Thickness of a sandwich composite panel, and a few other related questions?

Here is what it says about marine grade plywood.

From what I have read, Oukume is classified as structural but not marine grade. The list attached which are classified as marine grade.

 

As long as you use epoxy and do a couple of neat coats of resin and epoxy primers, I'd use the wood that failed the test at 6 hours. Any exposed edges get neat coated, so when you, for example, install a coaming, you bond solid timber coaming and close the plywood edges. Then a fairing fill above and below. Never allow the edges to be exposed. This is sometimes painful, but for that boat should be simple to achieve. At the deck interface, the deck to hull must be done so no ply is exposed. There are alternatives for this joint. Rx may even have a sketch. Or I may find one. But you can always laminate timber to close the edge as well.

But you must commit to leaving no wood edges or surfaces exposed to sun or water.

I'd want to better understand how the plywood does in a tear test as well. If you epoxied a piece of timber narrower than the ply and let it cure for a couple days, does the mdf shear easily? It may not matter, but I'd like the peace of mind. If it tears like paper, I'd not use it.

 

Here is a edge modification for honeycomb core. Here, no edge exposure of this plastic core material was acceptable. Your plywood can be treated same fashion if the core is exposed in any way.