A lot of extra weight will come from the epoxy. Getting a light weight job though comes with experience, there is really no short cut. If you get the epoxy too light there is no strength at all, so most new builders end up with a resin rich layer and that adds weight.

For a boat with a S&G plywood hull though, you don't necessarily need any glass on the ply surface itself, just in the corners, that will save considerable weight.

Bill H.