No, 2 layers of 1/8 in birch ply are not strong enough, ya have to glass the top and bottom and middle....you lose any wt advantage. 3 sandwiched 1/8ply layers are just strong enough with poly on the outside, perfect for mowing the lawn riding, but high jumppin flying, THEY WILL TEND TO BREAK. BUT, I do find if you use good 1 1/4in AC 3 ply sheet and 1 1/8 birch 3 ply sheet. Makes for a relative Boom proof LIGHT flexible board, go figure.
if still not sure, a bit of experimenting during your next visit to HD will add some hard numbers to all of the speculation. (or you could look them up from someone like 'plywood association' of wherever)
compare all your potential candidate materials.
one by one:
prop up any of your favorite choices between two carts.
toss a 50lb bag of cement in the middle
measure deflection, length width and thickness.
Deflection = wL^3/(48 E*I)
google 'beam deflection formula' for 3-point bending
the resulting numbers will be a good way to compare and give you enough to predict deflection for any of your design iterations.
It may also convince you that the cheepest and lightest crap you can find will be tough to beat with 1 or 2 layes of 4oz glass.
It retained the rocker better than I expected. Gluing up two layers seems to be the way to go for creating a controlled rocker. Now I think I've got myself a bit much rocker for a light wind board. I've tested it last night and it rides super sweet. More comfy then the old one of the same shape with less rocker. Now I have to find out if I get the same low end out of it. For now I am very pleased with the result.
After two sessions on it I have to say I am very pleased with the riding performance. The additional rocker makes this board way more smooth than the same outline with flatter rocker. No surprise here I guess.
The Baltic Birch ply gives this board a much better, snappier pop for jumps. I still think I lost a bit on the ultra low end but really not much. Huge improvement in riding down little waves, feels nice and loose. I was slashing them like on my smaller boards. And all that with a 16m kite, light wind and some ply under my feet. I am thrilled!
B/BB from now on for me. Thank you for the encouragement to use better quality plywood.
The original build on a few posts back was with Sande Ply, now re-built with Baltic Birch Plywood. I have to see how the thinner rails and the glued up layers of 1/4" sheets will hold up.