Use of plywood for internal structures

Hello, we are currently building a 6.5m powerboat in grp. The longitudinals are foam core but we want to make the bulkheads ie collision, fwd and engine room out of marine plywood and the transverse frames. We have designed the grp structure using the Lloyds SSC program which is fine, however there is very little guidance on the use of plywood. We are trying to find out what thickness of plywood would be suitable for the bhd's and transverse frames. We were thinking of using 2.5cm thick spaced at 40cm intervals for the transverse frames, should the bhd's be the same thickness or more? The sternplate is not really a problem as guidance is given from mercruiser in relation to the leg. The boat is expected to reach about 45 knots. Any help on the size of plywood to use would be great!

Many thanks.

 

use dry pressure treated ply. bulkheads 3/4inch, engine stringers 1 1/2 total width. glue dry ply with resin and screws. Tab stringers in with 2 or 3 layers of 1810 stitchmat.