> the thinking is that you are going to cover all with cloth
> and epoxy anyway so the strength of the doorskins may be enough?

If you're going to cover all with glass, probably. In that case your boat would be like a stripper, where the wood is carrying only shear loads and would have plenty of strength. Strippers are composite boats, while S&Gs are essentially plywood boats with composite joinery. They count on good wood.

If you plan to cover both sides of the plywood with glass you may consider using thinner plywood to keep your finished weight down. (I'm assuming your plans don't call for total glass sheathing See George Robert's site, or these archives, for some ideas.

And, on the subject of economy, by the time you add all that glas cloth and epoxy to the boats, that marine plywood will not look all that expensive.

If your plans happen to call for total glass coverage, never mind.