John,
I kind of figured you would go your own way no matter what anyone said.
OK, there is more.
There are distinct engineering qualities to every material we use to build boats. There is no way you can get the stiffness of a plywood panel at a comparable weight from sa strip composite. Strips work in a kayak because of the tubular nature of the stucture. If you build a flat panel of strips and glass it will be very flexible unless you lay on a lot of layers of glass/epoxy and/or make the strips very thick. In the end the weight will be the same or greater.
The same thing goes for foam core. Even if you use the most expensive marine foam panels it will take a lot of glass/epoxy to get the stiffness of a piece of plain old plywood. I got this straight from a naval architect who is constantly besieged with queries about building foam core versions of his plywood/glass designs. It costs twice as much and the hull will be every bit as heavy or heavier.
The PDR is supposed to be a big ungainly box that flies a ton of sail. It's hydrodynamics are so bad making it lighter will not get you much. Because of the outrageous amounts of sail used, making the box lighter may take away necessary ballast needed to stay upright. And, finally, if you want to fly a maximum of sail the box has to be strong enough to support the mast and everything that is attached to it.
You have a choice. You can spend the next several years trying to outwit the laws of Newtonian physics or you can build a plywood box like everyone else and be on the water in short order.

: Hi Charlie,

: I'm going to run some tests. A sqft of luan on a sqft of 1 1/2" thick
: blue foam. Vs a sqft of foam with a layer of glass, then a layer of
: 3/16" cedar toped with a layer of glass. Then weigh the two. You
: could be right. But I wonder if anyone has built two of the same type
: boats one a stripper and one luan, and what the weights were. I've used
: some luan on my two strippers and flinched at the weight of the luan.

: John