As I say, I'm no expert. I suppose I'll find out if it's strong enough when I pile onto the San Juan Rocks. I defaulted to 7-1/2 oz. cloth since my supplier had only that and 4 oz. I glassed the outside only. Also I probably have a couple more bulkheads than I really need, but they help make it stiff. I'm sure you're right about the weight contribution of glass and epoxy (I'm well into my third gallon, by the way), and you are certainly right that two 1/8" panels would be stronger than one 1/4" if one can pull off the lamination without any air pockets. I'm about a week away from launch and I haven't weighed the boat yet, but with seat, rudder and deck rigging I think it will come in at about 55 pounds. My guesstimate is that, with 1/4" in the hull only, it would have been in the 65#-70# range.

: 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.