hey David, why fiberglass and not a cotton sheet? fiberglass has a lot of compressive strength, think of a solid piece of glass. cotton does not,think of a compressed ball of cotton. now take either one of those fibers individually, cotton will fold over easily, glass will bend then break. Fiberglas has a lot of tensile strength,cotton has less (I think) So putting glass (in an epoxy matrix) over plywood provides for a strong skin that resist both compression and tension, cotton would only work on tension. So putting a fabric with both good tension and compression means that a panel(hull) can resist strong bending forces because one side is put into compression while the other is in tension. If you laminated the same plywood with cotton the plywood would bend easily ,although not as easily as without cotton, it just wouldn't give the strength for the amount of added weight compared to glass.

The other thing about epoxy in particular is that it soaks a little bit into the wood as well making the wood quite tough.