Feb 06

Stiffness, strength and shaping in wakesurf boards

We wanted to talk about some of the considerations when adding shaping features to wakesurf boards. We’ll have some amazing artwork again, so be prepared!

We’ve all bent a popscicle stick to the point of breaking. It’s a pretty uniform shape in terms of thickness and width. So when we bend it to the point of failure (breaking) that point is normally at the point of the greatest curvature. That is to say the ends of the stick are the furthest away from the normal plane or the flat area of the stick, but those ends are really bent much are they? They are still pretty flat, but some point in the middle of the stick is really curved and THAT is where the failure occurs. The only exception being if there is a weak spot on the stick. It could be the wood is weak and not uniform or possibly there is a notch. All things being equal, the failure will occur at the point of greatest curvature from the normal or resting plane.

curvature failure

So the next thing up is, are wakesurf boards uniform in shape? No, obviously we have curved outlines and typically skinny or thinner noses. If we were to change the shape of the outline our popsicle stick to look like the picture below, and then did the same bending test, where do you think it would break?

popsicle 2

It would break at the notches, if we applied equal bending forces at both ends and we’ve intentionally placed the notches off center so that we would know that by changing the shape of the popscile stick we can also change the point of failure.

Now in automotive design, this sort of thing is used to create crumple zones that absorb impact and keep drives and passengers safe in a crash. How does that impact us when we are adding or changing the shape of our wakesurf boards. Exactly the same. In the picture where we notched the imaginary popscicle stick, the notched area will be considerably weaker than surrounding areas and quess what happens when we go to bend it? The weaker area gets the brunt of those forces. The surrounding areas transfer most all of the forces over to the weaker area, which being weaker bends much easier but then also reaches failure quicker.

So uniform shapes and sizes do a couple of things. One is that they help spread the loads being applied uniformly across the entire structure. We also know that we can create thicker and wider structures between the points where we introduce bending forces to help resist those forces. BUT anytime there is a sudden change in the shape, that is it’s not uniform, we create a point of stress. The areas that are thicker or wider are stiffer and so resist those forces. Now that may be what we want, in terms of things like crumple zones in cars and most after market fins are designed to break off rather than destroy a fin box and the board. But is that what we want in the middle of our wakesurf board? No obviously not, the area between our feet is much like the popsicle stick example we used above. Out feet are creating the forces where the bending starts and the point of greatest curvature will be between those two extremes.

So…students of wakesurf board design and composite sandwich structures, see any problem with this?

concave deck skim

Oh yeah! Ok, we’ll talk more about that in tomorrows post! Thanks so much for following along, we really appreciate it!


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