: . . . The idea is that perhaps the ideal
: material for the skin of a kayak is different than the ideal for any other
: kind of boat I can think of. I agree that for a sailboat one wants a stiff
: material, therefore, the balsa/glass composite works fine. High E, high P,
: light: what I would want any boat to have.

: Except, I wonder, a kayak. The high degree of resistance to deflection that
: is so desireable in any other boat, including a canoe, is something I'm
: not sure I care about in the kayak. By replacing the 1/4" cedar with
: 1/8" hardwood, and replacing the 6 oz glass with 4 oz, the resistance
: to deflection would go way down, but so would weight, and compressive
: strength and fspl would go up. When I crunch against a big rock, the skin
: can deflect more easily, spreading the stress over a larger area, and
: minimizing the chance of crushing the outer wall of the wood and
: separating the outer layer of glass. . .

No angels, no dancing and no pins needed. Maybe a needle and thread.

The answer to your ruminations about kayak "skins" is contained in the very wording of your pondering: think skin. Original kayaks were not hard-sided, but were soft sided. They were covered with animal skins. Now we use woven fabrics.

There is obviously a great deal of difference in stiffness, strength and flexibility characteristics between woods and fibers. I wouldn't even try to compare them. Nor would I make any attempts at comparing the more complicated composite structures with the physical properties of a sheet of cotton or dacron fabric.

Next time you get out your sander for a few hours of mind-numbing work, consider the ultimate question: How strong is "strong enough"?

Since you like to do math in your head, try these numbers: Figure the average force exerted on the skin of a fully loaded kayak.

Since I'm not fond of doing calculus in my head, I'd estimate this number, starting by making a few assumptions. First, I'd work with an 18 foot long kayak that had a beam of 24 inches, and which drew 6 inches of water when loaded with 400 pounds of paddler and gear. That is fairly close to real life. Again, to keep the math simple I'd assume this boak had few curvesto speak of, and that the hull was shaped like a long diamond, and had straight sides rising from a flat bottom. The area of the bottom therefore could be calculated as teh area of a diamond 18 feet long and 24 inches ( 2 feet) wide. A little exmintion of the situation shows that this is the same as the area of a rectangle 1 foot wide and 18 feet long, so I calculate a bottom area of 18 square feet.

Now, I could further calculate the height of the sides that are submerged (6 inches or 1/2 a foot, multiplied by the length of one side of the diamond (that would be the hypotenuse of a 9 foot long and 1 foot high right triangle, or the square root of 82), and then multiply that number by 4. I'll multiply the 1/2 and the 4 first to get 2 and then fugure the hypotenuse as a tad over 9, so my total submerged sides would be another 18 square feet. So far I have 36 square feet under water, and suporting 400 pounds of gera and paddler. I'm going to now simplify things a lot, and remember that a real kayak has rounded edges ans not sharp square sides. Those curves cover quite a bit less area than what I've calculated, so i'm going to round things down to an estimate of 20 to 25 square feet of surface underwater.

With 20 square feet of surface and 400 pounds of contents being displaced, the pressure per square foot is 20 pounds. With 25 square feet of surface the pressure is 16 pounds per square foot. Now common measurements for pressure are in pounds per square inch or PSI. There are 144 square inches in a square foot, so, for the smallest area I'm estimating I can divide 20 by 144 and get a pressure on the hull of a whopping one-seventh of a pound per square inch. And for the largest surface area I'm estimating, it is 16 divided by 144, or about one-ninth of a pound per square inch.

Those are average loads, but I have tow other matters to deal with. One is water pressure. At a depth of 6 inches the water pressure on the bottom will be about 1/4 pound or 4 ounces per square inch. That is roughly twice the average load given above, and I figure it this way. From my diving experience I know that pressure of the water is about 1 atmoshpere, or about 15 psi at a depth of 33 to 34 feet (depends on salt- or fresh-water). That means that for each foot of depth the water pressure increases a bit less than 1/2 pound per square inch. with 6 inches of depth I take a half of that.

Then there is the possibility of connecting with an object. We have to know how much weight is going to be concentrated on how small of an area.

and here is where I leave you to do the rest of the math.

Have fun, and concentrate on the sanding

PGJ