Auxillary pontoons for old houseboat

The old houseboat I bought has had extra deck space added to the stern and a swim platform . This caused the stern to sag low. To try to remedy the issue the former owner attached 2 36" X 42" X 12" deep plastic floatation boxes using nylon tie-down straps.
I want to tryto redo it better. My idea was 2' X 2' X 7' long pontoons made out of plywood and fiberglass tape. I calculated the current floatation boxes are supporting about 400# a piece. I'd want my new pontoons to support upto 800#.
My question is, what minimum thickness plywood could I use? I want it to be as light weight as possible. I'd include baffles or braces. Would 1/4" fir plywood be acceptable.
Any design recommendations would be appreciated.

Thanks,

Mark mark8076@msn.com

 

Anyone?

 

Well...2x2x7 will displace 1748 lbs each when totally immersed so each will support 874 lbs at half immersion. With 3 frames of 2x2"s and 1x2 framing at the ends supporting the plywood I would use 3/8" ply for the build. I would also glass the entire outside using epoxy and 12 oz glass cloth. This should be pretty darn bulletproof. Each is going to weigh in at about 75-90 lbs so you are in the ballpark weight wise for 50% immersion. For best results...use 9mm BS1088 marine ply...it will last a lot longer.

Steve

PS: It usually takes 18-48 hours to start getting replys to posts