How could fin strength be tested without messing up surf sessions and endangering yourself and others?
If a fin is too weak, it will snap off just about the fin box, usually during a bottom turn. Correct me if I’m wrong, please.
If such a sudden fin failure occurs in a crowd, like at the Superbank, then injury to other surfers is likely, and possibly severe.
So how much lateral force must a fin be able to withstand? And how can it be tested and measured?
I suppose one way to answer the question is this: The fin should snap off before the fin box gets ripped out. But that never happens in my experience, unless you fall onto the fin or hit the bottom, or maybe run over another board or surfer.
Testing methods I’m pondering at the moment are these:
Clamp fin base in a vise, then push (or hang weights, or gradually fill a large container with water) until the fin snaps off, or until it withstands a force so great that you define it as ‘strong enough’.
Go surfing with the fin. The problem with that approach is that the fin will break at a very inconvenient moment, like during the most powerful bottom turn of your life, or when the waves are big and you might not have the strength left to get out the back again after sticking a new fin in the board. Not to mention the difficulties of surfing a finless board back to shore safely, particularly if there are rocks, swimmers, surf boats or anything else that might get in the way.
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So, what do you think is the best way to test fins (particularly single fins) in the ‘dry dock’ to determine if they are seaworthy?
Fin tab in vise, then mount a hydraulic jack with a set of scales in between jack and fin, so that it pushes the fin from the side until it snaps off.
Leverage will need to be taken into account, it is extremely important to consider at what distance from the fin tab the lateral force is being applied. I assume that the ‘centre of fin area’ would be the best spot for it.
Maybe not many surfers have experienced the problem of snapping fins, but I have.
Two fins have snapped off during bottom turns. And yes, I’m talking single fins with a box.
It all depends on how far you push it with newer, lighter fin production methods.
When I started to foil a fin out of a wood / cloth / epoxy lamination that I have made, I immediately found that method unacceptable due to the immense dust production and wastage of material involved.
And for various reasons, I want a fin that floats in water.
How many kg or pounds of lateral pressure do you think a large single fin needs to be able to tolerate?
I have these 50kg luggage scales that might be useful, but I doubt 50kg is enough.
Maybe 2 luggage scales in parallel, pulled by a ratchet operated cargo strap?
I could do a destructive test on one of my many unused fibreglass fins, but I think this will not neccessarily be helpful, because they are much stronger than they need to be. They always break the box out of the board, which I find highly undesirable.
Break the fin box out of an old but still sturdy board by applying force to a standard, very strong glass lamination fin. The result of this test would be the amount of ‘lateral force resistance’ that a fin does not need to exceed, because the box would snap out anyway.
Wow, that is a fully committed test. I wonder how it would have done with fiberglass cloth on the outside?
Have you thought about putting your broken molded fin bases and one of the 3D printed ones crosswise in a vise and test them against each other for a comparison?
When you say 23kg, is that just the weight used? Where/how was the weight applied?
What are the dimensions of the molded fin (foiled base, foiled depth, foiled thickness)?
the chance of snapping a glass fiber fin is somewhere between slim and nil, I have seen MANY glass fins with stress fractures from flexing beyond the elasticity of resin and glass.
The glass on will either tear from the bottom lamination or break out the box on a removable, Lexan, Polypro and Polycarbonate will also snap in two especially after age and exposure to prolonged UV
The fin that snapped was cast only 5 days earlier, from West epoxy 105 + 207 with about the same volume of Q-cel added and a few short silk ‘chopped strands’. No reinforcement in the fin tab area, just the same material for the entire fin. It may not have reached full strength at the time of the test, it’s been around 19-20degC in the garage.
I put the fin tab in a vise, put the loop of a digital 50kg luggage scale around the middle of the fin, then pulled with one hand while bracing against the vise with the other.
The photo shows a similar laminated fibreglass fin before tesing. I was wearing a welding glove in case it did snap off with sharp edges. It did not break at 50kg, but it felt like the fin box would have broken. I might be wrong about that. When actually pulling on the scale, the loop would have been higher up, around the area where the forward knuckle of the fin is.
I don’t have a suitable surfboard for destructive fin box testing at the moment.
