Hi Pete,

Thanks for the plot. It’s good to see that folks are interested in this stuff. Please pardon me if I’m a bit pickey, but you wrote about strength with orientation when the plot shows stiffness with orientation. It is an easy thing to show one while commenting on the other because, as you know, the strength curves have the same general shape. However, the strength difference between 0 and 45 degree orientations is not as great as suggested by the figure, about three times rather than almost ten times.

Regardless of the numbers, you bring up an interesting question – what is the best fiber (cloth) orientation? Here is a very abbreviated version of my take on the topic. First, we need to recognize the two main modes of loading on a kayak. The first is often referred to as global loading, which comes about through hydrostatic forces and is well distributed through the hull and deck (your bending forces, where the boat is a beam). The second is local loading that comes from, in addition to other things, hitting the rock you mentioned. The stresses that result from global loading are small because the loads are distributed over large regions of the boat (the total load is equal to the weight of the paddler, gear, and boat, plus some acceleration factor from waves). The stresses that result from local loading can be very large because they are concentrated in small areas.

In your boat hitting the rock scenario, the local stresses at the point of contact would be much, much greater than the stresses due to twisting. So a more specific question might be, what is the best fiber orientation to sustain local loading, like hitting a rock? The answer depends, to some degree, on the shape of the rock and how you hit it. We could make a number of arguments here, but for the abbreviated version, lets assume that the rock is uniform in shape, perhaps round’ish. Some would argue that in this case, the best way to orient the cloth would be to have many different layers having evenly distributed orientations. But when we look at the deformation (strain) that results, we find that it is uniform (locally). From your stiffness plot, we see that the cloth is stiffest at 0 and 90 degrees. Because we can treat glass/epoxy as a linear elastic material, that means the stresses are much higher in those directions. In addition, because the stiffness plot has a larger difference between 0 and 45 degrees than does the strength plot, the stresses are much, much greater at 0 and 90 degrees. The result is that as long as you have the same stiffness and strength at 0 and 90 degrees, you don’t gain by distributing that material at, say 0 90 +-45 degrees or every 30 degrees, by using more layers of lighter cloth.

Sure, there are a number of explicit and implicit assumptions here. We could discuss them if there is interest.

Sam