:. . . I am thinking 6oz glass on the outside. Extra strip along the
: bow and possibly the keel line. 4 oz glass on the inside cockpit area
: only. More for wear reasons than for additional hull strength. No cloth
: inside the hatches. Just a coat to seal the wood and then white paint to
: brighten it up. . .

: . . Anyone have a good reason that this is a bad idea? The lighter weight
: is a big plus for me.

Actually, what you are proposing would still be considered "overbuilt" by some people !

I usually calculate about 16 square feet of water contact for a kayak. If the total load (paddler and boat and gear) is 320 pounds (probably a fairly high estimate) then you have 20 pounds pressure on a square foot of hull. Being in water spreads this weight evenly over the entire hull. That is a paltry pressure of 1/7th of a pound per square inch. A well-supported piece of 1.7 ounce daron is strong enough to carry this load, as has been demonstrated by Platt monfort. Check out his site, Gaboats.com for a look at the Geodesic Airolite boat designs.

Assuming your fillets are large enough, properly braced, and strong enough, you could remove almost all the plywood and go with just the glass. In fact, you could use 1 ounce glass instead of 6 ounce glass. What you would end up with is a skin on frame kayak with a fiberglass frame.

The critical point is the strength of those fillets. If they are strong enough, then your plywood panels simply replace fabric, or animal hides, in covering the frame. The panels themselves add very little to the structural strength of your boat. In areas where they do add strength, you can still easily replace them with lighter frame components which will provide the same strength at a lighter weight.

Imagine that you have a stitch and glue kayak made with just taped seams and filleted joints holding your plywood panels together. now imagine that a colony of termites has moved in and overnight they have eaten away all the wood. What you have left is going to resemble a complete frame for a skin-on-frame kayak. Almost all the structural components of the skin-on-frame boat are there. Whats missing are the ribs and thwarts or deckbeams which provide crossbracing. In fact, the fiberglass fillets and the attached tapes are going to have a strength as great as, or greater, than the thin wood stringers used on some SOF boats.

If you go with very thin plywood, and your fillets are substantial, then you should go with bulkheads at the front and back of the cockpit. These can be full and watertight, or they can have big holes in them to lighten them.

A slightly "V" bottom, as opposed to a flat bottom, will allow you to cast in place a fillet of fiberglass to serve as a keelson. Or, you can use a wood chine along the interior keel line. This considerably stiffens the floor. If you don;t want a center seam, you can achieve a similar result by usng a curved bottom, with the center (along the keel line) being about an inch deeper than the edges. 4 to 6 wood forms, placed perpendicular to the keeline, along the length of the boat can define the shape of this curve, and they'll provide some crossbracing, too. If you build the boat over forms, you use hollow tubes of saturated fiberglass fabric instead of those wood forms. Once the resin sets, and the building forms are removed the floor shape will hold, and the sides will be nicely braced.

Boats under 15 pounds are certainly possible. They will be a bit more delicate, but should handle placid lakes, rivers and streams, and last for many years. If you want to shoot for a superlight boat, you'll need to spend a bit more time building it, and you should assume that you'll need to do some careful trials, and possibly some additional reinforcement, before calling the job "done".

But go for it. Take some pictures and let us see how the project goes.

PGJ