If you read the first line it says to use the full length 8' 2 1/2". So, am I correct in assuming I have to make it longer with non-marine ply?

Rough Cutting the Panels

Cut from 4 sheets of 8 foot (actually 8', 2 1/2") by 4 foot marine grade 4 mm Okoume plywood:

8 pieces that are 11 inches wide and 8 feet (use the full 8', 2 1/2") long. Four of these pieces are for the side panels and 4 for the bottom panels. These pieces will use up the first and second sheets of plywood.
2 pieces that are 12 inches wide and exactly 6 feet and 3 inches long. Lengthwise (6 feet and 3 inches long), the center panels are a final cut, not a rough cut, so these final cuts should be made cleanly at right angles. Each piece needs one straight edge lengthwise, so cut them out at the 2 long straight edges of the 8 foot by 4 foot plywood. Mark them as the center panels for the deck.
1 piece that is 22 inches wide and 7 feet long. Mark it as the stern panel for the deck. This piece and the 2 pieces from number 2 above will use up the third sheet of plywood.
1 piece that is 17 inches wide and 5 feet and 6 inches long. Mark it as the bow panel for the deck.
2 pieces that are 11 inches wide, and 2 feet and 6 inches long. Mark these 2 pieces as the bow end of the side panels. These 2 pieces and the piece from number 4 above will use the forth sheet of plywood and leave enough extra for the cockpit coaming lip and bulkheads.

: No. The marine ply is longer, but it has markings stamped on the
: ends for the grade of plywood and the BS numbers etc. You will
: want to cut the ends off so as not to have the markings on your
: yak. I built the PB 15'9" and cut them to the exact
: measurements in Duane's lofting. Came out fine.

: Charles Robinson