What grade plywood?

[snort]

Howdy,
I've decided to beef up the cabin walls where the portlights insert into the boat since the walls are only 1/4" thick. I'll add plywood rings around the port holes of another 1/4" , but was wondering if I can get away with using normal plywood instead of the more expensive varieties. The plywood ring will be in the interior and epoxied and about 1.5" in width.
Normal ply or better marine?
Thanks!

[Pauls]

First off, marine and exterior grade plywood both have the same specs for glues. Adhesives are not an issue. So the only difference between the marine rated plywood and exterior rated is the wood type (not necessarily different, for example you can get marine grade and exterior grade both in douglas fir), and voids between layers. Marine grade does not allow voids between layers whereas AB Exterior does. If you sort thru the plywood at a local lumberyard you may well find a sheet of AB exterior which has no voids, or has voids you can work around. For your application that will work just as well.

[snort]

Thanks! Paul. In researching the portlight on a thin cabin wall question, I see some people fiberglass over foam, so I think any decent piece of plywood would be a better option.

[Paul L]

By beef up the cabin walls are you saying you want to structurally you improve them? How wide an area is this? As Pauls says above the interior plies of marine ply do not have voids, while normal exterior ply do. If you are cutting relatively narrow pieces then you can see the voids and throw those out. In larger pieces you will not know how much void is in the interior.

Marine plies often also have more plies of thinner material.

[Cormorant]

Also check out MDO -- medium density overlay. It is exterior ply used for signs. It has a smooth resin coat on one side. I used it extensively in building my boat and it has worn very well over the last decade and a half. Scrap pieces laying around outside in the yard haven't delaminated in all that time. I like to call it "poor man's marine ply" . . .

Whatever you use, once you have the piece cut out, paint all the exposed edges with epoxy. That'll really seal it up and prevent any water intrusion, mold/mildew/fungus growth, etc.