: Swensons site has numerous kayaks that are old fashioned
: ply-on-frame which is just what you described. Why go there?
: There are many thousands of S&G kayks made from 4mm ply with
: no internal structure except shearclamps and bulkheads. If
: you're going to go to the trouble to dig up decent plywood why
: use it on some kind of home-brew design?

With a ply on frame, you don't have to loft the panel shape from dimensions/coordinates, you take the panel shape directly from the frame ribs, or chine logs. Done tightly, they don't need outside sealing, though the lower chines and keel line can be reinforced with F/G tape and epoxy where needed.

With chine logs, you could remove some of the framing in the cockpit area and use the next frames as bulkhead points. Put some ventilation system to let any moisture/water evaporate out when storing.

Built carefully, a chine log kayak wouldn't weigh any more than a S&G with glassing, often a bit less. With good surface ply, marine ply can look very nice varnished bright. Do the best varnishing on the deck and upper panes. That's what most people see. :D The finish on the lower panels WILL get scratched up. You could go for a painted hull and varnished deck and 'worry' less about scratches.

Years back, my uncle built a double hulled punt (crude catamaran) with 3/4" white deal frame & chine logs, hardboard* skin, and oil-based house paint inside and out. It lasted for at least ten years, stored outside, often upright and was light enough for us to carry and strong enough to survive us as kids.

Mike Savage
South West Cork
*(high density fibreboard, smooth on one side, dimpled on the reverse. Unpainted it will absorb moisture and wet quickly through the reverse side, a lot slower on the smooth side.)