Trimaran amas using extruded polystyrene with epoxy

Gonzo,
I have considered building a 10 ft tryst trimaran, but first, wanted to research using a parallel rig first with my existing 8 ft dingy. So I want to find amas that will support the stress of wind surfer rigs.
Thomas in Texas

 

I strongly recommend you read Dierking's book on Outrigger Canoes. @garydierking

I built outriggers for a canoe and I found it best to have multiple attachment points for the pylons, or at least two. The amas are not going to sink unless you walk on them, or have incredible wind.

@rwatson ought not be overlooked, if you perchance to load onto the ama; then it must have sufficient reserve to tolerate, but it should only be designed to just dance along otherwise, and, of course, aka as well

so design if for more, but do it as a reserve, this way the thing is not dragging in the water for little need; I can't tell you how with precision, but the aka also needs to be enough for the loading, and the reserve is important

 

Your logic is correct

BUT

IF you could guarantee that no-one will EVER move their bodies to the "outer limits", you could possibly reduce the target displacement.

From experience, any trimaran will be expected to support at least ONE crew member at some time. It could be while pulling up at a wharf or pontoon, or someone losing their balance and falling to one side or the other.
When launching from a beach, getting in over the rudder is way more unlikely than getting on at the side.

Also, in your case, if you have the sails on the Amas, you run the risk of broaching on such a short hull. A sudden gust of wind will push the lee Ama to the front, and down into the water. You will need reserve bouyancy to prevent a capsize.

On my project, I built the two Amas with little reserve buoyancy at first, but soon found out how awkward it made operations.

In any event, its up to you to make the judgement. I can only make suggestions based on my own experience.

 

Rwatson,
I got a kick out of your computer generated diagram, of me trying to measure the force necessary to submerge the end of a pole in the water. My wife laughed at the "bag of bricks" description and let me know that she was NOT going to sit in a boat while I tried to test the stability with a pole! I will put the results go good use in my ama displacement calculations...
Thomas in Texas

 

Fallguy,
I am looking at Dierkings book and hope to be able to read it soon. You gave great personal input based on your own outrigger construction, especially concerning adequate aka attachment points. Having enough reserve buoyancy is important and I am raising that number to ensure I can stand on the ama while docking.
Thomas in Texas

 

I'm trying to move to Texas.

 

The attachment points are indeed important. Here is just a run down of my own successful arrangements to start off your design process.

Based on my experience with the larger and heavier trimaran, if you can embed an aluminium tube, the depth of the Ama, and have 100mm stick out, with an inner diameter between 25 and 30 mm, and a wall thickness larger than 3.5 mm,
you can insert a longer aluminium tube like as sleeve, and pin it in place. This will be about 4 mm wall thickness.

This "sticking out tube" was very rigid, and was used to hold other key components.



This is a blown up shot of my Ama connection. Unfortunately, you cant see it joining the Ama here. but this full boat photo sort of shows its usage.



And here is a picture of the joining "unit".



You can see its just an aluminium tube, pushed through a hole in a 37mm x 37 mm square tube, of ~ 4mm wall thickness. You can see a bolts joining the two, but I got a local welder to do a couple of welds for extra rigidity and stiffness.

The square section fitted tightly into the end of an aluminium mast section, with a little bit of sanding of the sharp square edges, for 300 mm . You can just about see the arrangement in the full boat photo.

Over two dozen trips, in some fairly rigorous weather, it never gave any hint of a problem, and you can see I used an outboard with it as well. It was also really easy to disassemble for transport, as the "cross piece " just slid out of the tube in the Ama.

I'm not saying this is what you should use, but just give you and idea of materials and dimensions that have been shown to perform.
If you go down the the local metal shop, and get a few Aluminium samples of different sizes and sections, I am sure you will get a "feel" for their relative strengths.

But, I am recommending aluminium, as opposed to wood or f'glass because of its reliable, rot, rust free performance, and uniform strength. It should be easy to find a local welder to strengthen any tricky joining pieces, once you have glued or screwed them into their relative positions.

Have fun.

 

Rwatson,
You have given me a great deal of information on the mounting of the aka and the ama. I have some aluminum sections and will look at the options of using some of my existing stock on hand...
Thomas in Texas.