Decision: Composite

I found some foam, and it was a reasonable price, as was the delivery. Also some very reasonably-priced (for here) meranti marine ply, ¼".

"A sandwich construction of a core material faced on each side with a reinforcing glass/resin skin offers greater mechanical strength and stiffness, pound-for-pound, than any other structural option, regardless of the materials used."
-http://www.compositesworld.com/articles/wind-foam-sources-pet-san-pvc

I'll make the bottom and—since I intend to install a rudder—the transom, out of the ply. The rest of the hull will be foam/glass. The foam is PET: polyethylene terephthalate. I'm using 10mm (⅜" equivalent) for the hull, and 6mm (¼" equivalent) for the decks. The "composite" part of my post title is about using both plywood and foam. I want the plywood on the bottom as insurance against harsh groundings on sharp objects.

Here are the two bulkheads (green foam) and the temporary forms. I haven't cut the transom out yet. It would sit on top of the left stack:

Below: Laying out the bottom on the mahogany ply. That single sheet of ¼" ply was heavier than all four sheets of the foam, which of course won't all get used because of offcuts.

The bottom is over 8' long, so the after end gets a bit scabbed onto it:

I left the drawing of the final shape of the after end until after the whole length was there, to make sure the curves are fair between the two pieces. The after end is only 1" wide!

Building material, ama design

There doesn't seem to be an easy way to get the foam core that I wanted to build with. I would have to go to Vancouver, or have it shipped. The stuff is so expensive, the cost to get it here would just make it too much to be justified.

So I'm planning now to use marine grade mahogany ply. It will be heavier than the foam would have been, but at least a familiar medium to work with. I'll use lighter, thinner stuff for the decks, heavier stuff for the bottom and the amas. And even though it will be the best plywood one can buy, it will cost far less than the foam board. Hopefully I'll be able to manage the weight, using the noggin.

Speaking of amas, I made a life-size model of an ama out of cardboard, to test the concept. For the first one I made, the two sides were held together with tape. I didn't want to use my good Duck Tape on a model, so I cheaped out and used hockey tape. Well, the tape soon pulled away from the cardboard, so I decided to sew the pieces of cardboard together instead, wrapping each edge with tape to make sure the thread didn't pull through the cardboard.

I'm quite happy with how it turned out, and now it's hanging above my couch:

The painting that the model outrigger is sitting on shows a full moon rising over Cortes Island, as viewed from Rebecca Spit on Quadra Island. The last ferry of the day to Cortes is behind a tree. In the drawing on the right, the Toronto Island ferry captain is chewing out a hapless sailor.

Model

Since I found myself with two sets of plans, I decided to use the one I won't build (Toto) to make a model with, so I could get a hands-on as to the building sequence. Very simple, really. My only problem was that the white glue didn't stick the paper onto the balsa as well as it could have, so had to be removed in places, but not before I marked whatever station was going to be lost thereby.

Toto, the one I modeled below, has a bigger transom than Rio Grande, the one I will build. The top form in the top picture here is the transom, and you can see that the bottom plank is going to be 4" wide there, since the half-measure is 2". On the Rio Grande it's one inch wide, ½" half-measure, and the vertical side is a bit more than half the height of Toto's side, so she'll have a daintier bum: