Mac wrote:
I don't think the joints in the core NEED much strength. I mean, you can
use Styrofoam as a core, and that can be broken easily by hand. Also,
balsa core is not very strong laterally, but it is an excellent core
material (unless it gets wet).

You can use Styrofoam but you shouldn't.

With cores, the important thing is that it adhere well to the skin, and
that it have good compression strength. Butted plywood with, say, epoxy
filler at the joint, should make a good core.

With cores, the most important property is the shear
strength. Obviously if it doesn't stick to the skins it
isn't core... Transoms should have a higher compressive
strength - balsa is good, plywood is o.k., high density foam
(= 120 kg/m3 is ideal)

But then I am not a mechanical or structural engineer or naval architect,
so take my opinion for what it is worth.

--Mac

I am a m.eng. and naval architect

Evan Gatehouse