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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.

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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       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)

*    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.

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Hmmm, that’s quite a list of interacting factors!    

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 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.  

For several years, a local skier/sailor owned a Stiletto 27 cat with a beam of 14ft, giving an L/B of 1.93, or close to the theoretical sweet spot..  Coming from a Hobie-cat background, he’d often fly a hull as the boat was quite light at 1100 lbs with cigar berths in each hull and an open tramp between.  But after a strong gale rolled in and wrecked it on shore, he saw a new Gougeon 32 announced and without much thought, bought one.   The central cuddy looked cosy and the slim 32ft hulls sounded fast.   But this boat was designed to be trailerable without dismantling, so she only has the beam of a Hobie cat …. 8’ 4” … so her L/B ratio is an outlandish 3.84 !! .. nearly half the beam of what ‘the sweet spot’ would indicate.    Well, by day 3, its new owner had already capsized it and without much knowledge of how to right it, suffered a chilly 4 hour ordeal.     (He found out the hard way that ‘tacking with water-ballast on one side’ is full of perils and is really not for lake sailing as you cannot switch the water in time).  

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.

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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 xn to your basic calculator by first hitting the 3 dots [ ... ] that brings you to the Scientific Options, and then clicking on [ xn ] 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 xn 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 100ft, 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.

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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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