> The panel tests that originally started this discussion don't bear out
> what you are saying. Many of the posts in this discussion keep saying that
> it is a law of composites that the panel will be 4 times stronger with a
> core twice as thick. But in the tests this was not the case and in fact in
> one instance the thicker core failed first. This could have been an
> isolated anomaly, but the fact that the other test panels didn't show this
> increase in strength suggests that it was not.

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.

> I think that there is something else at work here.

> I have been told time and again, that the core is the strength of a
> composite but in the case of 1/4 inch strips it is the core that fails
> first. If the core fails it is still a failure, maybe not a catastrophic
> failure, but a failure .

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.

> The question I am trying to answere is this. If you make the core more
> flexible by making the strip thinner will this put more of the force into
> the epoxy which, won't raise the point of a catastrophic failure (hole
> through the hull), but would eliminate most failures (broken strips and
> delamination)?

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.

> Someone posted that if I wanted to save weight I should use kevlar. One of
> the properties of kevlar is that it will flex more before the fibers break
> than glass will. This is a moot point because most times the fiberglass is
> undamaged in a failure. So even if kevlar was used the strip would
> probably still fail.

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.

> Maybe what you mean by stronger is more rigid. The strips are already too
> rigid for the layup so we would have one of two choices; make the strips
> thinner and more flexible, or make the layup more rigid to prevent the
> current strip from breaking. What I am trying to show is that if we have
> way more minor than catastrophic failures, we should be able to change the
> composite in such a way as to eliminate most failures, the minor ones.

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.

> I was just looking through the archives and came across a post by George
> where he stated he used a layup of kevlar on the inside of his boats. This
> would really take advantage of the more flexable 1/8 inch strip, but
> wouldn't be necessary.

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, 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.

> I think that the forces would be the same on a 1/8 or 1/4 inch composite.
> A loaded 350# kayak hitting a rock gets the same impact either way. But
> the difference would be in the fact that the 1/4 inch strips would break
> before the force could be absorbed. The 1/8 inch could flex more and allow
> the force to be absorbed over a larger area for a longer period of time.

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.

> The weight savings of a 1/8" strip is only a minor bonus. Fewer
> repairs would be more important. A more durable boat would be the biggy.

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.

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.

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 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.