: Hallo all,
: I've been thinking it over: why not using 1/2 solid wood for the
: cross sections? Isn't it lighter than 1/2 marine plywood?

Weight isn't the issue here. If the difference were
3 ounces per frame/station and you used 5 frames in the boat the weight difference in the entire boat will be under a pound. That is trivial.

People built kayak frames, and other boat frames, for years from solid wood. They still do.

What you need to be concerned with are the dimensions of the stock so it is not too flexible or weak. You also need to think about how you are going to join the various pieces of wood which will make up each frame. There will be considerable stress at these corners. Pegged mortise and tenon joints have been used, as have lashed joints, and joints held with corrosion-resistant bolts and nuts. Nails won't be good for long-term use. Wood screws will work, but use glue too, as they'll get loose eventually.

If you are going to work with 1/2" stock but find that you need something stiffer, just glue on some additional pieces to build it up. Glued on (or laminated) stiffening pieces are common in the plywood frames used by Blandford in his design for a folding kayak (see plans description at www.clarkcraft.com).

Hope this helps.

PGJ