Outrigger canoe Ama design considerations

[pmudesign]

First modification started, adding an inspection port.

 

[pmudesign]

Updated ama design. LOA - 8'0", beam 6".

 

[Skip Johnson]

I'll cheerfully admit that I don't have a clue as to what's "best" for your outrigger design but it's an area that I've cogitated on for some time.
First what do you want the outrigger to do?

Just float down the river and not turn over.
Charge thru the surf with a touch of grace.
Beat all your competitors in a sprint race? In a marathon?

Most of my cogitation has been oriented towards the latter and best that I've come up with is to guess at desired displacement and the crafts speed and design for minimum drag at that speed.

An interesting outlier was a design for a fishing kayak with twin outriggers. In that case outrigger floats were designed to both be in the water at the same time and have a fairly narrow and sharp cross section so as to never splash and make noise.

 

[BlueBell]

I'd consider using wood for the beams but the only species available to me are fir, poplar and oak.

Nothing wrong with fir.
I've used 8' fir 2 x 2's for similar purposes. (3-man sit-on-top with kayak style paddling)

 

[pmudesign]

Nothing wrong with fir.
I've used 8' fir 2 x 2's for similar purposes. (3-man sit-on-top with kayak style paddling)

I'd love to see a photo of that boat!

 

[pmudesign]

Quick question: would a 1.25" OD, 0.050" wall aluminum (6063 T6) tube be sufficient for this application? Length will be about 5', and the unsupported length will be about 3'.

 

[BlueBell]

I'd love to see a photo of that boat!

I don't have any photos of the original 2x2 boat but here's what it evolved into.
That's 1/4" ply on top of two 2x2's screwed to the deck of both the ama and vaka.
2:03 shows the boat
5:00 is good footage
9:50 you can see the hydrofoil under the ama (9 degree angle of attack)
All made in 3 hours using only hand tools and $75 worth of materials.
(Turn the volume down, the videographer gets rather vocal.)

 

[pmudesign]

I don't have any photos of the original 2x2 boat but here's what it evolved into.
That's 1/4" ply on top of two 2x2's screwed to the deck of both the ama and vaka.
2:03 shows the boat
5:00 is good footage
9:50 you can see the hydrofoil under the ama (9 degree angle of attack)
All made in 3 hours using only hand tools and $75 worth of materials.
(Turn the volume down, the videographer gets rather vocal.)

Very cool. Looks like you folks had fun.

Did the hydrofoil provide any substantial lift? Was it shaped into a foil or just a slab of wood?

 

[BlueBell]

Did the hydrofoil provide any substantial lift? Was it shaped into a foil or just a slab of wood?

Winning's always fun.

Hydrofoil was 1/4" ply shaped with a block plane.
She was able to balance the boat with the ama out of the water while making way.
Hard to say how much lift it created, but it did give unexpected roll stability.

 

[pmudesign]

How bend-able are the Home Depot XPS insulation panels? I just had a thought: I was going to build a blank with multiple layers of 2" thick panels and then shape it like a surfboard. But that's going to produce a lot of waste, carving a rectangular block into a hydrodynamic shape. Then I had a thought, why not just use 1/2" panels and use them (like you would plywood) to build a simple, 8 foot LOA by 8 inch beam, double ended, flat bottom box? And then give the whole thing a layer of glass and epoxy? I could even build up a thick mix of epoxy / micro-balloons to fillet the inside. I'd have to reinforce where the cross beams connect, but I can't think of any reason why this wouldn't work. Thoughts?

 

[pmudesign]

I started work on the ama. I created a foam blank using a method similar to plywood "stitch and glue" except I'm using 3/4" XPS foam insulation and Gorilla glue. I'm pretty pleased with the outcome so far. The XPS foam is cheap and easy to work with. The resulting blank feels stiff but extremely light.

 

[BlueBell]

Nice

You can use a hot wire to shape it. Or even a bread knife.
Electric carving knives from the food industry work great!

Rounded is stronger and more hydrodynamic.
You could taper it vertically so it's narrower at the bottom.
Also, tapering the stem eliminates most snags.

Oops, I was thinking it was solid, disregard the above.
Round your edges tho so the glass fiber will stay down.

What are the dimensions of that ama?

 

[pmudesign]

I think working with the 3/4" thick panels is much easier than shaping a solid block, but it does have design limitations. The ama is 8'0" LOA, 6" beam and about 10" from keel to deck. A little bigger than I planned.

 

[BlueBell]

It doesn't have to be a solid block.
Laminating 2" thick shaped pieces works great for a rough shape.
Once shaped it can be cut into sections and 1/4" ply bulkheads inserted.
Good for joining to cross arms.
Look at the shape of the Hawaiian racing boat amas, beautiful and strong.

You'll be fine. Carry on.

 

[luckystrike]

Hello pmu,
is this blue hull just a proof of concept or your attempt to build the actual hull?

Sorry, I'am late to your thread.

If this is the actual ama, how many bulkhedas have you included into your hull? None? You will have difficulties with vacuum laminating because the vacuum will crush the foam. Its the same when you are sucking the air out of a light PET bottle. If you want to build it light then its best to make the hull round, that will stiffen the glasfibre. Round hull is also better for hydro- and aerodramatics. A flat bottom has more resistance in the water.
For my feeling your ama is too large 8' by 8'' by 10'' will give you more volume than nessesary for a paddle boat. Or do you want to sail?

So your initial idea shaping like a surfboard was the right idea. The XPS foam is the right stuff, XPS will not suck any water.

The added picture is a 16' ama from a sailing proa that I have designed. If the lines meet your taste I can make a design for you needs with advanced hydro- and aerodynamics as well as optimised shape for glasfibre construction.

Have fun, Michel