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(Publisher’s Note: The following discussion by Frank Smoot, from www.diy-tris.com, is really a continuation from the previous post about the new Uffa 10 trimaran. As you can see, that thread generated quite a bit of discussion in the comments area. Frank offered to add some photos to the continued discussion, so I’ve created a separate post here for him to do just that. To understand fully what Frank is writing about here, you should read the previous post, especially the comments contributed by Ian M and Stefano).

Smaller Trimaran” Discussion Continued…
by Frank Smoot

Great idea about teaching school kids to build boats, Stefano, especially here in boat-crazy Florida. Maybe I can help de-program them from a life of monohull monotony…

As usual, Ian, your post is insightful and informative. I couldn’t agree more about the wasted energy in those bouncy little dinghys. That’s the complete opposite of what I’m looking for in my sailing experience.

And re planing hulls and amas, I kind of hate to open up this can of worms, but here goes:

“What I’ve Learned (So Far) About Planing Hulls And Amas”

Since I don’t know anybody else who has tried half a dozen different ama shapes and styles on similar boats (my various 16′ tris) and in similar circumstances (good old Terra Ceia Bay and it’s ultra-fickle sailing conditions), I feel (borderline) qualified to share some of my observations conclusions with the elite sailors who read this fine column:

– Pros And Cons of Planing Main Hulls:

My 2-year-oild boat, “No Commotion,” has ended up being the default baseline for all my comparisons, mostly because it turned out to be so much better of a sailing experience than all my other efforts. It is fast enough (13 mph so far on a single 102 sf sail on an unstayed mast), quiet, smooth, and dry.

Trimaran Named - "No Commotion" with 102 Feet of Sail

I wanted to see if I could go faster, so I built a main hull that was specifically designed to plane. I probably overdid it, as the aft half of the hull tapered from a shallow V to nearly-flat (like the planing dinghys). While this hull was reasonably quick, it took a LOT of power to get it there. It was also noisy, wet, bumpy, and sluggish.

I actually recontoured the entire first ½ of this hull to make a smoother, finer entry,, but all that really accomplished was to move the noise, turbulence, and splashing farther aft. It was still too wet, too noisy, etc. By contrast, both No Commotion and my wife Laura’s tri were much quicker and quieter under virtually all wind and wave conditions.

My conclusion is that it just takes too much power to get a planing hull this size up on a plane. You need a very windy day, a big sail, and fairly flat water — a difficult combination to come by.

Of course, on the plus side, the shallow hull floated on almost zero water and turned on a dime. But like Laura’s flat-bottomed boat, it didn’t track worth squat or go where you pointed it if you couldn’t get at least a foot of the leeboard down into the water.

So on balance, I think a moderate V-hull of about 30-35 degrees dihedral makes for the best overall boat. Also, I’m not 100% convinced that a transom is really necessary on these boats, so I’m now building deeper V (45 degrees) double-ended, dory-type hull to see what that’s like.

– Pros And Cons Of Planing Amas:

Fortunately, my planing amas have proven a much more enjoyable experience than my planing main hull. I built them because my original amas (tortured ply) went underwater too readily (or so I thought) when I was trying to go really fast.

Even though those old amas had about 400 lbs of flotation each, it wasn’t hard to bury them when I was using the “big rig” (124 sq. ft. from a Laser II). And when they went deep enough for the akas to hit the water, it was like slamming on the brakes in your car — a jarring shock to me, and to the equipment as well.

The planing amas definitely eliminated that problem. Not only do they each have 800 lbs of buoyancy, they also have “powerboat” planing entries and V-bottoms that provide more lift the faster you go. So in that regard, they work very well indeed.

The downside is that the high-flotation planing amas make for a rougher and bumpier sailing experience precisely because they don’t want to go under. And when you want to go parallel to the troughs — where hearty beam breezes (and maximum speeds) are found, the boat pitches rather violently as the waves roll by. Ditto for when you encounter a powerboat wake.

The old tortured ply amas, on the other hand, have a very deep V-bottom and a very gentle entry, to they never cause a hard ride. They seem to split the water smoothly, like a knife, so they are also quieter. But perhaps most importantly, the tortured ply amas (with their nearly vertical sides) add significantly to the rig’s overall lateral resistance.

This was actually Laura’s discovery. She noticed how much better her boat pointed and tracked upwind when using the tortured ply amas I originally made for her boat vs. the round-bottom foam & fiberglas planing amas I later made for her boat. In fact, she now refuses to sail with the planing amas, saying they just compromise performance too much. And with the flattish bottom of her boat, the contribution to lateral resistance made by the tortured ply amas becomes all the more important.

If I want to ever take my own boat into bigger water with bigger waves, the high-flotation planing amas will be the first thing to go. Not only would the ride be too rough, but the stresses on the boat would be much greater.

However, for the kind of sailing I actually do, the non-planing hull of No Commotion plus the planing amas (currently attached to my new sliding aka system) makes for an ideal combination. Laura’s rig is almost the opposite — a mostly flat-bottom main hull and deep-V tortured ply amas — but that’s what works best for her and her boat.

Bottom line on all of this: Planing hulls and amas do have some advantages, but also some drawbacks, and anyone designing, building, or buying a trimaran needs to be aware of them.

Tris are not at all like monohulls, and not very much like catamarans. They are unique sailing vessels and play by a different set of rules. So prospective tri pilots need to know, first and foremost, where they plan to do most of their sailing, and what they want their sailing experience to be like.

How can you find out what combination works best for you? If possible, build two very different main hulls and two very different sets of amas, and go sailing. And let me know what you discover!

Cheers – Frank
www.diy-tris.com