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Do Trimarans Plane?

Question: I was wondering, can or do trimarans actually plane?

Answer: A lot of sailors ask this question. Some think they know the answer and some indeed may! Let's FIRST try to define what we mean by planing.

A full plane is when the vertical load is fully supported by a flat (or near flat) plane traveling over the water at a certain attack angle, so providing dynamic lift equal to that load.

If there is enough constant horizontal driving force, this can certainly be achieved and examples of this are easily found from well-designed motorboats and hydroplanes, that have ample power to lift all their weight on to a near flat bottom that finds the necessary attack angle to supply the lift. Power is not really an issue here and neither is the shape of the planing surface.

I mention the latter, because tests have shown that the ideal planing surface is with an L/B of around 2 in flat water or 3 in rough water. The longer and slimmer the surface, the less efficient the lift becomes, the more dragging surface stays in the water and the more resistance is created. By comparison, the main hull of a trimaran might theoretically have a planing L/B of say 6 or much higher and an ama would likely be at least double that.

We are nearly all aware that the leading edge of a foil needs to be long for most efficient lift but in the case of a multihull planing, that leading edge amounts to the narrow beam of a hull or ama. NOT very efficient to be sure.

We can even look as some figures:

As Lift = C×A×V², we can calculate the potential lift from such surfaces. The wider main hull 'might' under ideal design and circumstances come up with a lift coefficient (C) of 0.09, whereas the slimmer ama would be lucky to reach 0.05.

For a hypothetical tri of say 22', these factors might therefore give 400 lbs of lift at 20 kt for an ama and potentially up to 1000 lbs of lift at 20 kt for a main hull, assuming the planing surfaces could be flat and present themselves at the ideal attack angle of 2–4 degrees to the horizontal water surface.

In practice, 'only' getting 400 lbs from an ama (could be much less) would likely mean that the boat would heel more and press the ama down into a position of much higher resistance—unless sailing on a very broad reach. The fact that both the hull and ama shape would need to be compromised to cut wave resistance upwind, would further lower their dynamic lift capacity so even the above figures would not be achieved.

Now a trimaran COULD be designed to plane IF it had three short but wide, flat hulls AND only sail on a reach or downwind. So in that respect, it could be possible. But for any practical design of a tri designed to sail efficiently UP wind, then planing is hardly an option.

Let's also consider the buttock flow and angle. What is this, some may ask? Well, let's imagine that either the ama or main hull is cut longitudinally about ½ its beam out from the centerline. The line of that cut will represent the longitudinal flow along the bottom at that position—something designers call a ¼-buttock line.

In order for either hull to get dynamic lift from the bottom surface (i.e. to plane), that line MUST present an attack angle to the water surface, i.e. be 1 to 4 degrees higher at the forward end, compared to the aft or transom end. Contrary to motorboats whose stationary transom is well below the surface, sailboats have this line designed in exactly the opposite direction! To better be suited for the much larger range of sailboat speeds, it comes UP towards the rear, to release the water flow without a great change of pressure. Sure, some dynamic lift might come from a flat main hull if the trim angle is enough—but at what price to the overall design? Perhaps some designers are already compromising their shapes for something that is not possible.

So to summarize, a truly planing sailboat would not only need to have flat, wide surfaces, but it would also have to be sailed with an attack angle for the aft planing part, involving a MAJOR change of trim.
At sufficient speed, the flow might come clean from the depressed main transom but if this slowed, then a jump in resistance would occur. Being wind propelled, this could happen often, unlike a planing motorboat with continuous steady power. For the ama, it's much worse.

Pressing an ama with a significant transom (needed for planing) into full immersion—something that happens often—would increase its resistance VERY significantly.

For this reason, I believe that any ama that will be pressed underwater should have either a very small transom or not have one at all. I know this is contrary to some thinking but theory does not support a transom for an ama that is often pushed under water.

I leave you with this thought. After all, how many fish have you seen lately with transoms?

One design of boat that looks like it CAN plane, are the longer boats (ORMA-60) with very low keel rocker and with a foil to lift up the bow so that the flat stern half has the ideal 2–4 degree attack angle to the water surface. 'Banque Populaire' is a classic example and this video supports the concept. As explained earlier, such planing would be virtually impossible on a much shorter boat, due to its greater rocker and the correspondingly much greater trim angle required.

Even with very long 60' hulls, not only are forward-placed foils required to obtain a satisfactory planing attack angle, but for the most efficient and complete planing, the hull form would need to be significantly compromised for good low speed efficiency—unless of course, the boat were built only for high speed operation or some record-breaking attempt. In such a case, the stern half would likely be totally flat with hard chines and most likely also need some arrangement of vented bottom steps to add lift while reducing water surface for lower friction.

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