: For a whitewater kayak, I think you'd want a minimum layup of 25-30 oz. per
: square yard. This translates to 4-5 layers of your 6 oz. S-Glass. Mat is a
: cheap substitute for axial or woven cloth.

Even a with 4 to 5 layers of 6 oz the lay up would be quite flexible. I would look at 4 or 5 layers of 10 oz to get more stiffness. Substitute some of those layers with Kevlar to save some weight. Mat will increase weight with not much gain in strength although it will add stiffness.