When you haven't hit anything...

Don't know about the crack at the base of the trailing edge but the the cracks radiating from the barrel nut are probably from corrosion building up.

I disagree on the bolt bottoming out. Once the bolt has screwed as far as it can go, that's it your done. That's why there is a Philips head on the bolt, so you won't be able to over tighten it. If you do try to over tighten, it will start to strip out Philips head. Now if you modified the bolt with a hex head and tighten it with a ratchet wrench, you could find yourself in trouble, because there is more torque with that. But even then, you would hear the bolt starting to squeak and grind if you over tighten it. I'm thinking corrosion as well within the thread pitch adjustment screw.

Another possibility is maybe your thread pitch screw on the side of your finbox was just slightly out of alignment, but that would have still made you have to force the finbox screw to screw in.

That's simply put Mark, the finbox just got stressed out from usage and just cracked. Time for a new fin.

I doubt the bolt bottomed out (but it is possible), as I take a lot of care of fitting when I get new fins and boards. I also don't think the barrel nut was out of alignment as I have never felt any resistance when doing up the bolt.

Magic Ride said..
Shifu,

How old is that fin? How many years of sailing do you believe you have on that fin?

One year, around 2500 kilometres.

I had one do that ..but mine broke off flush ...I was sailing overpowered ..
never hit anything ..2 week old maui fin......i was out to sea ...it was fun getting in ...
i would say just a faulty fin .bad resin ...maybe the bolt did start a crack that kept going under stress.

Not an expert on fins. However, I can see stress fractures radiating from the barrel nut. Once cracks are formed, water will penetrate further and deeper to wherever it pleases. Of course, a weaken base also means the fin could be flexing around laterally in heavy loading. I would suggest checking the depth of the bolt. Also, it may pay to check in future if there was any rubbish built up at the bottom of the hole. Sand is one possible suspect, I reckon.

I think its just overloading caused by excessive fishy smell. Rather obvioius actually.

Zooming the photo up .i don't reckon its the bolt at all .it has fractures on the top and a long the fin .
its crap resin ...braking under stress..
Its stuffed.

Its a powa box, its supposed to break. There is your problem.

is it a Makani fin? If so, they've sold some advertised as g10 that aren't g10 even though the fin is marked with g10 (apparently they changed the manufacturing process?), and some might have a defect. I've had one labelled as g10 that fractured approx 1cm below and parallel to the board in less than 3 months with tame use.

If it is, give them an email and they should be able to send you a replacement. It was about $25aud for the replacement. I chose to replace it with a different model that was definitely more like a g10 construction which they confirmed before sending it. It's been working well since.