Marc I used 7mm ply which is about 5/16 with a 20x20mm gunwald and had no trouble but the frames were full with no hole. I was making a S on F using ply (dare I say it doorskin!) for the hull skin. I did use Okhune for the deck as I needed better lengthwise flexibility. I was making a design Id been playing with for a couple of years just to satisfy myself it was OK and my dreaming wasnt wasted - it turned out pretty well. Maybe cutting the inside hole on the frame after its in place may have helped? Or cutting the frame on the diagonal? Okhune was about $US 65 for marine 4mm here in New Zealand so $120 a sheet in the US sounds a bit toxic as some others others on the board seem to be charged.

: Niven,

: I tried to go to 1/4" ply to reduce weight. The #3 frame
: buckled at the top when bending the gunnel around the frames. I
: had to reinforce it to keep it straight.
: Tom Yost's Sea Tour 17 EXP