Everybody needs to read this. The Plywood industry is changing. For years we (all wooden driftboat builders) have been able to buy high quality AA Marine Grade fir plywood. Fir plywood used to be the strongest stuff you could buy. It might not have been the most handsome plywood, but it was the strongest. And if you planned to paint your hull anyway, then the appearance didn't matter. Many a beautiful boat has been built with a fir-core hull, covered in fiberglass and painted (perhaps with a high gloss automotive paint) and then finished off with bright, natural wood finished seats and gunwales. That's how I have always built and how I liked it the best.
But that high quality fir plywood is now (very suddently) no longer available. The industry specification standards have changed, and what they now call AA Marine Grade Fir now has fewer plys (fewer layers of venier to make up the panel). It isn't worth buying.
The best affordable plywoods now are mahogony-based. The cheapest is known throughout the industry as "Meranti Hydrotek" Meranti Hydrotek plywood (usually made in the Phillippeans) is the best deal in plywood now. Okoume plywoods can be used too. Sepele is probably the best, but also the most expensive. Take your pick--as long as it isn't fir you'll do just fine. This is a sad day for me. But that's the way it is.
Enter Polypropelene Honeycomb Core I'm not a commercial boat builder. Not anymore anyway. I build boats because it's my hobby. I like to do it. In fact I get nervous if I don't have a boat project in the works. :-) What I really like to do is to experiment with new ideas. In the 1980s I built driftboats with end grain balsa core. That made a very light boat, but it was not a successful technique, because balsa core soaks up too much moisture, if and when the boat's fiberglass skin ever gets punctured.
Jason Cajune and me and a few others have started building driftboat bottoms with polypropelene honeycomb core. Plastic honeycomb core makes the best driftboat bottom. It's light, it's stiff and it does not soak up water.
NidaCore and Plascore are the two primary brands. You can use Google to find the 800 numbers and go from there. It isn't cheap. Each 4x8x3/4" panel will cost approximately $100 each, with the shipping included anyway. Both brands can ship directly, or direct you to a local distributor, perhaps in a place like Salt Lake City, Spokane Washington, Seattle, etc.
Using honeycomb core is almost the same as plywood. You do join two sheets of 3/4" together in a different way than plywood. Instead of the slanted scarf joint used for plywood, Plascore is joined by a simple 6-8" wide box joint 6-8" deep. But from that point on the rest is the same. You can think of honeycomb core a extra-extra-light plywood. The only other difference (besides scarfing) is extra fiberglass. You will need to have a minimum of 4 layers glass on the outside bottom and 2 layers on the inside bottom. Kevlar is good stuff, but not required. If you do want to use Kevlar, use it as the first inside-bottom layer only. I do not recommend Kevlar on the outside surface. You can still build a boat with a plywood bottom. Thousands have been made. Just don't use fir plywood anymore. Use 1/2" (or 12mm) mahogony of some kind.