This is a Select Viper Speed fin that I bought second hand.

Upon inspection I noticed the hairline crack that you can see in the attached picture, and it looks to me like that's a pretty serious crack that could deposit my fin into the water unwantedly pretty soon...

Is it serious like I think it is? And is there a way to fix this?

This fin took 2 months to arrive from my country to Boracay where I currently live so if I would order another fin the season would already be over. So I need to make things work with this fin.

Any tips? How can I fix it or at least make it last longer?

Oh yes. I had exactly the same problem.

For my case it was so worn away that the brass nut inside could just fall out. Everything around it was so grinded away because of wear. And ontop of that was a hairline crack just like yours.

What i did is mix some epoxy. I think it was some kind of metal filler 2 component epoxy. I then carefully put the epoxy around the nut and smeared it over the cracks aswell. Eventually i got the nut covered and some of it between the sides of the nut and fin body.
For your case i think i would dremel away a little bit around the cracks and nuts to make more binding surface for the epoxy.

So far i have surfed 2 seasons and even hit a rock ever so slightly and it hasn't fallen off.

I've read that some racing people even tighten their fin bolts with drill & wrenches and whatnot. But I think doing this for a powerbox type of fin is insanity. Maybe they were running some special racing fins.

But just keep the tightness reasonable and it should be ok.

Ah ok, the nut inside my fin is still sitting tight and when I put a bit of pressure on the fin (by hand when it's not in the board) everything still feels solid.

Doesn't sound too bad after all, considering you went on for 2 seasons with a worse case :)

I'll bring it to a surf center tomorrow, see what they can do with it. Thanks for your reply.

I would not worry about it at all - If you put a rubber fin bolt rubber under the stainless washer just nip it up rather than overtightening and you will be fine. Order a spare fin at some point and then do a proper repair in the off season if you wish.

Looks like the crack is confined to the base that is likely, from what I can see in the pictures, moulded around the blade of the fin. Are there any obvious cracks appearing in the blade?

The blade looks 100% intact, I don't see any damage there. It's just the base like you say that is cracked.

As long as it's in the board, the box is stopping the cracks from opening up, so you shouldn't lose the fin. However if another crack develops, (and it's not very likely), the nut could come of of the fin, then you'd lose it.

I'd grind away a mm or so under the nut, and fill that up with fiberglass and epoxy, that'll hold the head together. But then I just love fixing things.
Just make sure you don't get any epoxy in the thread.

The crack happened as a result of the fin being way over tightened by someone, I've seen this before.
But once in the fin box, assuming it's a nice snug fit, there is no place that it can go. The only way you'd loose the fin is if you were to break it off on a rock, or smack it with a sludge hammer, or throw it off a high cliff.

"The only way you'd loose the fin is if you were to break it off on a rock, or smack it with a sludge hammer, or throw it off a high cliff."

That is in order from least likely to most likely I assume :)

I actually had a crack like this before that was less severe, then one day I hit a rock at full speed... It shaved my fin clean off. I didn't feel a thing, just heard a clunky sound and then had a spinnout.

^^^ if you mean the whole fin, then that is designed to happen. The brass barrel nut has a loose fit with the screw so the threads break out easily.

Ah ok yeah makes sense, better my fin them my board and me flying a 100 yards. Not sure if it was whole fin including the nut though, don't remember