Tunnel Hulls

I am looking at building a new trimaran and am thinking of using a tunnel hull for the main hull. Does anybody have any queries or facts or whatever that may turn this idea from a possibility into a fallousy.



What do you mean by a “tunnel hull”
Sounds interesting, but I have no idea what that could be. Do you mean a tunnel between two keel lines, like on offshore powerboats?

If that is what you mean, I think, the main argument against it is the wetted surface. It will take a lot of speed to get to hull out of the water far enough to overcome this drawback.
I would think, that you might be able to fly the main hull a little earlier than on other Tris, but you would be a lot slower than the others before you get the hull out.


Tunnel hull (fullsize Laser Vortex) :-

They only really work upwind and on a reach in above a force 3, the concept is sound but the thing is just too heavy and underpowered for it to really fly, and downwind you cant get an airflow under the hull so it kills it anyway.

As to a model tunnel hull I thought a cat would work if you could make it light enough and somehow stop it heeling too much (wind getting under the thing is going to flip it real quick).
I dont see the point of building a tunnel main hull into a tri (or should that be quad?), firstly as soon as you start flying the main hull its pointless and just heavy, secondly its going to make the boat harder to tack as you are tacking a catamaran main hull.

Luff 'em & leave 'em.


The full size class I used to race in (the M-20 scow) has a tunneled hull. The main advantage to it is that as you heel it up, you can dramatically lower the wetted surface area. It is not as radical of a tunnel as the Vortex. Here is a picture:

Download Attachment: M20_616_s.jpg

Now since the main advantage is to reduce wetted surface are when heeled, I think this is probably not a worthwhile idea for your trimaran. Because of the amas, the center hull will never really heel before it leaves the water altogether.

So I agree with Matthew that this is not a good idea for your tri.

  • Will

Will Gorgen

In response to your post, I build these boat’s so that the platform isn’t flat. By that I mean that there is around 11 degrees angle in the beams. When the boat is sailing the windward hull in light airs is about 4-6 inches clear of the water. By your comment will this make much of a difference in your opinion?

Hey Peter,

When you say 11 degrees angle is that total from beam to beam or is that one beam relative to the main hull?

In order for a tunnel hull to work, you would want one side of the hull to lift completely clear (sort of like what is shown in the picture I posted) when the leeward ama was in the water. But what do you gain by doing this? Your righting moment has not been increased since you have not lengthened the moment arm between the ama and the CG. You have reduced the wetted surface, but the tunnel hull will have more wetted surface than a traditional hull, so in reality you are probably back to the same wetted surface as a traditional hull. All you have really done is effectively shifted the Center of boyancy of the main hull a little closer to the ama. I’m not sure what the point of that might be…

In addition, the tunnel hull will be wider which will hurt its ablity to slice through waves. Scows are only effective on small inland lakes where the waves are quite small. Scows do not perform well when the waves start coming over the bow. The tend to slap the waves rather than slicing them and this is slow. In addition, the waves tend to come over the low wide bow rather than getting pushed to the side. This causes a lot of water to get into the boat. So anything bigger than 2’ chop and you are risking swamping your boat!

Getting back to your tri: the tunnel hull will be wider than a traditional hull. This might allow you to more easily package electronics. So perhaps this is an advantage. But I really can’t think of any other good reason to do it…

You have obviously given this some thought and have a lot of experience with traditional hulled trimarans. Why do you want to try this? Have you thought of some advantages?

  • Will

Will Gorgen

By the shape that I am looking at the hull is going to be about as wide as the hulls that several over here are now using. The reason behind the tunnel idea is that provided I could get the tunnel to be clear of the water while sailing I’m imagining that the wetted surface will be less as the boat should sail on one side of the tunnel cheifly. I realise the tunnel will touch the water on both sides but the hope is that the wetted surface will still be less than a “normal” hull. The other thought is that with the tunnel that the main hull will lift earlier than they do now, and we have found that tri’s that sail on the leeward hull can sail upto ten degrees higher than tri’s with the main hull in the water without losing speed. Even if only on one hull for 10 seconds or so being able to sail that much higher has so many advantages around a course. Currently there are boat’s here that quite easily one one hull for extended periods to windward, and to be able to match them on less wetted surface must be an advantage.

To answer your question concerning the angle of the beams. Each beam/float is around 11 degrees up, this means that it’s impossible to have three hull’s in the water at the same time, thus there is less wetted surface and it is much quicker.

To have this tunnel hull idea with the windward float on that much of an angle would seem to me that the tunnel would work properly. (in thoery anyway)