Re: Method and associated cost for stringer and deck repair
Keep in mind that, in theory, the plywood acts as a core and the fiber-resin matrix provides the strength. The core is there to act only as a shear web.
In this case, the plywood is GROSSLY over-strength, and some small voids have absolutely no bearing.
In practice, many parts of our boats are almost like "stitch and glue" structures, where the plywood provides stiffness and strength, with chopped strand mat attaching pieces together and providing some percentage of the strength of the overall sandwich, and with the CSM adding considerable stiffness (not to be confused with strength) and of course waterproofing. In this case, any suspect underlying material could be reinforced by throwing one more layer of CSM on top of it, if in doubt, or you could fill the void with any structural filler.
From a practical standpoint, these structures aren't even remotely stressed to the point that you would experience a failure due to an interlaminar void in the plywood.
The ONLY application where marine ply (voidless ply?) would mean squat would be in the case of compression forces on the sandwich that would somehow occur over the void. An example could be through-bolts with a large amount of tension on them. In that case, normal sandwich hard point techniques could be used and all would be well.
It wouldn't be difficult to make a transom out of a fiberglass-styrofoam sandwich; the hard points would have to be placed appropriately, and the stresses calculated in order to design an appropriate layup schedule. Aircraft wings capable of withstanding 10+ G's are made this way, so a boat transom is well within the capability of these materials.
Debates about whether to use marine ply in our boats is like arguing whether a shotgun or bazooka is best for fly hunting, while dismissing the flyswatter as grossly underpowered.
