I found this awhile back. What causes this, (flex, impact, time) and how concerned should I be. Thanks for the responses.

Where is the location of the crack,is there another crack on the opposite side of the boat?

furthest outside strake ahead of engine bulk head by 15 feet. No matching crack on other side

By the amount of small hazing cracks around it and what looks to be yellowing it appears that it has been developing over a fair amount of time.,not just recent

Also the brown at the bottom indicates there is water behind it

Most likely caused by a impact with something in the water doesn't look like a structural fracture.

GThank you for the response.

Should be lightly ground back until you don't see gel coat anymore . From there you can see if there are fractures in the glass also.

I would look inside the cabin at that spot maybe drill a small test hole in the stringer. Or it could be from a hard spot from something pushing on that spot like a bulk head or fridge mount. Tap on the hull around that spot listen for a change in the sound. Maybe it is just a air pocket. What is that next to it spider cracking in the gel coat. Something is moving further investigate to determine the cause.

I had heard of some structural issues in some BT's but it started above the rubrail and right about mid ship. Saw a pick of one that looked like it wanted to crack in half from the top down. Had not heard of anything from the bottom up though . . . .

I've seen what you are talking about. It happened on a 97 flat deck and had to be repaired at the factory. Split the deck right in front of the engine bay bulkhead about 8 inches on both sides. My boat thank goodness has not shown any stressing in that area.

The issue I noted on my boat is on the outside of a strake and runs vertical. After taking a die grinder tonight to investigate the area, my concerns are less as it appears to be a superficial wound. Only skin/gel deep.

Next question is what should I purchase to fix/seal this back up?

Endeavor... Get a angle grinder with 24grit green disks (I use 2" 3M) grind out effected area. Grind a decent amount of glass away. There will be white cracks in the glass. Grind until those go away. Reglass with chop strand and poly resin unless it's not a normal glass boat (ie, carbon, Kevlar) wet up the sheets one at a time. Stacking them after each on card board. Pull apart the chop strand to get the shape and size don't use sissors. It will bond easier and stronger this way (Wet one layer, stack another, wet it thoroughly, stack another wet it. 3-4 layers should be good for that size unless you grind real deep. Once it cures use the same 3M disks to grind out the access glass. DA sand with 36g then 80 grit. Once FLAT use US COMPOSITE light filler. It's in a tube. Most PPG paint stores carry it. (I'm sure other people use other Brands but this is what we use and have never had one issue. Put a thick coat on it'll make the process faster. Block it down with 36grit. Then 80 grit. But make sure once sanded there is not much filler on it... Make it end up being a thin coat.. If that makes sense. Then 180 DA, then 320 DA. Wet sand with 1000 a large area. Whipe clean with a paint cleaning solovant. I can't remember off hand what I use. It's like "all clean" or something like that. Spray it into a thick paper towel and whipe clean. Don't spray onto the boat then whipe.. We've had issues of fish eyeing when doing it that way. Blend it in. And wah la! Your done!

Now I'm sure there's other ways other places or people would do... But this is how we do it, and what works..

Hope this helps!