I was looking at a site that showed a home builder's sailboat. He had some
plans, set up a frame on the ground and made a cross piece frame ..

He used plywood to sheath the boat. Epoxy on the plywood.

Two things ... when building a hull, do you first need a backbone,,,
keel,,, or whatever??

Another ,,, if you use plywood, can you glass over the ply and the ply
becomes a core?