Wood pontoons

I am looking at building a plywood pontoon boat about 18 foot long out of 1/2 plywood. I have to motor its a 105 hp outboard. has anyone done this before? what is the bests way to start? Thanks for your help.

 

The pontoons are essentially box sections except at the bow. If bulkheaded off every 4 ft or so the pontoons will have good redundant safety characteristics without the use of foam.