Testing fin strength

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:

  1. 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’.

  2. 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.


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? 

Method 3: Car jack and personal scales?

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.

You are asking about single fins? … They should not run “tabs”

Running the fin into something/bad glass on aside, I have personally yet to see any modern single fin just snap in half or at the base. 

You are seeking the solution, to a problem that does not exist.       Very much like trying to determine how many angels, can dance on the head of a pin.

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?


Method 4: Luggage scales.

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. 

Method 5: I think this is the one to use:

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.


I’m glad I did not break this fin in the surf…

23kg lateral force snapped it off.

The photo shows the parts of the deliberately snapped fin, above the 2 bits that remained in the fin box when similar fins snapped while surfing. 

The similarity of the fracture lines boosts my confidence that this testing method may have some validity.

Mr. Mik-

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

My thoughts exactly.

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.

I have made fins that float. One of the first I did in '70.

For a center fin box I’ve layed up 11 layers of 6oz for a center spine.

Then applied high density foam to either side.

This what I would call a plug. Once foiled a bit foam is taken away

at the base of the fin at and above the fin tab and several layers

of glass laminated to either side. However if the central spine of the fin is layed

up with some carbon cloth breakage will not be an issue.  In any case

the tab has to be built up with solid laminant for obvious reasons.

Once the fin is foiled and fitted to the fin box a shell is laminated to

the foiled result with a couple layers of glass either side. The more narrow

the spine is the lighter the fin will be, so carbon in the spine will provide strength.

It’s a very time consuming process. There’s a glass-on on the 9’11" attached.

I believe there’s one in the fin archives I did for a 11’6" Lance Carson some time ago.

The customer wanted it to be an inch thick. It was and surfed sooooo smoooooooth.



Stay Stoked, Rich