: Why on earth would you want 1/4" plywood?

Interesting point ... not sure if the 1/8" is sufficient in itself, but 2 sheets would probably give you the same as the 1/4" (5.5mm) luan that some people use, but without that mediocre thick core ply surrounded by the paper thin outer layers ...and also without the need to scarf joints.

Wonder what the weight of a typical kayak is in terms of its components? I think the epoxy is 10 lbs/gal. Did you glass the interior and exterior? I personally was thinking of a S&G hull, with a stripped deck, but haven't weighed a sheet of 1/8" ply to see how much that will contribute. True, you lose some epoxy weight with sanding, but I'm betting that at least 2 gals. of epoxy was used to wet-out that 7 1/2 oz glass and build the rest of the boat.
So about 1/2 of the boat's weight is epoxy and fiberglass? Seems like there was a compromise made by using heavier glass than usual to compensate for thinner plywood. So maybe the weight issue balances out between more epoxy and less plywood. In any event, the 1/8" stuff times two is less expensive than the 4mm marine ply, and maybe 1/8" by itself is sufficient ...when I have an accurate measuring tool, I'll try to verify the actual thickness in millimeters of this "1/8" luan plywood.

I don't think the goal was to make the "lightest boat", but rather one that could be built inexpensively, with readily available plans and materials.
It still should come in at a reasonable wight, perhpas 15-20% heavier than one made with the recommended $$ marine plywood.