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.
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