HELP: hull stress cracking

WordMan · #1 12-31-2016, 10:29 AM

Long time lurker, first time poster.
I've got some stress cracking that concerns me, but my experience is limited.
Hoping this is minor, but expecting the worst.
Seeking group wisdom, how bad is it, repair methods, etc.

A little history if that helps.
Bought my '71 20SF from a guy on THT 3.5 years ago.
For 2.5 of those years the boat was lift kept.
The last 10 months it was bottom painted and left in the water. It was used regularly the whole time, and is now back to being lift kept.
When I pulled it out it had some blistering, but what concerns me are the long stress cracks along the first chine up from the keel.
The boat has an Armstrong bracket, and a 200 Merc.

Thanks up front

Bushwacker · #2 12-31-2016, 02:40 PM

Do those cracks run most of the length of the hull or are they just localized? If localized, maybe the boat was not properly supported at one time, such that all the weight was concentrated at one location, which could cause a local one-time over-stress situation. Gel coat is very brittle and gets even more so with age, so it doesn't take much stress to cause very tight cracks. I've seen 60's vintage boats with hundreds of tight craze cracks just in the gel coat due to aging. I understand Carl Moesly intentionally built the SeaCraft 21 prototype with about half the laminate he intended to use for production, and then used gel coat cracks to identify the high stress locations!

If you have a structural defect like a detached stringer, you'd get repetitive flexing which would cause the crack to grow, both in length and width. If they don't grow in length or width, it may mean that a one-time over-stress was relieved by the cracks, and there is no mechanism currently present to propagate the crack. Such cracks are best repaired by routing out with a V-shaped tool and then filled with an epoxy filler, which is much more ductile than polyester resin.

WordMan · #3 12-31-2016, 06:28 PM

Thanks Bushwacker. The cracks don't exactly go the full length, but they are longer than I would call localized. Approximately from the fishbox drain to just forward of the stern end. The pictures are of the starboard side where the cracks are more pronounced, but they are present on the port as well. Same Chine.
I can only see the inside edge of the stringers. That side at least does not appear detached.
Would you route and fill as you suggest, and monitor, or would you tear into the boat more attempting to locate the cause?

WordMan · #4 12-31-2016, 06:33 PM

I am acquainted with the West System Epoxy. The wife and I repaired and painted the top side two years ago. Approximately how big is the routed "v" you suggest? Can I dremel with a small bit?

cdavisdb · #5 12-31-2016, 06:47 PM

If you are willing to wait and see, you can monitor crack growth by using a sharpie to put a dot at the end of every crack. If it grows, you can see it.

Bushwacker · #6 12-31-2016, 07:02 PM

I'd be inclined to mark the ends of the crack with a permanent marker and then use the boat for awhile to see if the cracks grow. If they don't grow, I'd consider them just cosmetic. The V only needs to go down to the bottom of the crack and width doesn't need to be more than 2X the depth, which is probably the bottom of the gel coat. A Dremel tool would work, or you could just drag a sharpened V-shaped punch type can opener or file along the crack to open it up.

kmoose · #7 12-31-2016, 10:04 PM

If that is bottom paint the cracks could be more superficial then they appear and could be confined to the paint and barrier coat. The weeping seen could be from moisture underneath the bottom paint were some delamination has occurred between the paint layers and not be in the gel coat at all. It all depends on how thick or how many coats are on it. My hull bottom had been sprayed with gel coat and developed similar cracks. I sanded it down in a small area and found the original gelcoat underneath in good condition with no cracks.

NoBones · #8 12-31-2016, 10:56 PM

They are just old gelcoat stress cracks....

My ole' 1970 20 SF had them all over the place.

Nothing to be worried about.

If you ever decide to give the old girl a doo, they can be sanded down
before you re gel or Awl grip!

After 25 + years after my re-do there are NO signs of cracking.