably under power and be easy to board from both the dock and from a dinghy.

That is one reason why I don't now like canoe sterns. They make boarding so much harder than a boat with transom steps (the acid test I always use - "could my mother get on board?"). Safe maneuvering in a small harbour is another reason I like small boats. I also find a trimaran much harder to handle than a catamaran when coming alongside, as it is so difficult to reach the outrigger bows to fend off, especially when compared to the big wide decks of a catamaran. Successful designs are ones that work in every situation, not just those that sail or motor fast in a straight line.

I always try to visulise what a particular design would be like when sailing to windward at 2am in the rain. Or when reefing. Or of course when drifting downwind on a very hot humid day.

I tend to own a fleet of multihulls. Sometimes I just go for a day sail, sometimes I race for the weekend, and most years I spend a long time living on board (I spent every Christmas living on board a boat from 2001 - 2009). All this experience means that I have personally faced nearly every situation you can meet when sailing and I use that experience in my designs.

Hull Shapes and Performance

In this article I will talk solely about hull shapes in relation to performance. Comfort, seakindliness and load carrying are also major factors affecting hull shapes and will be discussed in more detail in future articles.

People try to simplify hull design and performance predictions, formulae like the Bruce Number and KSP spring to mind. These coefficients rely only on basic sail area, displacement and length dimensions yet purport to give an accurate indicator of performance. It's easy to show that these formulae cannot be relied on if you consider that a Tornado would have the same rating whether it was sailing forwards or backwards! I suspect the latter is slower! Its probably as accurate as predicting car speed from the kerbside weight and engine horsepower. In fact hull design is a hugely complex subject while different sailing conditions require different solutions. For example, inshore boats can have a flatter rocker while offshore cruisers should be more veed forward to prevent pounding when sailing to windward in waves.

Some factors affecting yacht design are based on scientific principles and are unalterable, so always apply, whatever ones basic design philosophy and regardless of cross section shape (ie whether one uses a Deep V or round bilge hull for example). Everything else is just styling or dressing up the same proven concepts in a slightly different way. As with all moving objects, speed is the result of the combination of resistance to movement (drag) and available power. In sailing boats the power is related to the sail area while in simple terms drag comes in two forms - friction drag and wave making drag.

Frictional drag is primarily dependent on the Wetted Surface Area (WSA). Less is always better than more and WSA is the biggest factor affecting lightwind speed. The minimum WSA for a given displacement (or boat weight) is the hemisphere (eg half an orange). A longer, thinner hull has proportionately more WSA and so in light winds suffers from more drag and thus is slower but conversely is significantly faster as the wind gets up. In fact this is one reason why monohulls - which are much more orange like, do well in light winds. Spray also adds to wetted surface, one reason why powerboats have spray rails. Lots of spray makes a boat look as though it is sailing fast, but it is actually very inefficient. As an example, because of their heel and deeply immersed lee outrigger, trimarans make a lot more spray than catamarans. But we usually find that they are actually slower, particularly reaching, than an upright, low spray producing catamaran. Round bilge hulls have the lowest WSA and deep V hulls the most.

Many people think that, because multihulls have relatively thin hulls, wave making drag is non-existent, but in fact, nothing could be further from the truth. The size of waves that a hull makes depends on several factors. The Slenderness Ratio (SLR) or Displacement Length Ratio (DLR) is a measure of the fineness of a hull and is the technically correct coefficient that naval architects use. However, it is easier to visualise the hull waterline length/hull WL Beam ratio (LWL/BWL), so that is more commonly used. That's acceptable, as for a given cross-section shape, the SLR is directly related to LWL/BWL. Higher ratios result in smaller waves. Typically the LWL/BWL ratio will vary from 10:1 for a good cruising boat to 16:1 for a racing boat. (Team Philips has a LWL/BWL ratio of 35:1!) Finer hulls are more efficient at high speeds, but as we've just seen suffer from more WSA and so for normal cruising catamarans in average conditions a ratio of 11:1 - 13:1 seems optimum.

The Prismatic Coefficient (Cp) is a measure of the fullness of the ends of a boat, the higher the number the fatter the ends and - surprisingly - the more efficient at high speeds. Intuitively you'd think that a diamond shape would cut through the water best, but that's not actually the case. A high Cp also has the advantage of reducing pitching. But to complicate matters, the lower the SLR the lower the Cp can be. Typically a monohull has a figure around .56, while a properly designed multihull will be about .63. Although such a shape is slower than a lower Cp in light winds that is not a problem as one then has the sail carrying power to add extra sail to compensate. It is when sailing fast in strong winds that you need an efficient hull because you then don't have the stability to carry more sail. As an example a 30' boat with a Cp of .63 will be 1/2 knot faster than one with a Cp of .55 when sailing in 25 knot winds for EXACTLY the same sail area (and crew effort etc). Boats with a low Cp try to race faster by keeping too much sail up and it was these types that often used to capsize 25 years ago. Add in the fact that the high Cp boat won't pitch as much and its clear which is going to be the better boat.

In simple terms the Half Angle of Entry is the angle that the waterlines make to the hull CL at the bow. If it is too low then the boat is wet to sail, and, in extremis, if it is hollow there is a pressure build up further aft which slows the boat. If too fat it is also wet to sail as the bow wave goes vertically up the sides of the boat. All sailors, no matter how skillful, sail slower if they can't see where they are going because they are being blinded by spray! In both cases the correct Cp has to be maintained. So a 10 degree angle seems a good compromise. Vertical bows look fast but its actually very difficult to draw a vertical bow on a hull with both the correct Cp and one that has good reserve buoyancy. Read my articles about the Cape to Rio race to discover what I learnt first hand about the perils of vertical bows!

When I was a design student I took the opportunity to do some tank testing on catamaran models and I investigated the drag from the wave interference between the hulls. I found that this interference caused significant drag at certain speeds - in fact up to 20% when compared to hulls at an infinite spacing. So it's vital to reduce this interference as much as possible. The simplest way is to have a hull spacing wide enough so that the bow waves meet at the stern rather than under the boat. This has the added bonus that there is significantly less bridge deck slamming. In the past designers said that the optimum L/B ratio was 2:1. (In fact they were talking about overall length and beam when obviously it is waterline length and beam that are the crucial measurements.) The reasons given for this ratio were the theories that wider boats would break up and be hard to tack. In practice I've heard that limiting beam had more to do with the width of the boatyard doors! Our Strider Turbo has a LWL of 6.6m and a hull centreline spacing of 4.2m yet I've always thought it was a better sailing boat than the standard Strider. So the general trend is to go as wide as one can. But structural strength is still a problem, even with modern techniques and materials. Wide boats are heavier than narrow ones and that ultimately limits the LWL/BWL ratio to about 2:1 on cruising boats with full bridgedeck cabins.

Turning now to the hull above the waterline, vertical topsides reduce space inside dramatically and in addition are not good news when sailing offshore. As a boat moves in waves so it heaves up and down, causing discomfort and slowing the boat. Flared topsides help counteract this heaving because as the boat sinks the buoyancy picks up more quickly than with vertical topsides. The result is a smoother ride and as a bonus, better load carrying ability for, by the same token, the hull sinks slower so there is less increase in WSA and wavemaking as the boat is loaded. Clearly, freeboard adds to windage and slows the boat. Traditionally yachts had low freeboard because they were large (J Class etc) so people could fit in the accommodation easily regardless of freeboard and it's easy to make a low freeboard yacht look elegant. More importantly, it was hard to make a conventionally caulked and planked boat strong and watertight if it had too many planks (ie too much freeboard). As boats got smaller and as grp took over freeboards had to, and could, increase. Adding a few centimetres (inches) of freeboard adds enormously to interior space and at the same time results in a boat that is drier and more comfortable to sail. Fortunately, in practice I have never found that high freeboard slows the boat down appreciably and certainly not by enough to worry any except the most ardent racer.

Load carrying considerations are an important factor for cruising boats. In general it's natural for people to add weight aft because it is easier to load stores near the companionway than forward. Engines and their associated tanks, generators, a/c units etc are also always aft. So I always try to add extra buoyancy near the stern. That means that when empty the boat will probably float stern up. Too many poorly designed multihulls float stern down and drag their transoms.

Pros and Cons of popular hull shapes

The deep V is a simple to build hull shape that matches the human body as it is narrow low down and wider high up so it is a good choice if the accommodation is only in the hulls. It can make to windward without keels or boards - just - but it's more maneuverable and makes less leeway with them fitted. Deep V hulls pitch more than any other hull shape, particularly if they have canoe sterns. Hull asymmetry is needed to reduce pitching, a canoe stern is obviously as pointed as the bow so it's bound to pitch more. They have more WSA than any other hull shape so are slow in light winds.

The flat bottom hull is also easy to build and has the added advantage that it is self supporting during building and transport. It needs Veeing forward for offshore sailing or it will pound. Then it becomes a hard chine/single chine hull. Carefully drawn such a shape is a close approximation to a round bilge hull, but without any complicated building.

The round bilge hull is the only hull shape that can be varied over it's length so one get exactly the shape one wants. It has minimum WSA, and so is also the optimum hull shape but it is the slowest to build. A topsides knuckle helps deflect spray, adds interior volume and makes it easy to join flat topsides to the curved bottom. It also makes a nice styling line.

From the start of my design career I have always tried to design balanced, undistorted hulls that sail easily on all points but are not too extreme. However, I have made a few changes to my hull shapes over the years. First I have increased freeboard (in common with most designers, monohull and multihull). I have also increased the centreline spacing and where appropriate, drawn a bigger knuckle. I haven't designed any deep V boats for a long time because of the pitching and light wind speed problems. I have found the flat bottomed or single chine hulls are as simple to build and are more efficient hull shapes.

Finally, I am one of the few designers who use all feasible hull shapes and so can choose the most appropriate one for the intended use. I'm not committed by dogma to any one hull design. The performance differences between different hulls are easy to see, however I have not noticed any practical difference in seaworthiness between them.

The following sketches are typical hull cross sections. Please note, these are not to scale and are not real boats, instead they are just examples of the different hullshapes we use in our designs. (For those not familiar with lines plans: Only half sections are shown. The forward half of the boat is shown to the right, the aft half is to the left of the vertical centreline.)

This is the "Dory" hull used on the Janus and Gypsy as well as the Strike trimaran mainhulls. Note that the Janus does not have a V'eed area forward (as shown) as the bottom is narrow enough to prevent slamming on such a small boat.

This chined hull is used on Flica, Mirage and Romany and is a close approximation to a round bilge hull, but built in flat panels

This is a deep V hull used on Surfsong, Windsong and Mira (deep V version)

This is the chined V hull used on Meander, Rhea and Ondina. If these larger boats had a conventional V hull then either the gunwale or keel panel would have to be very wide so that the hull had the correct displacement. By adding a soft chine the lower hull section can be well flared, while the topsides remain nearer vertical. This hull shape has the added advantage that the hull panel is stiffer and, as each section is smaller, it can be easier to make.

This continuously curving hull shape is used on Wizard, Sango and Wizzer. It has a similar below-waterline shape to a Strider hull (for example) but the bulge in the topsides allows a vertical bow to be drawn while keeping a good flare forward to prevent nosediving. It also adds to the interior room, especially at shoulder level. These hulls can be built in strip planking or foam sandwich but it is harder to build than the small knuckle hull shape.

This "Small Knuckle" round bilge hull is used on Strider, Shadow, Merlin, Gwahir, Skua, Gypsy (round bilge version), Mira (round bilge version), Scylla, Nimbus, Rhea (round bilge version) and Cirrus. This shape is easier to make than the one below. The knuckle is small and is usually made from solid timber (eg 2" x 1"). Even so it has proven effective at reducing spray and slamming. The hull bottom can be double diagonal plywood or strip plank. The topsides of both this hull shape and the one below can be strip plank or sheet plywood. Alternatively both knuckle designs can be built in foam sandwich with a flat panel topside panel.

This is the "Large Knuckle" hull used on the Scorpio, Javelin, Sagitta, Eclipse and Transit. It is the most sophisticated shape I draw, and takes the longest to build. The large flare increases space inside and cuts down on spray. The angle of the hull at the WL is actually higher than on other hull shapes. That means that it sinks relatively slowly as you add weight. A big advantage for cruisers. But it also means the boat doesn't pitch and heave so much, ie vertical movements are reduced. That is because it is (slightly) harder to make the boat sink as it goes through waves. All minor differences but they add up if you are looking for the best all round shape.

Having said that, if you are planning on using LAR keels rather than daggerboards then you will probably be better off with a flat panel hull. There is no point in taking just one part of the overall design to the limit, you have to balance the trade off for the whole boat. So don't fit daggerboards and cheap sails! Makes no sense to me.

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In comparison with Moore's Law , the nonsilicon world's progress can seem rather glacial. Indeed, some designs made of wood or metal came up against their functional limits generations ago

The length-to-beam ratio (LBR) of large oceangoing vessels offers an excellent example of such technological maturity. This ratio is simply the quotient of a ship's length and breadth, both measured at the waterline; you can think of it simply as the expression of a vessel's sleekness. A high LBR favors speed but restricts maneuverability as well as cargo hold and cabin design. These considerations, together with the properties of shipbuilders' materials, have limited the LBR ratio of large vessels to single digits.

If all you have is a rough wickerwork over which you stretch thick animal skins, you get a man-size, circular or slightly oval coracle —a riverboat or lake boat that has been used since antiquity from Wales to Tibet. Such a craft has an LBR close to 1, so it's no vessel for crossing an ocean, but in 1974 an adventurer did paddle one across the English Channel.

Building with wood allows for sleeker designs, but only up to a point. The LBR of ancient and medieval commercial wooden sailing ships increased slowly. Roman vessels transporting wheat from Egypt to Italy had an LBR of about 3; ratios of 3.4 to 4.5 were typical for Viking ships , whose lower freeboard—the distance between the waterline and the main deck of a ship—and much smaller carrying capacity made them even less comfortable

The Santa María , a small carrack captained by Christopher Columbus in 1492, had an LBR of 3.45. With high prows and poops, some small carracks had a nearly semicircular profile. Caravels , used on the European voyages of discovery during the following two centuries, had similar dimensions, but multidecked galleons were sleeker: The Golden Hind , which Francis Drake used to circumnavigate Earth between 1577 and 1580, had an LBR of 5.1.

Little changed over the following 250 years. Packet sailing ships, the mainstays of European emigration to the United States before the Civil War, had an LBR of less than 4. In 1851, Donald McKay crowned his career designing sleek clippers by launching the Flying Cloud , whose LBR of 5.4 had reached the practical limit of nonreinforced wood; beyond that ratio, the hulls would simply break.

A high LBR favors speed but restricts maneuverability as well as cargo hold and cabin design. These considerations, together with the properties of shipbuilders' materials, have limited the ratio of large vessels to single digits.

But by that time wooden hulls were on the way out. In 1845 the SS Great Britain (designed by Isambard Kingdom Brunel , at that time the country's most famous engineer) was the first iron vessel to cross the Atlantic—it had an LBR of 6.4. Then inexpensive steel became available (thanks to Bessemer process converters), inducing Lloyd's of London to accept its use as an insurable material in 1877. In 1881, the Concord Line's SS Servia , the first large trans-Atlantic steel-hulled liner, had an LBR of 9.9. Dimensions of future steel liners clustered close around that ratio: 9.6, for the RMS Titanic (launched in 1912); 9.3, for the SS United States (1951); and 8.9 for the SS France (1960, two years after the Boeing 707 began the rapid elimination of trans-Atlantic passenger ships).

Huge container ships , today's most important commercial vessels, have relatively low LBRs in order to accommodate packed rows of standard steel container units. The MSC Gülsün (launched in 2019) the world's largest, with a capacity of 23,756 container units, is 1,312 feet (399.9 meters) long and 202 feet (61.5 meters) wide; hence its LBR is only 6.5. The Symphony of the Seas (2018) , the world's largest cruise ship, is only about 10 percent shorter, but its narrower beam gives it an LBR of 7.6.

Of course, there are much sleeker vessels around, but they are designed for speed, not to carry massive loads of goods or passengers. Each demi-hull of a catamaran has an LBR of about 10 to 12, and in a trimaran, whose center hull has no inherent stability (that feature is supplied by the outriggers), the LBR can exceed 17.

This article appears in the August 2021 print issue as "A Boat Can Indeed Be Too Long and Too Skinny."

Vaclav Smil writes Numbers Don’t Lie, IEEE Spectrum 's column devoted to the quantitative analysis of the material world. Smil does interdisciplinary research focused primarily on energy, technical innovation, environmental and population change, food and nutrition, and on historical aspects of these developments. He has published 40 books and nearly 500 papers on these topics. He is a distinguished professor emeritus at the University of Manitoba and a Fellow of the Royal Society of Canada (Science Academy). In 2010 he was named by Foreign Policy as one of the top 100 global thinkers , in 2013 he was appointed as a Member of the Order of Canada , and in 2015 he received an OPEC Award for research on energy. He has also worked as a consultant for many U.S., EU and international institutions, has been an invited speaker in more than 400 conferences and workshops and has lectured at many universities in North America, Europe, and Asia (particularly in Japan).

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Length-beam ratio

L/B = length divided by beam.

Units: Dimensionless.

Usually, the waterline dimensions L WL and B WL are used for monohulls, or for a single hull of a multihull.

What it's used for

Performance.

Larger L/B indicates a slimmer hull. This usually implies less wave-making resistance, and thus more efficient high-speed performance, but also suggests reduced load-carrying ability for a given length.

If a boat can plane, smaller L/B often suggests more efficient performance at low planing speeds. The balance generally tilts in favour of high L/B for fast boats.

Typical ranges of L/B are:

2 to 4 - Small to mid-size planing powerboats.

3 to 4 - Most small to mid-size sailboats and motor yachts, the longer ones generally having higher L/B. Some "skimming dish" racing sailboats also have L/B in this range; their wide beam gives them more initial stability so that they can fly larger sails.

4 to 6 - Fairly long and lean for a monohull. Some large, efficient long-range cruisers fall in this range, along with many racing monohulls.

6 to 10 - Large freighters; main hulls of cruising trimarans; a few very portly cruising catamarans; the lightest and slimmest of large sailing monohulls.

10 to 16 - Fast cruising cats and tris; a few racing multihulls.

Over 16 - Racing multihulls. Such high L/B is conducive to very light, low-drag hulls for race boats, but makes it very hard to get enough room inside the hulls for equipment or living space.

Living Space

If a boat is going to spend most of its time in a marina or at anchor, relatively low L/B implies a larger, more spacious interior and increased carrying capacity when compared to slimmer competitors of the same length. For a boat that must entertain guests at the dock but will rarely be used in rough weather or at high speeds, this may be a significant advantage. The slimmer boat, though, will generally have the advantage when fuel is expensive or when the weather picks up.

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Catamaran stability, foreword (james wharram, 2004).

It is 50 years since I designed my (and Britain's) first offshore Sailing Catamaran. The accepted opinion at the time, expressed in Yacht Magazines, was that the offshore catamaran would break up in high sea waves, that their motion on the high seas would be so violent as to render the crew helpless and that the double canoe/catamaran could not sail to windward.

Well, the voyage of Eric de Bisschop of France who sailed his 38ft. KAIMILOA half way around the world in 1937/39 and his two by him inspired 'Sailing Sons', Rudy Choy of Hawaii in the Pacific and James Wharram in the Atlantic proved these 'theorists' wrong.

What is interesting on looking back is that no critic at the time mentioned 'capsizing' as a possible fate of the historic offshore double canoe/catamaran. The reason is that early in the 1950s, the wartime experience of hundreds of men who had survived, sometimes for weeks, in small open boats, in rubber life rafts, cork Carley rafts or even floating wooden hatch covers, encountering severe storms with big waves without capsizing, was in seafaring circles common knowledge.

That you could capsize at sea on a form stable sailing ship (which is what a catamaran is) through having too high a mast and too much sail area was at the time also common knowledge amongst seamen, as it affected all commercial sailing ship design. There were many people around in the 1950s who still had knowledge and practical experience of such ships. Their knowledge certainly influenced the mast heights and sail areas of my first seagoing catamaran designs, as did the writings on Form Stable ships by Howard Chapelle, the great American naval architect.

In the late 1950s, the Prout Brothers were developing a 16ft. racing day boat catamaran. It was fast and outsailed all monohull racing, dinghies of the time. Like racing sailing dinghies, without skilled handling, they capsized frequently. Still, with the attendance of the patrolling Race Guard Boats no one died.

Offshore catamarans began to first develop in the 1960s. From the beginning, there were some designers, like myself, who saw them as Form Stable boats following traditional Form Stable Stability values i.e. boats which 'looked after their crew'.

There were also designers (not many), drunk on the speeds of day racing catamarans, who used the sail areas, mast heights and stability values of the day racing catamarans on Offshore Cruising Catamarans, i.e. at all times the Crew looked after the stability of the boat.

Unfortunately, designers of these low stability catamarans have nearly always tended to imply in yacht magazines, to the public, that they are more skilled designers "Because their boats sail faster"?

Equally unfortunate was that by 1976 many of these low stability catamarans were publicly capsizing, when their trained crew got tired or, particularly, when sold to unsuspecting monohull sailors. Suggestions in England and America were made to 'ban offshore multihulls'. Hanneke Boon and I wrote our first article on cruising catamaran stability ("The Stable Multihull") in 1977, and things settled down again.

However, around the late 1980s another group of young designers from racing background or using racing catamaran concepts moved into Cruising Catamaran design and, once again, capsizes with deaths occurred. So, again we wrote in another article on our observations on safe stability for Cruising Catamarans.

This article was first published in 'Practical Boat Owner' (UK) in August 1991 and since then in several other countries. The I.S.O. has recently also published formulas for calculating catamaran stability as part of the Recreational Craft Directive. On examination, their formula is the same as the one published by us in 1991 with a slightly smaller safety margin for Dynamic Stability (70% to our 60%). So far, the I.S.O. has not yet given recommendations as to what is a 'safe' stability for offshore sailing.

Introduction

In November 1989, the British Multihull Club, M.O.C.R.A., had an International Symposium on multihull design to celebrate its 20th anniversary.

During the lunch break, one very pregnant lady asked me: "Why don't they discuss capsizing? That is what I want to know about. I do not like heeling monohulls, but I do not fancy swimming with my baby out of an upside down catamaran."

Unfortunately, what I had to tell the pregnant lady is that they never seriously discuss capsizes at Multihull Symposia except in a self-congratulatory way, saying that an "upside-down catamaran floats as against a monohull that sinks". Ignored are hypothermia, broken limbs, lost crewmembers and mothers frantically trying to find their children to say nothing of at least half the value of the boat, i.e. the interior destroyed by the inrushing water.

It is a very emotive subject between designers and their followers because it touches not just on sales and profits, but also on masculine subjective attitudes like high tech., low tech., modern, traditional, taking risks, being cautious...etc. Symposium organizers realize that free discussion would lead to uproar. For the monohull sailor wishing to buy a cruising catamaran to suit his/her family's needs, Multihull Symposia and Multihull Magazines so far have given no real information.

Fortunately, the formulas that most catamaran designers use nowadays to calculate stability are not all that difficult to understand, and with them a prospective owner with a little background knowledge on sailing ships in general, can make his/her own decision as to whether the cruising catamaran they desire has the stability to be safe for their intended usage.

If you have forgotten most of your mathematics since you left school and, like most of us, hate to admit it, do not be frightened of the word formula. Calculators now do most of the work, and everyone knows or has some bright adolescent only too eager to use and demonstrate his/her latest calculator acquisition. However, the calculator only produces figures. To relate these to our needs, we do need to know some sailing ship history.

Polynesian Origins

The historic catamaran is the workboat of the Polynesian Pacific. Archaeological excavations, legend and early Western observers have shown that they had been in use hundreds of years - perhaps thousands - for fishing, coastal trade and ocean exploration, a background usage similar to that of the Chinese junk types and our own traditional Western sailing boats (before the development of the modern ballast keel yachts). Catamarans have exactly the same stability behavior as Junks and the traditional Western Sailboat.

Joshua Slocum's SPRAY is a typical example of a workboat of the late 18th and early 19th century. (See Fig.1)

What kept the SPRAY and traditional sailing ships from capsizing under the pressure of the wind on the beam were the wide hull beam, flattish bottom shape (i.e. "form stability"), and a selection of heavy rocks (i.e. "ballast stability"). In addition, the masts were kept short to lower the heeling moment of the sails.

Extra sail area for light winds was achieved by spreading the base of the sails out by means of bowsprits and bumpkins rather than raising the sails higher on a longer mast, creating a greater heeling/capsizing force.

According to Chapelle, in his book "The Search for Speed under Sail", if traditional sailing ships heeled much more than 55º, then they were in trouble. The loose rock ballast, about 10% of the total displacement, could break loose. A complete capsize would then occur and the boat would remain upside down. Capsizing, until the advent of the modern ballast keel yacht, was the theoretical possibility of ALL seagoing sailing vessels. Designers/ Boatbuilders have been able to design boats stable enough to stay well away from the possibility of capsizing for at least 3000 years. (ULU BURUN SHIP -Nat. Geographic Magazine, Dec. 87)

Racing developed the modern ballast keel yacht. To sail closer to the wind the rigs got higher. To balance that, the ballast changed from rocks to heavy iron (this became cheaper with industrialization), and finally, to be able to use even higher masts, the ballast changed to the heavier lead and moved from inside the hull to the outside in a deeper keel. As a side result, and not intended by design, the modern self-righting yacht was born.

Those who observed this development towards self-righting yachts did not regard it as a total blessing. They commented on how these "new" yachts plunged and rolled, which made sailing very uncomfortable and caused seasickness.

Even so the modern ballast keel yacht is still a relatively broad-beamed vessel, i.e. with a waterline length about 3 times longer than its beam - in technical terms, a length/beam ratio of 3:1.

Beamy hulls of 3:1 have to push a lot of water around them when sailing. This produces the well-known drag waves. (See Fig.1) and limits the maximum possible speed to approx. 1.4 x √WLL (in feet). Thus with a waterline length of 25 feet, your average speed will be about 5 - 6 knots.

The catamaran's unique speed potential, greater than that of the equivalent size monohull has arisen because it developed out of two ancestral boat types of the Pacific. Around the Pacific Ocean of antiquity there were various maritime peoples. Some used large paddling canoes up to 60 feet long for coastal trading, fishing, and whale hunting. (See Fig.2) Their long slim hulls with length/beam ratios of 12:1 to 20:1 allowed the water to part and run around them without creating drag waves at √WLL. They could reach speeds as high as 2 or 3 times the √WLL. So a canoe of 25 foot waterline length could reach speeds of 10 knots and over.

Hard paddling men with their food and water add up to weight. Even the toughest men can only paddle for a few hours.

Other Pacific maritime people had sailing rafts. Thor Heyerdahl's Kon Tiki expedition of 1947 used a modern replica of this type of craft. 45 ft. long, 18 ft. wide, rigged with a squaresail, manoeuvred by daggerboards, it could sail sufficiently against the wind to be a true sailing craft. It carried a crew of six in basic, though surprising comfort across the Pacific. (See Fig.2b)

It was not a speedy craft, but by its beam and weight, it was practically impossible to capsize and thus had stability, an essential part of seaworthiness.

Long ago some genius in the Pacific joined two fast, easily driven canoe hulls into a beamy raft shape, giving a new type of sailing craft with the stability of the broad beam raft and the high-through-the-water speed potential of the single canoe. (See Fig.2c)

Fig.2c is an approximation of a traditional Polynesian sailing craft and how it developed from its two ancestral types. It has a raft-like deck platform that could house people, and ample room to move around. From early European explorers' descriptions, the crew sailed with families, friends, lovers, singers and dancers in one joyous group from island to island - a marvellous way of life.

Efficient Crab Claws

Modern wind tunnel tests, as done by Tony Marchaj, of Southampton University, have shown that the Polynesian sail shapes were highly efficient to windward. With efficient sails, a hull form that allowed the boats to sail faster than the maximum speed of 1.4 x √WLL of Western ships and enough raft stability to be uncapsizable, (i.e. the sails would rip before the ships could capsize), the Polynesian catamaran was a remarkable sailing craft and worthy of being developed as a modern pleasure sailing craft.

Though to-day's yachtsman increasingly accepts the concept of the double-hulled ship, he/she places modern urban attitudes on the catamaran. These are: 1) to get the maximum speed potential out of the catamaran form. (Faster is always equated with being better, no matter what the cost.) 2) to alter the hull form to get as close to modern urban style accommodation needs as possible, which was described in the recent RYA (Royal Yachting Association) 'Competent Crew Handbook': "The typical modern cruising yacht has....interior design principles....much in common with a caravan".

The quest for speed

It is Demand 1) which creates most controversy for, in order to reach the maximum speed potential of a catamaran, you have to carry a large sail area, which reduces its inherent stability to the point, where with the average cruising crew, it is in danger of capsizing well before the average monohull suffers a knockdown.

With a sense of realism any would-be catamaran owner, once he/she knows how to calculate stability, can make his/her own decision when viewing a cruising catamaran design, whether they want maximum speed or maximum stability. As the formula will show, you cannot have both at the same time. Fig.3 shows how to calculate catamaran stability. Fig.4 and Fig.5 are helpful to learn how to determine the position of the center of Effort and the Center of Lateral Resistance.

In 1976, catamarans built using this stability formula were capsizing all over the world at mean wind speeds a lot lower than the wind speed the formula predicted.

In an Article called "The Stable Multihull", published in 1977, Hanneke Boon and I demonstrated that the given formula was a static formula for static state conditions.

However, wind is a turbulent, gusty, dynamic force. Gusts can be as much as 40% to 60% greater than the mean wind speed, so the static formula has to have built in a safety factor for dynamic, natural state wind conditions to allow sailing craft to absorb the extra wind gusts without immediately capsizing in the manner of a dinghy.

Since 1977, this dynamic formula concept, after much initial argument, has been accepted. It has now been generally agreed amongst designers, that taking 60% (x 0.6) of the Static stability allows for a suitable safety factor. So, the Dynamic Stability (i.e. maximum mean wind speed it is safe to sail in before reducing sail) is found as follows:

At the M.O.C.R.A. symposium were the designers of two 34-35 ft. catamarans about to be placed on the market. We will use them as examples of two opposing design attitudes towards speed and catamaran stability. Their dimensions, obtained from yacht magazines and brochures, are given in Fig.6a .

The first noticeable points from Fig.6a are that catamaran B has a wider beam than catamaran A, but carries 33% more sail and has a much lighter construction weight.

If you asked the opinion of the designer of catamaran A with reference to design B, he would say that he has been designing and building catamarans for thirty years, that his sail area to weight ratio to beam etc. had evolved to provide the maximum stability, Which adds up to sailing safety.

The designer of catamaran B, a more recent designer in the cruising catamaran field, would point out, that his design had much more beam (which is a feature of catamaran design over the last ten years) and. thus has the stability to carry the extra sail area.

You, the would-be catamaran purchaser, without the aid of the given formula would be at a loss to know:

• The Static Stability of either design, which can be described as the "Moment of Truth" when the boat is on the edge of capsizing i.e. when the windward hull lifts out of the water.
• The Dynamic Stability , when it is safe to sail along with all sails up and have sufficient reserve stability to meet safely any wind gusts that lie under that lovely white cloud or just along the coast where a narrow, scenic valley opens to the sea and down which the wind unexpectedly gusts.

Fig.6b shows the working out of the Static and Dynamic stability of both designs, using their lightly loaded weights, sometimes described as racing trim. From the formula we can see that catamaran B has less stability in spite of its wider beam than catamaran A. So what!

These figures must be related to the real world of sailing. To do this I will use a book by the sailing meteorologist, Alan Watts, called "Instant Weather Forecasting" (published by Adlard Coles).

On pages 10 and 11, Alan Watts describes the behavior of dinghies and deep keel monohulls in various wind strengths. (I have extracted these details for wind forces 3 to 6.). See Fig.7 .

Catamaran A with a Dynamic stability of 18.2 knots ( Fig.6 ) needs to be carefully sailed or reefed by the middle of force 5 (remember this is a lightly loaded catamaran). Alan Watts describes the deep keel monohull in a force 5 wind as "Craft's way somewhat impeded by seaway. Genoas near their limit. Yachts approach maximum speed.'

Force 5 is a well-known wind state that to the average yachtsman draws attention to itself by strong audiovisual signals of waves and wind, which leads him naturally to take particular care in sailing, changing headsails or reefing.

Therefore, if a monohull yachtsman handling Catamaran A is slow at reefing in force 5 and is hit by a strong wind gust, he would have approx. a 60% stability safety margin to absorb his slowness and the gust, for his windward hull would not begin to lift from the water (the Moment of Truth) until 30.5 knots of wind hits the sails (i.e. a force 7 gust).

Similar sail handling

Conclusion, the same sail handling habits of the monohull cruising sailor can easily be applied to the lightly loaded catamaran A, without fear of immediate capsize. Catamaran B with a Dynamic stability of 13.3 knots (lightly loaded) will need to be carefully handled, aware of wind gusts, or reefed, to preserve its 60% stability safety margin in the middle of force 4 (11-16 knots). THIS IS AT ONE FORCE lower than monohulls or Catamaran A.

Watts describes force 4 for monohulls as: "Best general working breeze for all craft, genoas at optimum". With the necessity to reef to preserve the 60% safety margin, this description does not apply to Catamaran B. However, his description of force 4 for dinghies does, for he writes: "Dinghy crews lie out...", i.e. are attentive to stability to prevent capsizing. As dinghies, so Catamaran B.

Catamaran B continues to echo dinghy-handling characteristics at increasing wind strengths. Catamaran B's Static stability, i.e. hull lifting point, lies on the borderline between force 5 and 6 (22.5 knots). Watts' dinghy handling descriptions for force 5 and 6 are as follows: Force 5: "Dinghies ease sheets in gusts...some capsizes." Force 6: "Dinghies overpowered when carrying full sail. Many capsizes." Conclusion: Lightly loaded Catamaran B, above wind force 4 can only be sailed safely by skilled dinghy type sailing techniques or should be reefed at force 4.

Why not reef?

The enthusiasts for the Catamaran B type argue that for general family cruising, you can reef Catamaran B and give it the same monohull type stability as Catamaran A. At other times, with a trained crew and all sails up, you have the benefits of fast exciting sailing.

This is true, and Fig.8 shows the use of the formula to see how much sail you would have to reef down to give Catamaran B the same stability as Catamaran A with all sail up. This is a sail reduction from 750 sq.ft, to 511 sq.ft.

Providing that reefing was a rigidly applied rule when there was not a fully experienced dinghy technique skipper standing by the sheets or helm, it would be effective.

If you feel a little conspicuous, sailing reefed in a Force 4 breeze, you can carry more load to stabilize Catamaran B. Again, the formula (see Fig.9 ) shows that if the boat weight is increased to 11787 lbs with extra stores and equipment (in fact, its full cruising payload), your stability would again equal that of Catamaran A in its lightly loaded condition.

However, if Catamaran A increases its payload to the designed maximum (approx. 11050 lbs) its Dynamic stability goes up too, and it would require a gale gust of 34 knots, to lift one hull out of the water. This conforms to the wind stabilities of traditional sailing craft throughout the ages. A cruising catamaran designed to these principles gives no stability problems to the average yachtsman and his family, enjoying its broad decked upright sailing.

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Dear Readers

Comparing trimarans & catamarans.

Trimarans tend to be more performance oriented than catamarans. In part, this is because it’s easier to design a folding trimaran, and as a result Farrier, Corsair, and Dragonfly trimarans had a disproportionate share of the market.

In spite of this and in spite of the fact that many are raced aggressively in windy conditions, capsizes are few, certainly fewer than in equivalent performance catamaran classes.  But when they do go over, they do so in different ways.

Trimarans have greater beam than catamarans, making them considerably more resistant to capsize by wind alone, whether gusts or sustained wind. They heel sooner and more than catamaran, giving more warning that they are over powered. 

Waves are a different matter. The amas are generally much finer, designed for low resistance when sailing deeply immersed to windward. As a result, trimarans are more susceptible to broach and capsize when broad reaching at high speed or when caught on the beam by a large breaking wave.

In the first case, the boat is sailing fast and overtaking waves. You surf down a nice steep one, into the backside of the next one, the ama buries up to the beam and the boat slows down. The apparent wind increases, the following wave lifts the transom, and the boat slews into a broach. If all sail is instantly eased, the boat will generally come back down, even from scary levels of heel, but not always.

In the second case a large wave breaks under the boat, pulling the leeward ama down and rolling the boat. Catamarans, on the other hand, are more likely to slide sideways when hit by a breaking wave, particularly if the keels are shallow (or raised in the case of daggerboards), because the hulls are too big to be forced under. They simply get dragged to leeward, alerting the crew that it is time to start bearing off the wind.

Another place the numbers leave us short is ama design. In the 70s and 80s, most catamarans were designed with considerable flare in the bow, like other boats of the period. This will keep the bow from burying, right? Nope. When a hull is skinny it can always be driven through a wave, and wide flare causes a rapid increase in drag once submerged, causing the boat to slow and possibly pitchpole.

Hobie Cat sailors know this well. More modern designs either eliminate or minimize this flare, making for more predictable behavior in rough conditions. A classic case is the evolution of Ian Farrier’s designs from bows that flare above the waterline to a wave-piercing shape with little flare, no deck flange, increased forward volume, and reduced rocker (see photos page 18). After more than two decades of designing multihulls, Farrier saw clear advantages of the new bow form. The F-22 is a little faster, but more importantly, it is less prone to broach or pitchpole, allowing it to be driven harder.

Beam and Stability

The stability index goes up with beam. Why isn’t more beam always better? Because as beam increases, a pitchpole off the wind becomes more likely, both under sail and under bare poles. (The optimum length-to-beam ratios is 1.7:1 – 2.2:1 for cats and 1.2:1-1.8:1 for trimarans.) Again, hull shape and buoyancy also play critical roles in averting a pitchpole, so beam alone shouldn’t be regarded as a determining factor.

Drogues and Chutes

While monohull sailors circle the globe without ever needing their drogues and sea anchors, multihulls are more likely to use them. In part, this is because strategies such as heaving to and lying a hull don’t work for multihulls. Moderate beam seas cause an uncomfortable snap-roll, and sailing or laying ahull in a multihull is poor seamanship in beam seas.

Fortunately, drogues work better with multihulls. The boats are lighter, reducing loads. They rise over the waves, like a raft. Dangerous surfing, and the risk of pitchpole and broach that comes with it, is eliminated.  There’s no deep keel to trip over to the side and the broad beam increases the lever arm, reducing yawing to a bare minimum. 

Speed-limiting drogues are often used by delivery skippers simply to ease the motion and take some work off the autopilot. By keeping her head down, a wind-only capsize becomes extremely unlikely, and rolling stops, making for an easy ride. A properly sized drogue will keep her moving at 4-6 knots, but will not allow surfing, and by extension, pitch poling. 

For more information on speed limiting drogues, see “ How Much Drag is a Drogue? ” PS , September 2016.

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Principles of Yacht Design Sen

Its Loa/Bmax is thus 3.25. For an Lwl in the light condition of 9.85 m this corresponds very well with the median line of Fig 5.32. In fact, for a new design the hull is slightly narrow, since new hulls are often a bit beamier than the median according to the figure. The data in the statistical analysis of this section may be considered representative of the yacht fleets in Europe and the United States in the early 1990s, and may therefore represent an average of design trends in the 1980s and to some extent in the 1970s also.

Length of water tin e/draft (LyyL/T)

LWI/T is plotted versus Lwl in Fig 5.jj. Obviously, this ratio increases with length as well. A larger yacht has a larger ratio, ie a smaller relative draft. In fact, beam is a better scaling factor than length for the draft of a sailing yacht, and a good approximation is BMAX = 1.6 - T, which is valid more or less for all lengths. This relation corresponds very well to the median line in Fig 5.33. The upper and lower limits in this case are 15% from the median line.

Fig 5.33 Length/draft ratio

The choice of draft for a cruising yacht is a trade-off between performance and practical advantages, like the possibilities of entering more shallow water areas, ease of handling ashore etc, while for a racing yacht draft is penalized to cancel the performance advantage. The YD-40 has an Lwl of 9.85 m and a draft of 2.04 m in the light condition. This yields an Lwl/T of 4.83. According to Fig 5.33, the median for this size is 5.2, which yields a draft of 1.89. The extra 0.15 m will give the YD-40 an edge upwind, consistent with the desire to create a fast cruiser/racer.

Length of Since most modern yachts have fin keels it is possible in most cases to water line/canoe body define the canoe body draft. This seems to scale very well with length, draft (Lwl/TJ as can be seen in Fig 5.34. A typical value of Lw,/Tc is 18 for a medium displacement yacht. The ultra light dinghy type racing

machines may reach values up to 26, while heavy displacement, narrow hulls may have as small an Lwl/Tc as 12. For the ultra light hulls data are available only for large waterline lengths. The YD-40 has an Lwl/Tc close to the medium.

As explained above the length/displacement ratio is a very important

Fig 5.34 Length/hull draft ratio

Length/displacement ratio (Lwl/V'a)

quantity for the resistance of the yacht at high speeds. To enable the yacht to exceed a Froude number of about 0.45, ratios above about 5.7 are required. In Fig 5.35 the length/displacement ratio is plotted versus waterline length.

Since beam and draft do not increase linearly with length, displacement

Length/displacemenl ra tio

increases slightly slower than length cubed. In fact, with the same assumptions as above, the displacement increases as (length)7/?, which means that the length/displacement ratio increases as (length)2/». Increasing the length by a factor of two increases the ratio by 17%. The increase is not quite as fast in the statistical data, as may be seen in Fig 5.35.

As was the case in the length/beam ratio the spread is asymmetric. The lower limit in this case is some 12% below the median line, while the upper limit is put 20% above the median. There "are.- however, certain kinds of hulls outside the limits. Thus, some extreme ultra light yachts have considerably higher ratios, and since the statistics are based mainly on yachts which may participate in some kind of racing (performance handicapping system, IMS or 10R), some heavy cruising yachts may have been missed.

The length/displacement ratio is, of course, quite different between a racer and a cruiser, since the equipment required for comfortable living on board is rather heavy. In the case of the YD-40 we have tried to create a cruiser/racer with full comfort. Its length/displacement ratio is 5.16, which is close to the median for a 10 m Lwl yacht.

The overhangs of modern hulls have decreased as compared to hulls designed before the 1960s. To a certain extent this is a matter of fashion, but there is also an attempt to reduce the longitudinal gyradius as much as possible for a given (effective) waterline length. The inverse slope of the transom is another effect of this effort.

A medium value of Loa/Lwl for modern yachts is 1.23 with a spread of 0.15 up and down. There is no discernible trend with hull length . The YD-40 is very close to the median: L0A/LW, is 1.22.

Freeboard height It is a well-known fact that the relative freeboard height decreases with hull length. Obviously this is due to the requirements of the accommodation. Even on very small yachts headroom for moderately tall people is required. The trend is shown in Fig 5.36, which shows the freeboard forward versus the waterline length. No upper and lower limits arc given, since the statistical basis for this graph is smaller than for the others above (only about 50 yachts).

A typical value of freeboard forward/freeboard aft is 1.3. As compared to older yachts this is lower, so modern yachts have a more horizontal sheer line. Both the forward and aft freeboards are higher however, and the camber of the sheer line, the 'spring', is smaller. The YD-40 is representative of modern cruiser/racers and has somewhat higher freeboards than the statistical mean value, which is influenced to a certain extent by some older designs. The freeboard forward/ waterline length is 0.144, while the mean value is 0.138 for this size of hull, and the ratio of the two freeboards is 1.22.

Length overall/length of waterline

Ballast ratio

The ballast ratio, ic the ratio of keel weieht to total weieht, varies

considerably on modern yachts. A good average value is 0.45 and most yachts lie within the range 0.35-0.55 (see Fig 5.37). There docs not seem

Fig 5.36 Freeboard forward/ length ratio

Continue reading here: Keel and Rudder Design

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Readers' Questions

What length boat is considered a yacht?

There is no specific length that officially categorizes a boat as a yacht. However, generally, a boat is considered a yacht when it is at least 33 feet (10 meters) in length.

What length is considered a yacht?

A yacht is generally considered to be a recreational watercraft that is at least 40 feet (12 meters) in length.

What is beam length on a boat?

The beam length of a boat can vary, depending on the size and type of boat. For example, the beam length of a small sailboat may be 9-10 feet, while a large powerboat or yacht may have a beam length of up to 25 feet or more.

How to design ship's draft boatbuilding?

Select a hull type: The first step in designing a ship's draft is to select a hull type. Common hull types for shipbuilding include displacement hulls, semi-displacement hulls, planing hulls, and catamaran hulls. Choose a draft: The draft is the depth of the waterline below the surface of the water. Generally, the deeper the draft, the larger the ship and the more cargo it can carry. Calculate the stability: Stability is an important factor in ship design, as it impacts the ship's center of gravity and its ability to resist leaning or tipping. Calculate the trim: The trim is the angle at which the bow and stern of the ship sit relative to the waterline. It is important for ships to have the proper trim to ensure both stability and performance. Calculate the position of the center of buoyancy: The center of buoyancy is the point at which the ship maintains buoyancy and stability in the water. The position of the center of buoyancy must be calculated in order to effectively design the ship's draft. Calculate the displacement: The displacement is the amount of water displaced by the ship when it is in the water. The displacement of a ship is directly related to its draft and is used to calculate its total weight. Calculate the resistance: Resistance is the amount of force that a ship experiences when it moves through the water. The resistance must be taken into account to ensure that the ship is able to move efficiently through the water. Calculate the manoeuvrability: Manoeuvrability is the ability of the ship to change direction quickly and smoothly. The manoeuvrability of a ship is directly related to its draft, so it is important to consider when designing a ship's draft.

What is the average draft ratio of a yacht?

The average draft ratio of a yacht can vary significantly depending on the type and size of the yacht. Generally, it will range from approximately 1.5 feet to 8 feet.

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> > Please support our sponsors and let them know you heard about their products on Cruisers Forums. 19-05-2020, 10:36   Check out John web site. Look at his Articles tab. Info is a bit old but would think the engineering calcs are still valid. His are wider than most. French designs are usually narrow. You might want to also dig into info on cat stats. So many are narrow that get caught in a squall. In my experience, no design is perfect - its always a compromise. So figure out your application (off shore, coastal, lazy cruising in big comfort, , etc.) and try fit design to it. Ya, all cats have trouble at due to width. I always try for outside berth and talk marina out of 'catamaran rates'. 19-05-2020, 11:08   Boat: Alerion Express 38 Yawl (former) 19-05-2020, 11:23   Boat: 50ft Custom Fast Catamaran 43 1.8:1 TS42 1.75:1 Prout 37 2.45:1 500 1.85:1 KH56 1.7:1 Iroquos 2.24:1 Formula 40 1.63:1 The fatness vs centre line beam will also have an effect on the measurements. trimarans are typically 1.5:1 When you get very big Playstation, Explorer 2:1 is more common but the difference between CL beam and OA beam is very small. 19-05-2020, 13:06   19-05-2020, 13:14   19-05-2020, 13:24   Boat: TRT 1200 19-05-2020, 13:25   19-05-2020, 13:29   Boat: TRT 1200 19-05-2020, 13:55   19-05-2020, 13:57   Boat: Shuttleworth Advantage (Boards Down) 8' 9" / 2.67 M 4000 LBS / 1814 KG 808 SQ. FT. / 246.3 M 86 SQ. FT. / 26.2 M 1055 SQ. FT. / 321.64 M I 30' 0" / 9.14 M J 8' 6" / 2.59 M P 66' 4" / 20.24 M E 19' 3" / 5.85 M Materials Pre-preg carbon fiber 19-05-2020, 14:13   Boat: Shuttleworth Advantage 19-05-2020, 16:46   design now covers a wide spectrum from off the beach surf cats, small cruisers/weekenders/ club racers/ condo cats/ cats to 50 foil screamers. The designers clearly must be doing their homework. A very poor use of an an old adage but "a lot of water has gone under the bridge (deck)" Sorry. Lockdown is getting to me. 19-05-2020, 17:53   Boat: FP Belize Maestro 43 and OPBs 19-05-2020, 18:32   Boat: 50ft Custom Fast Catamaran 19-05-2020, 19:03   Boat: Simpson 11m Catamaran with its tonnes of lead at midships doesn't just stop the boat from falling over sideways, it also limits the boat from pitching in choppy seas. Lightweight catamarans rely on relatively narrow, long hulls to minimise pitching (as well as trying to concentrate mass midships). Shorten those hulls (in relation to the beam) and the boat will become a pitch-bitch in choppy conditions. Thread Tools Rate This Thread : Posting Rules post new threads post replies post attachments edit your posts is are code is are are are Similar Threads Thread Thread Starter Forum Replies Last Post Srah 1953 Monohull Sailboats 107 25-05-2024 08:48 Chotu General Sailing Forum 8 19-01-2020 09:57 sailabroad Dollars & Cents 33 16-04-2018 07:12 black_sails Multihull Sailboats 15 04-05-2016 08:11 rockerdar Multihull Sailboats 4 10-02-2009 17:33 No Threads to Display. - - - - - - - Privacy Guaranteed - your email is never shared with anyone, opt out any time. Catamaran Hull Speed Calculator For Beginners (Table and Free Spreadsheet) As an Amazon Associate, we earn from qualifying purchases. We may also earn commissions if you purchase products from other retailers after clicking on a link from our site. Speed is important, it can get you out of harm’s way, and it makes sailing much more fun, but figuring out how fast a catamaran will be able to sail can be tricky. One important aspect is to understand maximum hull speed. In this article, I have calculated different hull speeds for different lengths of boats; this includes both monohulls and catamarans but focuses on the latter. Here is the catamaran maximum hull speed table: Table of Contents Catamaran Max Hull Speed Calculator Table Length on Water Displacement Max Hull Speed Semi Planing Beyond Max Hull Speed 0% 10% 20% 30% Feet Meter Knots Knots Knots Knots 26 8 6.9 7.5 9.1 11.8 30 9 7.3 8.0 9.6 12.5 33 10 7.7 8.4 10.1 13.2 36 11 8.0 8.8 10.6 13.8 39 12 8.4 9.2 11.1 14.4 43 13 8.7 9.6 11.5 15.0 46 14 9.1 10.0 12.0 15.6 49 15 9.4 10.3 12.4 16.1 52 16 9.7 10.7 12.8 16.6 56 17 10.0 11.0 13.2 17.2 59 18 10.3 11.3 13.6 17.7 62 19 10.6 11.6 14.0 18.1 66 20 10.8 11.9 14.3 18.6 69 21 11.1 12.2 14.7 19.1 72 22 11.4 12.5 15.0 19.5 75 23 11.6 12.8 15.3 20.0 79 24 11.9 13.1 15.7 20.4 82 25 12.1 13.3 16.0 20.8 85 26 12.4 13.6 16.3 21.2 Table Explanation Length on the waterline (L.W.L.): Length of the boat when measured on the waterline, not to be confused with length overall (L.O.A.), which is the boat’s total length (above the waterline) including bowsprit, etc. Displacement max hull speed: The max speed of a boat whose L.W.L doesn’t change when underway and where the vessel’s bow wave is the limiting speed factor. Semi/Light Displacement or Semi planing hulls speed: A boat where the bow waves speed limiting factors can be partially overcome and therefore exceed the displacement hull speed. These hulls usually overcome hull speed by 10-30% . How to use the Catamaran Hulls Speed Table Choose your length on waterline in the left-most column, either in feet or meter. Continue reading to your right and stop either at “Displacement hulls speed” or continue to “10,20, or 30%”, depending on your estimated hull efficiency. This will be your calculated maximum hull speed for a semi-displacement catamaran. The Formula First of all, we need to know the maximum hull speed for a displacement hull, and from that number, we will be able to calculate how much faster the semi-planing (or semi-displacement) hull will be. This is the formula for Maximum Hull Speed on a displacement boat: Now we need to add the increased efficiency (loss of drag) of a semi-displacement hull, usually, this is somewhere between a 10-30% increase. Note: “1.3” is the increase in efficiency, if you believe you are on the lower end of the scale this would be 1.2 or 1.1. How to Exceed Hull Speed This calculator offers a theoretical perspective, but many other factors such as sail plan, weight, and sailor skill, of course, have a profound impact on speed. As we have seen, a semi-displacement hull can exceed maximum hull speed, but we can also see that it isn’t by much. The next step is to reduce drag even further by utilizing a planning hull. Catamaran Hull Speed Spreadsheet If you want more info, calculate other lengths, or see the speeds in Km/h or Mph then I suggest you check out this free spreadsheet. Catamaran Freedom Hull Speed Calculator Note: If you want your own copy just click, File->make a copy. Common Questions About Catamaran Hull Design Below I will answer some of the questions I receive concerning catamaran hull design. The list will be updated as relevant questions come in. Is a Catamaran a Planing hull? As we have discussed above, a catamaran can definitely have a semi-planing hull, but can it be designed in a fully planing configuration as well? Catamarans can be configured as planing hulls, although most sailing catamarans are set up as either semi-planing or hydrofoil. Due to the high speeds needed to get a boat to planing speed, this is only possible on racing sailboats or motor-powered catamarans such as high-speed ferries. Owner of CatamaranFreedom.com. A minimalist that has lived in a caravan in Sweden, 35ft Monohull in the Bahamas, and right now in his self-built Van. He just started the next adventure, to circumnavigate the world on a Catamaran! Leave a Reply Cancel reply Your email address will not be published. Required fields are marked * Save my name and email in this browser for the next time I comment. Recent Posts Must-Have Boat Gear for Catamaran Sailors! Sailing is probably the most gear-intensive activity I've ever done; there are so many decisions to be made about what gear to buy now, for tomorrow, and what to definitely never buy. The gear on... 6 Best Trailerable Trimarans For Bluewater and Coastal Sailing Having a boat costs a lot of money, even when you are not using it, marina fees, etc. And once it is in the water most sailors never go very far from their "home marina" and sailing will be somewhat... Sail Calculator Go Directly To The Sail Calculator Here What Carl’s Sail Calculator Does: Physicist and sailor Carl Adler developed this online Sail Calculator for comparing sailboats  and its database has grown over a number of years to almost 3000 boats. It should be one of the first places you go on the Web if you want to know the vital statistics about a sailboat, including Length Overall (LOA) , Length on the Waterline (LWL) , Displacement and Sail Area . The Sail Calculator will also give you valuable performance numbers for any vessel in its database or any numbers you enter, including the Displacement / LWL ratio, Theoretical Limiting Hull Speed, Sail Area / Displacement ratio, Length to Beam ratio, Motion Comfort value, Capsize Screening value, sailing category and Pounds per inch immersion value . Naval architects use these values when they design a new boat, and from them you can determine a conventional displacement hull boat’s purpose and predict its performance. Note that planing hulls, catamarans and hydrofoil vessels are not defined in the same way. Here’s what the performance numbers mean: Displacement/LWL ratio – Heavy boats (D/L above about 300) will carry big loads but require plenty of power to drive. Light boats (D/L below about 150) are generally quicker and more responsive but are affected by loading. Most boats have moderate displacement and they compromise the conflicting virtues of the extreme designs. Contemporary racing boats often have D/L ratios well below 100. Hull Speed – A conventional hull, which moves through the water rather than rising atop it and planing across the surface, is limited in speed by length of the waves it produces; long waves travel faster. This wave length can be calculated and the top speed of the hull predicted. Long boats make long waves. Sail Area / Displacement ratio – The SA/D ratio is like the power/weight ratio of an automobile. A high SA/D ratio (> about 18) indicates a powerful rig, while a low ratio indicates a more docile boat. Length / Beam ratio – A long, narrow hull with limited interior space is easier to drive than a short, fat one with plentiful capacity. Compare L/B ratios to gain insight into the purpose of the boat. Motion Comfort value – Not as widely used as the previous numbers, the Motion Comfort value tries to predict whether a boat has a quick, motion through the waves or a slow, easy motion. Note that some people get more seasick with a slowly rolling motion than a quick, jerky one. Your mileage will vary. Capsize Screening number – Developed after the Fastnet Race tragedy, the Capsize Screening number is a quick way to judge if a boat is seaworthy. Values below 2.0 are desirable for offshore yachts. Do not put too much faith in the exact number, as it is an approximation only. Pounds / square inch Immersion – When you load a boat, it sinks deeper into the water. This Immersion value indicates the weight carrying capacity of a vessel. There is also a Prop Sizing section which will calculate the optimum propeller to use on any displacement-hull boat, based on noted naval architect Dave Gerr’s formulas.  To The Sail Calculator 19 Comments on “ Sail Calculator ” I corrected it. Thanks! S2 7.3 specs from factory brochure (visible at boatbrochure DOT com SLASH products SLASH s2-7-3-meter-brochure the free preview is pretty legible) LWL is 18.5 not 18.73 Beam is 8.0 not 8.5 Displ is 3250 not 3373 S.A. is 255, not 261 Thank you so much for your work maintaining this web page; it is tremendously valuable resource that I refer to often! Tom, The Colgate 26 has a sail area different from that published on your calculator. It’s listed as 338 SF per https://www.colgate26.com/specifications/ Chris, Thanks for the note; I’m glad you find the Sail Calculator useful. I’ll change the value on the database on the next update, since your source is probably more accurate. I rarely know what the sources are when a user submits data, so there are definitely errors in there. It’s possible that one of the numbers is based on the 100% foretriangle measurement and the other is with a larger jib, which could be either the working jib or a Genoa. I get this question from time to time and probably should add something to the description about it (www.tomdove/blog/sail-calculator/). –Tom Hi Tom, thanks for Carl’s calculator alive. I have a Tayana 48 DS and from their website, I get a different sail area. 1316 sq ft vs 1048. Regards, Chris Hi Tom, Looking at your specs the Marieholm 26 literature does not match what is posted. There were 3 versions of this boat built with the Marieholm being the middle one. The Folkboat website shows this: loa 25.83, lwl 19.83, beam 7.17, s.a. 280 sq. ft., draft 4′, disp. 4740, ballast 2750. The 1st model was built from wood, the last (3rd) model is called the Nordic Folkboat built from fiberglas but made with lapstrack design to look like wood. It was heavier in weight than the other 2 with less s.a.. Google “Folkboats Around the World” and the info is there on the main page. Hope this helps. Like to see values once new info is inserted. Wish I could figure it myself but not sure how to. Thank you, Sam Thanks for catching that. I’ll correct it on the next update. — Tom The Goletta Oceanica De Biot 39 is missing a decimal point in the LWL so it is throwing off all of the calculations. David, There’s no simple answer to that. If you enjoy sailing the boat, it’s a good one. When you put the numbers into SailCalculator, it will return some basic information that can be very useful, but note that small, lightweight boats like the American 23 are sensitive to loading. The working displacement is actually the “Light Ship” displacement plus the weight of the average crew. Try adding the weights of you and your crew in SailCalculator and see how that affects the performance numbers. The people I have known who have the American 23 like it. It looks like a nice, stable daysailer. Enjoy! — Tom I have American sailboat 23 ft. sailboat with a displacement 3500 lbs my keel is a 1000 lbs with a beam 7ft and 11 inch just wondering how good is this boat for sailing thanks Mark, Very interesting. I can see why the Length/Beam ratio at the waterline would be the defining characteristic for hull speed. That can be an evasive number, I think. Multis with very narrow hulls will sink deeper into the water quickly as the boat is loaded, so the LWL/BWL could change dramatically. It seems that you’d have to be careful about specifying the displacement that produces a specific LWL/BWL ratio, don’t you think? Is there an issue of one hull being submerged more than the other when the boat is under sail? This seems especially important in trimarans, which often have one hull flying and the other deeply submerged, but a long, narrow cat would have some of the same response to a breeze. Keep me posted on your thoughts. I think you’ve hit on a key element here. — Tom Hello, I’m a mechanical engineer and experienced multihull sailor that has long thought multihulls need a better performance parameter for comparison so sales guys can’t hoodwink people! I have some graduate school education from Dr. Marshall Tulin (UCSB) who has published many works regarding high-speed displacement mode for long slender hulls for naval/military applications and I think this work is very applicable to sailing multihulls. The critical parameter as far as hull drag for catamarans is really L/B at the waterline since other parameters as far as hull form go (prismatic coefficient) are generally within a narrow range. It has the benefit of implying displacement and waterline length as well, since a heavy boat must be either fat, or long to carry the displacement. As a result, I’ve been working on a parameter that includes both sail area and L/B at the waterline for performance comparisons. The trouble is Schionning is one of the few designers that cites L/B in all of his designs but it would be an easy measurement to take dockside, when the true displacement isn’t known. Steve, I’ve never seen that formula but would love to have it. The speed of a multihull is largely a factor of the hull shapes, and most multis are not limited by the “Displacement Hull Speed” that determines the maximum speed of most monohulls. The hulls are generally long and narrow and do not create the speed-limiting waves. There are exceptions, and I think any formula that predicts the speed of a cat or tri would have to incorporate prismatic coefficient (“sharpness”). Most boats are not speed-limited by their sail area. I’m looking for a formula that predicts potential performance of a cruising catamaran, in teh same way that SA/D does for monohulls. I saw teh formula years ago – it uses sail area and the second power (i.e., the square) of a factor, but I don’t recall anything else. Can you help me? -Steve Charlie, Thank you for the compliment. I enjoy running the site and meeting so many people who love sailing. Good luck on your boat search; there are many good deals on used, mid-size cruising boats available now in the U.S. because builders flooded the market with 40-footers a few years ago. Now that so many Baby Boomers have finished their lifetime sailing adventures, the boats are for sale. I’ve just been introduced to your site by a good friend from the US. Im looking for a retirement live aboard that can take me around the world. He gave me a potted history and speaks very highly of Carl Alder in this site in general. What a great tool. I’ll be flying to the states to view some boats that otherwise wouldn’t have even been on my Radar. Thank you Tom, Thank you Carl (Thank you Harvey). Thanks for keeping Carl’s program alive, Tom. I sent him hundreds of small boat specs over the years and found quite a few errors from other inputs that Carl tried to correct. Between his poor vision, a lot of incorrect input (especially the difference between LOD versus LOA for most people) and the vague info from boat builders it was a long process. People should have supported him with far more donations, he was a good guy. Les Hall, San Antonio Thanks for catching that, Paul. Yes, that would be a mighty powerful boat. It appears that the displacement should have been 14,500, so I corrected that. The SA/D ratio still looks a bit high, but I don’t know what the submitter used as a source. Enjoy the site and please send any other corrections you see. — Tom Cavalier 37, LWL=30, Sail Area to Displacement=2314.05 Cant possibly be correct Great calculator, thanks for keeping it available. Cheers Leave a Reply Your email address will not be published. Required fields are marked * This site uses Akismet to reduce spam. Learn how your comment data is processed . Log in or Sign up You are using an out of date browser. It may not display this or other websites correctly. You should upgrade or use an alternative browser . Beam to length ratio for power catamaran Discussion in ' Stability ' started by Carlazzomark , Jul 12, 2022 . Carlazzomark Senior Member Should the beam to length ratio of a power cat be the same as a sailing cat (about 50%), or can it be narrower? Thanks   fallguy Senior Member It is a decent standard for a cruiser, but for a planing hull; weight might get up awful high. Need more context for anyone to answer well.   TANSL Senior Member The beam to length ratio has to do, above all, with the stability of the boat. In a catamaran this 1 hull beam to hull length ratio has less to do with stability, which is more influenced by hull spacing. Therefore the answer to your question is, in my opinion, little to do with each other.   Alik Senior Member Depends on speed, first of all. The close the catamaran to hump of resistance, the wider it should be. Then, depends on requirements for space, accommodations, etc. Generally yes, powercat can be (and usually is) narrower than a sailing cat.   Sorry, I should have offered more detail. This will be a very small boat, 12-15’, and no more than 9 hp. The hulls are displacement. It is for lake use for two, possibly three, people. Top speed perhaps 10 mph.   Carlazzomark said: ↑ Sorry, I should have offered more detail. This will be a very small boat, 12-15’, and no more than 9 hp. The hulls are displacement. It is for lake use for two, possibly three, people. Top speed perhaps 10 mph. Click to expand... Ad Hoc Naval Architect Carlazzomark said: ↑ Should the beam to length ratio of a power cat be the same as a sailing cat (about 50%), .... Click to expand... messabout Senior Member For the use that you have described, it is likely that a monohull will be a better choice. A 14 footer will handle the 9.9 nicely and can push the boat into the low twenties. The mono will have more people space and will be more comfortable than a small cat would be. Single engine cats can have problems with the convergent waves from the hulls. At certain speeds the wave conflict between the hulls will make the engine very unhappy....and sometimes have the boat become unpleasantly and even dangerously wet.   sailhand Senior Member Interesting comments here. I have been designing, building and testing small displacement cats since 1996. I have sold heaps of plans for these small boats and have many happy customers. I use one for my tender and you can see this dinghy on youtube doing more than ten knots with a 3 hp, search magic carpet 3.5 with a 3hp. It is 3.5 metres long and 1.75 wide, exactly 50%. It can comfortably deal with two people and groceries fuel etc. . Essentially the length and beam of your hulls tends to dictate your displacement. Put simply the general concept is bigger boat bigger load. If you want speed in a displacement cat you really should keep the waterline beam length ratio of each individual hull above 10 to 1. 10 ft hull 1 foot wide on the waterline. This is a really simple explanation and does not include hull shapes etc. If you want to carry a particular load in a small powercat its probably a good idea to calculate that weight and then calculate your hull size, ie load carrying capacity using the 10 to 1 minimum rule. You will easily achieve 3 average people along with minimal gear in a 14 footer in a lightweight displacement cat. They are however not the best hull shape to carry high loads at this length so will not tolerate overloading and still maintain your target speed. Goodluck, cheers   Scuff Senior Member sailhand said: ↑ Interesting comments here. I have been designing, building and testing small displacement cats since 1996. I have sold heaps of plans for these small boats and have many happy customers. I use one for my tender and you can see this dinghy on youtube doing more than ten knots with a 3 hp, search magic carpet 3.5 with a 3hp. It is 3.5 metres long and 1.75 wide, exactly 50%. It can comfortably deal with two people and groceries fuel etc. . Essentially the length and beam of your hulls tends to dictate your displacement. Put simply the general concept is bigger boat bigger load. If you want speed in a displacement cat you really should keep the waterline beam length ratio of each individual hull above 10 to 1. 10 ft hull 1 foot wide on the waterline. This is a really simple explanation and does not include hull shapes etc. If you want to carry a particular load in a small powercat its probably a good idea to calculate that weight and then calculate your hull size, ie load carrying capacity using the 10 to 1 minimum rule. You will easily achieve 3 average people along with minimal gear in a 14 footer in a lightweight displacement cat. They are however not the best hull shape to carry high loads at this length so will not tolerate overloading and still maintain your target speed. Goodluck, cheers Click to expand... Advertisement: No I am retired and not interested in maintaining a website. You can view my dinghy on youtube and there is an email address on there. I have a bit of an ongoing problem with email addresses at the moment which will hopefully be sorted soon. I see no reason a slightly larger version wouldnt suit your needs. We have 7.8m x 3.5 metre catamaran doing 19.8 knots, thats right so close to 20 yet so far, with 4 onboard and full fuel with a pair of 30s. Thats a lot of boat in a hurry for 30s. Hull shape in displacement boats is critical. In my opinion, and thats all it is an opinion, planing hulls are less critical in hull shape. I used to barefoot waterski when I was a kid and I can assure you my feet arent an elegant shape but they got me planing at around 37knots if I remember correctly, I was 210 pounds then. Get the hull shape wrong and youll be stuck with adhocs formula speed = square root waterline length x 1.3, and at 14 feet thats definitely closer to 4 knots than 14. Cheers   Beam, length, fishabilty, New England Maxsurf was not correct to calculate the beam water line of catamaran Beam increase Beam Winds Combined With Rolling in HSNC 2015 Ship stability in bow/beam seas ? Inclining an stability length less than 24 m ship Will a full length rail help? floodable length calculations floodable length & damage stability floodable length curve No, create an account now. Yes, my password is: Forgot your password? Forum Listing Marketplace Advanced Search About The Boat Sailboat Design and Construction SailNet is a forum community dedicated to Sailing enthusiasts. Come join the discussion about sailing, modifications, classifieds, troubleshooting, repairs, reviews, maintenance, and more! Catamaran Hull Speed Add to quote The hull speed formula for displacement monohulls is 1.34 x the square root of the waterline length. When planing, a monohull will exceed it's hull speed. Obviously this formula doesn't apply to catamarans. Even when not planing, they will travel much faster than what the hull speed formula predicts. Is there a formula or general guideline for catamaran hull speed related to waterline length and / or hull length:beam ratio?   Here, read this ...   Few Catamarans are actually planning vessels. Most are semi-displacement boats (like most modern IMS/IRC derived monohulls). Both forms of semi-displacement boats achieve speeds that are higher than normal hullspeed (i.e. 1.34 x the square root of the waterline length) by minimizing wave production. In the case of multihulls, this is done through a very narrow waterline beam to length ratio, and minimal interference between the waves produced by the other hull(s). Semi-displacement mono-hulls cheat a bit by using very fine entries to minimize the size of the bow wave that they produce. In both cases, clean hull forms and minimal drag is critical to overall performance, but properly designed, semi-displacement boats can achieve sustained speeds that can literally exceed twice their theoretical hull speed without planning. Of course as Alex points out, and as the article that he linked to explains, few cruising cats achieve passage speeds that exceed or even match those of modern performance mono-hull cruising boats. Respectfully, Jeff   Thanks guys.   Damn multis. They leave me standing.   Under body shapes: keels and centerboards, Beam & hull-beam ratios, Beam overall – overall wide beam versus standard beam, Displacement but I can`t find any mention of the optimal hight of the bridge deck cabins, Giuliettas post mentions under deck clearance and it`s extremely important but if you build the bridge on top off the bridge deck would that make it extremely unstable as the center of gravity would be to high making the cat useless in any swell.   sctpc said: Under body shapes: keels and centerboards, Beam & hull-beam ratios, Beam overall - overall wide beam versus standard beam, Displacement but I can`t find any mention of the optimal hight of the bridge deck cabins, Giuliettas post mentions under deck clearance and it`s extremely important but if you build the bridge on top off the bridge deck would that make it extremely unstable as the center of gravity would be to high making the cat useless in any swell. Click to expand... Yes but what would the optimal height formula be I have been reading for over 4 hours since I read this post and lots of designers mention it but is there a rule to it, 'deck should be 2 foot making cabin 9 foot for a 30 foot cat or deck 4 foot cabin roof 10 foot for a 50 foot cat. I find it interesting that all the other design stuff is quite well out explained but none on windage problems ect.   The less cabin, the better from a windage and weight standpoint. There are various rules of thumb for bridgedeck clearance. .06 x DWL is one. As far as hull waterline beam goes, some say 16 to 1 is as good as it gets from a resistance standpoint. A fast cruiser will be about 12 or 13 to 1, but the shorter the boat, the more beam there will be in the hulls in order to get a reasonable amount of room inside and carrying capacity. 8 to 1 hull beam to DWL length is common in small to medium size cruising cats. Overall beam is usually about one half of the overall length. Recent designs are often somewhat beamier overall-BOA 55% of LOA is not uncommon now. Tim Dunn - 65 Foot Sailing Catamaran Design by Tim Dunn   there isn't a basic rule for bridge deck clearance and cabin height. Some designers avoid the problem entirely by not using the bridgedeck form living accommodations. Wharram's design often don't have any living space on the bridge deck.   Check out Performance Cruising Inc. - The Official Gemini 105MC and Telstar 28 Web - I find the boat to be a best of both worlds design in a smallish cruising catamaran. I am of course partial and biased. The hulls are roughly 8:1 length: beam. I've had mine at 10.3 knts SOG in 16 knts on a close reach (50 degrees off the wind) going up current in 1.25 knts of current. Other owners have reported (and photographed) top speeds of 18 knts, and routinely get 10+. From the side the gemini looks like a moderately high windage mono. That's hard to do with a queen sized bed in the master cabin and twin staterooms aft with double beds. It has 39 inches of clearance at the front it takes chesapeake chop well on the nose, but with only 18 inches at the stern we do get the occasional 'thumb' on the floor of the cockpit. Reportedly they do well in typical ocean swell and have crossed the Atlantic and Pacific Ocean quite well. Several reside in Hawaii, having sailed there on their own bottoms, a couple are currently circumnavigating ( theslapdash.com . By making the salon area standing height for only the walk in area and sitting height at the salon table Tony Smith (the designer) was able to give it a nice profile. Otherwise, headroom is 6'2" throughout. I routinely dock my boat in a cross wind of 5 or so knots and I can tell you it parks and tracks as surely as a car with very little problem caused by windage. Mine:   Attachments Fixed it, thanks.   Bridge deck, pounding, and windage So, 3 feet between the bottom of the bridge deck & the deep blue sea. In some cases long and thin is good but can it be taken to extremes? For example a 50' cat with a waterline beam of 2.5' mathmatically will produce a vapor trail at 35+ kts. In reality though, will it? Since I hate doing time in the hulls of any boat I am going to keep the boat light in terms of gear and have a tapered bridge deck encompassing all accomodations with a profile to the wind of about 3'. Add a nice sail plan and I'm having fun! Or at least that is the current plan. So, any thoughts on the long skinny hull thing?   One other point is the overall beam of the boat, especially in the case of a catamaran matters as well. If the beam is relatively narrow for the boat size, the hull wake from the two hulls can interfere with performance.   ?             176.2K members Top Contributors this Month 104 COMMENTS Catamaran Beam to Length Ratios Explained: For Beginners Most modern catamarans have a beam to length ratio of >50%. You can easily calculate this on your own by following the steps below. But first, let's check out some more terminology to make sure we really understand this ratio. ... Hull Fineness Ratio (HFR) is another name for Hull length-to-beam ratio. This is basically the same as the ratio ... Catamaran Design Formulas I found his paper easy to follow and all the Catamaran hull design equations were in one place. Terho was kind enough to grant permission to reproduce his work here. ... While the length/beam ratio of catamaran, L BRC is between 2.2 and 3.2, a catamaran can be certified to A category if SF > 40 000 and to B category if SF > 15 000. Length to Beam ratios for Multihulls A motor catamaran can have less beam, with a clean flow between the hulls now taking prominence over high beam for sailing stability, so L/B ratios of 2.5 to 3 are now more appropriate. Hulls may need to be asymmetrical with a straighter side on the inside to avoid unfavorable hull wave interaction between them. HOW TO DIMENSION A SAILING CATAMARAN? The beam between hull centers is named B CB (Figure 2). Length/beam ratio of the catamaran, LBRC , is defined as follows: LBRC LH BCB:= . If we set LBRC:= 2.2 , the longitudinal and transversal stability will come very near to the same value. You can design a sailing catamaran wider or narrower, if you like. Catamaran Hull Design Weight and length can be combined into the Slenderness Ratio (SLR). But since most multihulls have similar Depth/WL beam ratios you can pretty much say the SLR equates to the LWL/BWL ratio. Typically this will be 8-10:1 for a slow cruising catamaran (or the main hull of most trimarans), 12-14:1 for a performance cruiser and 20:1 for an extreme ... Length-beam ratio Definition L/B = length divided by beam. Units: Dimensionless. Usually, the waterline dimensions LWL and BWL are used for monohulls, or for a single hull of a multihull. What it's used for Performance Larger L/B indicates a slimmer hull. This usually implies less wave-making resistance, and thus more efficient high-speed performance, but also suggests reduced load-carrying ability for a given ... Sailing Catamarans The Slenderness Ratio (SLR) or Displacement Length Ratio (DLR) is a measure of the fineness of a hull and is the technically correct coefficient that naval architects use. However, it is easier to visualise the hull waterline length/hull WL Beam ratio (LWL/BWL), so that is more commonly used. PDF HOW TO DIMENSION A SAILING CATAMARAN? The engine power needed for the catamaran is typically 4 kW/tonne and the motoring speed is near the hull speed, so: Powering While the length/beam ratio of catamaran, LBRC, is between 2.2 and 3.2, a catamaran can be certified to A category if SF > 40 000 and to B category if SF > 15 000. SF 82 10 3 SF := 1.75 ⋅mMOC ⋅ LH⋅BCB = × Catamaran Design Guide The Hull Fineness Ratio, known as the hull's beam-to-length ratio, is an interesting number. ... 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. lena Boats Should Be Sleek—But Only Up to a Point The length-to-beam ratio has risen over the centuries, but there are still practical limits ... Each demi-hull of a catamaran has an LBR of about 10 to 12, and in a trimaran, whose center hull has ... Length-beam ratio Length-beam ratio. Definition. L/B = length divided by beam. ... are used for monohulls, or for a single hull of a multihull. What it's used for Performance. Larger L/B indicates a slimmer hull. This usually implies less wave-making resistance, and thus more efficient high-speed performance, but also suggests reduced load-carrying ability for a ... Building Cruising Catamaran: What is the ideal beam width on each hull Based on Terho Halme's suggestions, the "length/beam ratio" of each hull should be between 9 and 12 for a comfortable cruiser. 8 would increase wave making and should be avoided. But if I aim for a ratio of 9, that means the beam width ends up only 3ft 3in wide (way too tight for accommodation, not to mention for retractable daggerboard). Catamaran Stability Even so the modern ballast keel yacht is still a relatively broad-beamed vessel, i.e. with a waterline length about 3 times longer than its beam - in technical terms, a length/beam ratio of 3:1. Beamy hulls of 3:1 have to push a lot of water around them when sailing. This produces the well-known drag waves. Question about beam to length ratio on a catamaran So the ratio you want is LWL/BWL, but better still would be the slenderness ratio, or displacement -length ratio. I am one of the few designers to have experimented with varying the hull spacing on my own boats. I found no disadvantage when sailing with the wider beam, in fact I prefered it. That was 22ft WL and a 17ft overall beam, so it was ... HOW TO DIMENSION A SAILING CATAMARAN? The beam between hull centers is named BCB (Figure 2). Length/beam ratio of the catamaran, LBRC , is defined as follows: length of hull LH divided by beam between hull centers B CB. If we set LBRC:= 2.2 , the longitudinal and transversal stability will come very near to the same value. You can design a sailing catamaran wider or narrower, if ... Comparing Trimarans & Catamarans Because as beam increases, a pitchpole off the wind becomes more likely, both under sail and under bare poles. (The optimum length-to-beam ratios is 1.7:1 - 2.2:1 for cats and 1.2:1-1.8:1 for trimarans.) Again, hull shape and buoyancy also play critical roles in averting a pitchpole, so beam alone shouldn't be regarded as a determining factor. Principles of Yacht Design Sen Fig 5.34 Length/hull draft ratio. Length/displacement ratio (Lwl/V'a) quantity for the resistance of the yacht at high speeds. To enable the yacht to exceed a Froude number of about 0.45, ratios above about 5.7 are required. In Fig 5.35 the length/displacement ratio is plotted versus waterline length. Since beam and draft do not increase ... Best L/B ratio for power catamaran hull efficiency The optimal L/B seems to be around 20 for each hull. Achieving 20 knots with a 40' displacement CAT is fully realistic. Google Leo Lazauskas + Multihulls and you should get a link. Leo's case study was for 300 kg and 6 m length, which would scale to 2.4 tons at 12 m. I don't know what displacement you assume. Beam vs length on catamaran Once you remove those variations, I think you will find that the hull centerline width to waterline length ratio is typically closer to 1:2.5 for almost all boats My Simpson 36 is certainly one of the "older designs" with a beam of 5.3m and LOA of 11m. Ratio of 1:2. Waterline length to centreline width is about 2:6 Another consideration is ... Catamaran Hull Speed Calculator For Beginners (Table and Free Max hull speed= √((Length on Water Line x g) /(2 x pi)) x 3600/1852. Now we need to add the increased efficiency (loss of drag) of a semi-displacement hull, usually, this is somewhere between a 10-30% increase. Semi Displacement hull speed = Maximum hull speed * 1.3. Note: "1.3" is the increase in efficiency, if you believe you are on the ... Sail Calculator Sail Area / Displacement ratio - The SA/D ratio is like the power/weight ratio of an automobile. A high SA/D ratio (> about 18) indicates a powerful rig, while a low ratio indicates a more docile boat. Length / Beam ratio - A long, narrow hull with limited interior space is easier to drive than a short, fat one with plentiful capacity ... Beam to length ratio for power catamaran The beam to length ratio has to do, above all, with the stability of the boat. In a catamaran this 1 hull beam to hull length ratio has less to do with stability, which is more influenced by hull spacing. ... If you want speed in a displacement cat you really should keep the waterline beam length ratio of each individual hull above 10 to 1. 10 ... Catamaran Hull Speed 87689 posts · Joined 1999. #1 · Dec 28, 2007. The hull speed formula for displacement monohulls is 1.34 x the square root of the waterline length. When planing, a monohull will exceed it's hull speed. Obviously this formula doesn't apply to catamarans. Even when not planing, they will travel much faster than what the hull speed formula predicts. catamaran gulet motorboat powerboat riverboat sailboat trimaran yacht