: I don't know about jumpin' up and down, but I often stand there at least
: partially and I am rigging for a small mast too, so I am looking for a way
: to distrubute these stresses over a larger area.

If I recall correctly, you are using 6 ounce glass fabric. Once the strips are covered with one layer of that on each side you will have a composite that will hold several hundred pounds per square inch. Each of these layers of fabric is already much stronger than a single layer of canvas, such as used on most skin-on-frame kayaks, ansd would be strong enough for most uses all by themselves. The wood-strip core is made of materials as thick as those used for over a century in canvas-covered wood canoes. In short, the conventional design is already overbuilt. I'm not saying it is indestructible, but it is pretty darn strong as is.

If you want to make it stronger, line 2/3 or more of the length of the bottom of the boat with a second layer of glass cloth. You probably can do this for almost no cost by piecing together some of the left over cut-offs from when you glass the deck and hull. A 12 to 14 foot piece centered on the 17 to 18 foot length of the boat will allow the hull to flex a bit under stress. In contrast, a piece of plywood in the center of the boat will be a rigid component that may concentrate stresses at either end of the plywood, and actually weaken that area of the boat. It will also look ugly, while the two layers of glass cloth will be nearly as transparent as a single layer, and their thickness will not be apparent.

Let's assume you are building a boat to be used in water. That sentence may sound strange, but you'll understand in a second. When the boat is on dry land it amy appear to be rather fragile, but when it is in the water, it responds to stresses in a totally different way -- it shares them with the surrounding water. I like to think of my inflatable boat. The skin on that is a floppy piece of thin vinyl, but put in some air, at about 2 psi, and it becomes very stiff. When I step into that boat, the bottom sags a good 6 inches, but the boat does not collapse. What is happening is that my weight is being distributed over a large area, and displacing a great volume of water.

Now look at a typical kayak. Just pulling some numbers out of the air, the hull of such a boat would be about 18 feet long and easily average something over 12 inches wide. That gives me an area of boat in the water that is 18 square feet. With 144 square inches to each square foot I have somewhere over (round numbers) 2500 square inches of hull in contact with the water. If I put 500 pounds of weight (me and my gear) in such a boat the loading on the hull would be somewhere around 1/5th pound per square inch. Now that single layers of fiberglass/woodstrip/fiberglass composite has a strength of well over a couple hundred pounds per square inch. So it is already 1000 times stronger than it absolutely needs to be. With the second layer on the inside you'ld probably double that. And these are very conservative numbers! It wouldn't surprise me if one of the structural engineers who frequent this board could show that fiberglass/wood composite strength was well over 1000 psi.

Why the heck do you need somthing more reinforced than this? And why the cockpit?

Sure, a sharp rock will put a lot of stress on a tiny area, and if you hit it hard enough it might do some damage, but that is a temporary condition. You can cover that area with duct tape (It only needs to resist something less than 1/5 psi) If you hit the same area again the duct tape rips. Replace it. If you hit a nearby area of the boat, you put on a second patch. If you should severely bash into a couple dozen sharp rocks and puncture the hull several dozen times, it will still float well enough to get you home. Actually, this kind of damage is not the norm. Mostly the boat will slide off of rocks, and just get some superficial scratches which do not penetrate the exterior glass.

If you are mega worried about bashing into rocks, consioder this: you'll undoubtedly hit them with the bow of the boat rather than the middle of it. Therefore, you should reinforce the front, not the middle. A 3 to 4 foot long piece of additional glass cloth applied to the bow will be the recipient of 60 to 80 percent of the dings.

Now, let's suppose I have not convinced you, but I have upset your line of thinking. Try this. Build the boat and glass it conventionally. If it is not strong enough then you can add your plywood to the middle (or anywhere else) after the glass is on. You can even put another layer of glass over the whole thing.

But I really don't think you'll want to.

PGJ

It will be 6 to 8 times thicker than the skin on an aluminum canoe.