So - last hollow board I did, I used a jig saw and a router in a router table to build the spine/ribs. Today, I got a scroll saw. Holy crap-cake-coolness! I can cut circles in marine plywood now! I am so stoked - glued my pattern down to some sweet 6mm Okoume marine ply – gonna cut it out tomorrow. 
The board is a 5’10" x 24" x 2 1/2" planing hull.
