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Can a good Trimaran main hull work as well for a Catamaran ?


I like the easy-to-build, flat, low-rocker hulls of your W17 / W19 trimarans. Would this also be an efficient hull shape for a larger cruising catamaran ?    Please explain/qualify your reply.       Giuseppe-V, Rome, ITALY


The short answer is ‘no’ … so this article will explain the ’why not’, as the reasoning can apply to other situations.     In my humble opinion, catamaran hulls justify to be quite different from a trimaran main hull, so let’s first consider why.

Typically, a trimaran hull is the principal displacement supporter as well as the main accommodation area.    This means that its L/b (Length to waterline beam) ratio will typically be up in the 7 to 10 range and with a cruising tri, often needs to spread out on both sides above the waterline to find adequate living space.   Any ratio below 9 will compromise wave-making resistance, but is the trade off with space.     In the water, it heels equally to each side over its life.    This generally applies to tris of all sizes.

(In the case of catamarans, beach-cats and cruising cats cannot be considered the same, as beach cats are light, sport boats that are often sailed on one hull, with the resulting risk of capsize not generally considered a major problem as such crews typically are (or should be) prepared for this).

The inevitably heavier cruising cat is typically designed to sail with both hulls in the water, so they can share the weight to carry and required buoyancy.  This typically results in a hull with a slightly higher L/b ratio that benefits low wavemaking.

Let’s now look at this graph (first included in an article on Simple Shapes published in PBB #169 Oct 2017) and compare the W17 main hull with the needs of a catamaran hull that will also be longer.

The Red dot indicates the approximate situation for the W17 travelling at 8kts.  Notice that the skin friction will only be about 30% of the total at that relative (and common) speed and that wave-making will be the dominant resistance.    The near vertically-sided box/scow shape of the W17 centerhull handles this very well and the extra wetted surface it carries will have less effect and as also noted in the above PBB article, can even be offset when sailing upwind with asymmetrical amas of special design that improve ground-made-good to windward by significantly lowering leeway slip.

With a longer catamaran at 8kts, not only is the L/b ratio now closer to 15 but the relative Speed/Length ratio will now drop to say 1.3 for a 36ft catamaran hull, so placing its operating situation near the Green spot on the chart as far as its hydrodynamics are concerned. At this point, the resistance due to wetted surface has now gone up to 50% of the total and the wave-making dropped from around 60% to about 45%.  Even if the graph cannot be 100% accurate for all shapes and forms, there’s sufficient change indicated that the hull design clearly needs tweaking to better suit the different situation.   

So to better handle the relative rise in wetted-surface resistance, yet not give up too much on the wave-making, a form with near parallel sides at the waterline but with a semi-circular bottom will likely give the best form for such a catamaran hull.      This is reflected in many catamaran hulls today, but was also very present in the long, slim and fast hulls of Don Karmin back in the 1960’s, when he used a semi-circular pre-cast fiberglass ‘shoe’ below sides of plywood … yet another way to build a fast multihull that is cost effective. The pre-laminated flat sheet KSS hulls of Derek Kelsall also achieve the same effect.

See these two links for more info on these two hull shapes:


 Footnote:  The above graph was created by the author back in 2015, based on a combination of standard series ship model resistance tests plus model test data for catamaran hulls of varying L/b ratios.  Such model tests, especially being in the 12-20ft range, suit this application well.


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