Nick, BEWARE!!! Long post.

> I take it from this that you have tested some panels. Is this true? Your
> original post did not make that clear. Can you tell us about the tests.

The tests I was refering to were the Bougeon Bros. test results.

> There are failures and there are failures. What most people are interested
> is what kind of accident can they paddle away from. A failure of the outer
> layer of glass or the core is not a good thing, but if you can keep
> paddling until your next chance to fix it, most people will be satisified.

Agreed

> I'm not sure why you feel this won't change the point of catastrophic
> failure. In the breakage I have seen, the wood does not fail
> catastrophically until the glass gives way. If the epoxy and glass is
> under higher load, which it would be on thinner strips, it will fail
> earlier.

The test showed that the thinner panel didn't always fail first. So both panels fail at roughly the same point, and a catastrophic failure only occurs when both layups and the core failed. The difference is that the 1/4 inch strips seem to fail long before the point of catastrophic failure. This would mean that it is the epoxy and glass that is taking all the force. A 1/8 inch strip will flex about 4 times more than the 1/4 inch strip. The epoxy and glass will still take the same amount of force. So the 1/8 inch strip will not break until closer to and maybe after the failure point of the epoxy has been reached. Therefore, you could theoretically eliminate all failures except scratches, or at the other end, catastrophic failures. (Also if the 1/8 strip is still intact the composite is now stronger than the broken 1/4 strip and could have a higher failure rate. The trick is to get the strip and the layup to fail at the same time. Thicker strip/thicker layup-thinner strip/thinner layup, probably to an "ideal" ratio for the core material.)

> I don't agree that the glass is undamaged in a failure. Almost all the
> damage I have seen involved failure of the epoxy and glass.

You have far more experience here than I do. All I can go on is what others have said and one picture of a catastrophic failure. (Don't worry, he lived.) I would be very interested in the details of various damage you have witnessed.

> Eliminating the minor failures is only worth it if you don't increase the
> number of catastrophic failures. It is possible thinner strips will do
> this, but so far I am not convinced that would be the case.

Agreed, and these are simply observations based on limited info, with no testing on my part. I am getting geared up to build though! In the meantime I will have to be content to be a theoretical builder.

> I used Kevlar in several boat I made about 7 years ago. Since then I've
> decided it wasn't worth the effort. I have not done any stringent testing, on
> but of the sample panels I've made, the one with the Kevlar is the only
> one that has shown any sign of failure do to hand bending. It has a
> compression failure of the Kevlar due to bending with Kevlar on the inside
> of the bend.

Once again, I don't have much experience with kevlar, but my understanding is that even if the epoxy fails the fibers remain intact to provide a measure of structural support. Bullet proof vests use this property without epoxy.

> I don't take it as a given that flexing will allow distribution of the
> force over a larger area. Stiffness would seem to be the required to
> create that distribution. Imagine poking a pencil into a water balloon vs
> poking an egg. While it is hard to say, all-things-being-equal, which is
> stronger, but it is clear that the egg distributes the force over area
> better than the waterballoon. I don't think we are dealing with enough
> flex that the skin can significantly wrap around a rock for larger force
> distribution.

Hit that same balloon and egg with a hammer and the results would reverse. So we would have to decide what we are building for. I must admit that I have been thinking in terms of the impact being like a hammer blow, but it could be thought of as a pencil push which would change things. (More thinking!!)

> The damage I have seen in my boats involve two primary forms: Scratches
> and more rarely bruises.
> Scratches are not going to be effected much by the strip thickness. I
> suppose if the skin flexed a little more the scratches might not be as
> deep, but I doubt it. Scratches are usually from a sharp object moving
> slowly down the length of the boat with the full weight of the boat
> pressing on the object. The boat would have to flex enough to lower the
> effective weight on the sharp point. I don't think we are talking about
> that kind of flex.

Agreed.

> By bruises, I mean an impact compressed the skin resulting in some failure
> in the epoxy. These happen from bouncing off some obstruction with a good
> deal of force. The damage is generally right at the point of impact. This
> would seem to the kind of damage effected by flex of the skin, but again I
> don't think it would make much difference. I don't foresee the flex of
> thinner strips being enough to absorb and distribute the force right at
> the point of impact fast enough to change the surface damage. Both the
> thick and the thin strips will see approximately the same forces right
> where the glass meets the rock. How those forces are distributed around
> the hull will be different, but I don't forsee the local forces being
> changed much. I may be wrong, but I don't think changing the strips will
> effect this kind of damage.

On this point we disagree, which is why we are having this discussion. I only have experience in fiberglass, SMC and various plastics used in the automotive trade, and respect your many years of experience in kayak building. I have seen many many failures of fiberglass and SMC,but they don't deal with a core material. One interesting thing I have seen regularly though is a very flexable urethane or similar plastic ( they use so many)used as bumper covers on cars that when subjected to a collision are only scuffed or slightly torn when the metal reinforcement behind them is destroyed. Hmmmm. This is kind of like those indestructable rotomoulded whitewater kayaks, to the extreem.

> The way to reduce the damage of both scratches and bruising seems to me to
> be thicker glass to absorb the localized forces right at the point of
> impact. Also the mechanical properties of the epoxy are very important.
> Some epoxies are harder or more brittle. This will probably have more
> effect on the typical damage than the thickness of the strips. Different
> cloth will also make a difference.

I agree that this would also reduce damage, as long as the failure points of the materials was close.

> I am most interested in catastrophic damage. Scratches and bruises can
> usually be fixed when you get home. I am interested in how the strip
> thickness will effect the ultimate strength of the boat. It does not seem
> logical that thinner wood and the same glass would ultimately be stronger.

I had trouble with the idea until someone in another post mentioned toughness. This would, I think more aptly describe this concept, than strength. Chromoly is much stronger than mild steel but when it fails it has very little strength. Mild steel gets stronger as it fails (work hardening). If I was going to build something I knew would eventually have a catastrophic failure (cars), I would build it with the steel, not for strength, but for toughness.

Thankyou for a very interesting discussion, and we haven't even touched on kayak shapes as a component of strength! HaHa. Just kidding, I am just very curious about this idea. Ian