Planing DIY Trimaran Amas
In this post, Frank Smoot shares some great information about how he came up with new amas for his DIY Trimaran. What I love about Frank’s work is that it’s based on “real-world, real-experience” small tri sailing.
After reading the below, you can see all of Frank’s ama construction photos beginning on this page at his website –> www.diy-tris.com/PA-1.htm
( Thanks again for sharing with us how you came up with the new amas Frank :-)
Amazing DIY Trimaran Planing Amas
by Frank Smoot
My original amas were built more or less according to what I saw as “common practice” in the examples I found online. But I found that they submerged too easily when really pressed with a lot of sail power. Not only did this submerging cause the mast to lean sideways more than necessary (thus spilling wind needlessly), but this also brought the akas (crossrubes) in contact with the water — which creates exactly the same effect as putting on the brakes in your car.
So I set out to design some amas that would not only not submerge when pressed hard, but would actually rise higher as they went faster — like a powerboat hull. My goal was to get 15 mph out of a boat that cost me less than $1500 to build, and the amas were clearly a speed-limiting factor.
Like most everything I do, it was a matter of trial and error. I sail my little tris several times a week, so I get a ton of real-world experience. Theory is great, but practice is the real source of understanding.
The very first amas I made (tortured plywood) only had about 200 lbs of flotation each. They were slippery and quiet, but submerged WAY too easily. My next set of amas was the same style, but bigger, with about 400 lbs of bouyancy each.
Unfortunately, I could also make them submerge when pushed really hard. But my new planing amas, which have about 800 lbs of flotation each, have solved that problem once and for all. I can’t make them submerge no matter how hard I try, and they actually rise farther out of the water the faster they go.
BTW, that 800 lbs of flotation on each side came in very handy when my (very under-designed) mast step blew out right through the side of my hull, back around Memorial Day. It made a hole about a foot square, which very quickly sunk my main hull. But those 1600 lbs of ama flotation saved the day!
I’d say it took me about 1-2 hours a day for a about a month. So maybe 45 hours all together? Of course, there were many more months of mental design work before I ever picked up a saw.
It took me that long to come up with a design that would (a) plane, (b) not slap when making the kind of intermittent contact with the surface that the windward ama always makes, and (c) provide maximum bouyancy at minimum immersion — my real #1 goal.
I will confess to buying ultra-cheap 1/8″ (3 mm) “doorskin” lauan ply (for bottoms and sides — the top is 6 mm ply), and using ultra-cheap porch & floor enamel (which works just great with just a single coat).
Since everything I do is experimental, and none of it will probably be around next year, why spend more? But I do fiberglass the sides and bottom (4 oz) because they take a lot of abuse. And I really, really hate it when things sink…
They have exceeded my expectations. I am just delighted with them. Since I first used the planing amas, I have’t been remotely tempted to use any of the old ones.
I can easily sustain 13 mph in any decent wind, even with a single 102 sf sail and freestanding (unstayed) mast. The only thing I’m not thrilled with is the weight. They came in at just over 30 lbs each. I’m sure I could knock off at least 5 lbs by going to better ply (okoume), which is also 30% lighter, and by not making these amas quite so strong (I need to be able to attach shrouds to them when I want to use my stayed masts).
As the size and power of our sails has increased this year, so has the speed and the “push” on the amas. My wife Laura (who has her own, lighter tri) has been using those original tortured-ply amas I mentioned.
In the beginning, she was a bit timid with her tri. But now that she’s not afraid to really push her boat, she finds herself burying those little amas all the time. So after seeing me pull away from her boat one too many times, she decided that she wanted some planing amas, too. So I made her an experimental set using foam, copying very closely the shape of my own planing amas.
Her planing amas are only slightly smaller than mine (because her boat has a bit less freeboard), and they weigh just 19 lbs! They will be field-tested next week, and I’m 100% confident she’ll love them. But now that she has her own high-tech amas, how will I keep her from passing me?