> Ian, you would be pretty much correct that the cedar strips add almost
> nothing -- if you're talking about a tensile load only. In other words, if
> you built a lay-up of only fiberglass and epoxy and submitted it to a
> tensile test (a tug-of-war with the panel) against a typical
> cedar-strip/glass/epoxy layup, the wood version would only be slightly
> stronger.

> But a kayak never sees pure tensile loads. The loads we are most concerned
> with are bending loads. Let's say you had 2 different panels; one has 2
> layers of cloth seperated them by 1/8" of air (let's assume we had a
> weightless spacer) and the other is 2 layers of cloth seperated by
> 1/4" of air. Now we subject them to a bending test where 2 ends of
> the panels are supported and we put a weight in the middle. Not to get
> cocky, but I'll bet you my engineering sheepskin that the 1/4" panel
> will be 4 times stronger than the 1/8" panel, even though both panels
> contain the exact same amount of fiberglass. It has to do with the moment
> of inertia of the panel.

> The heavy stuff: The stress in the panel is inversely proportional to the
> moment of inertia. Max stress=(bending moment)*(1/2 panel
> thickness)/(moment of inertia) The moment of inertia is proportional to
> the square of the distance from the neutral axis of the body, which is the
> center of the lay-up. Moment of inertia=(area)*(distance from axis)^2.
> What this means is that if you double the distance between layers, the
> force quadruples. If you quadrupled the distance between panels (say we
> went from 1/8" spacing to 1/2"), the panel would actually be
> 16(four squared) times stronger. In bending.

> George's 1/8" panels will see higher stresses than the typical
> 1/4" panel sees, under similar loads. This is just basic engineering.
> George's claim is that he can apply a stronger fiberglass and epoxy lay-up
> that will withstand these increased forces. And give a substantially
> lighter boat, to boot.

> Dean

Dean,

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 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 .

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)?

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.

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.

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 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.

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.

Very interesting discussion! Ian