I've been converting this old tuttle fin to powerbox. I didn't use it much when it was a tuttle and don't remember hitting anything with it. Tonight i noticed this "crack" near the head. It look like just s fine crack in the finish coat, but I'm not sure. Could it be something more serious? Hate to have put in all that work on converting it only to find that it would be unsafe to use.
on a more serious note... probably just a fracture in the top coat. You could try and slightly but very carefully sand it back to see how deep the fracture is
I'd try some fine sandpaper/wet&dry over it, see how deep it is.
Could put the base in a vice and load the fin up, see if the crack opens up at all.
Looks a bit suss to me, I wouldn't be going out a long way with it like that, unless you're sure it's superficial
Looks like just the finish coat as it appears some more has come away from just above the crack.
Try a light sand.
Maybe try marking the ends to see if it's growing when you use it.
Thanks for your responses guys.
I sanded back the finish coat and the results are not good. I don't think i can trust this fin.
You haven''t sanded right down below the gelcoat / paint yet so maybe all is not lost?
I'd just
(1) drill a 1mm hole at each end of the crack (stops it going further) and drill maybe halfway thru the fin. If the crack was deeper than halfway it would surely have failed totally?
(2) grind it out the crack in a V shape say 5mm wide and 1/3 to 1/2 fin depth
(3) fill with epoxy resin and glass. Lots of glass and not much resin, as long as the glass is wet out properly
(4) sand and finish
(5) sell it to some schmuck 
no seriously if the crack can't extend and you have removed the cracked material and replaced with resin and fibre it should be fine for reasonable close inshore use.
If it was mine, I'd cut it off at the crack and rebox it, just ends up a bit shorter.
If you want to fix it, I disagree with Mark, you need to go much wider. I'd go 30 or 40mm either side of the crack, in other words all the way thru the base. All the tension forces are right where that crack is, at the surface, and around the base. a narrow bandage of 5mm is just not going to have enough grip on the fin to be any good. But he is right in making it a "V" shape, only a much shallower "V". A lot of work, to get it back to a nice smooth foil.
Much easier to rebox, as long as you can use a smaller fin.
Ahhh but I didn't suggest a bandage, I suggested a fill.
But I do prefer your amputation idea more 
Air bubbles in the resin are the cause of the cracks; very hard to spot in a painted fin..that fin doesent look that bad, buy a new one if you are worried to much about it,otherwise if you are stranded with a snapped fin, tie your harness through the back strap and get back craig masonville style
I got a dodgy fin and am going to the speed challenge at burrum heads today.I have got a lot of surface cracks in it but it is just the coating i think!
I sort of worked out that my camel waterpack can carry a spare fin and a screw driver in it so i am going to use the fin till it breaks then attempt to change the fin either in deep water or where ever it lets go!!!!
