It looks like the bolts became loose which caused them to ride up against the edge of the holes. A properly designed shear joint will take all of the shear in friction, in this case between the chain plate and the inside of the hull. If the bolts are not torqued properly, the load will be taken by the bolts riding up against the side of the hole which can cause stress cracking.
At this point, I'd take it apart and inspect the fiberglass from the inside. If the glass is damaged or shows any evidence of cracking (not just the gelcoat on the outside) then it will need to be repaired.
Yes, the bolts attaching the chainplate to the hull look loose. Definitely not snugged down. That's not the way it was meant to be, for sure.
It's been a long time since I took a course in Mechanics, so what I'm about to write may be off, so please excuse me if I'm wrong
It's not clear to me that chainplates-to-hull joints function primarily as slip-critical joints. I've worked on bulkheads attached to fiberglass with bolts, and they exhibit wear patterns consistent with bearing-type joints.
Rigger Brion Toss' book
The Rigger's Apprentice discusses chainplate design, an he does it in the context of a bearing-type joint. His book offers skantlings for calculating the bearing surfaces between the chainplate and the bolts and shroud pins. He doesn't address the requirement of sizing the faying (contact) area of the chainplate-to-hull joint at all. His book doesn't mention prepping the faying surfaces for a slip-critcal joint, nor the force required to create the friction. He's an acknowleged expert in the field, so if it functions primarily as a slip-critical joint why doesn't he mention the surface friction between the chainplate and the hull?
Here's an alternative, hypothesis about the cracks in the pictures, in the context of a bearing-type joint. The chainplate to hull bolts weren't snugged down, allowing the chainplate bolts to be misaligned with the holes in the hull. The chainplate was free to move up and down a fraction of a millimeter or two with each wave and gust and tack. This lead to point loading of the bolts laminate on the top inner edges of the holes, deforming the laminate. Over thousands of cycles, the laminate flexed and fatigued, and cracked.
In any case, if that were my boat, I'd remove the chain plates and inspect the laminate for cracks. If the gelcoat is cracked like that, the laminate has been flexing in that exact location enough to worry me. If I found any cracks in the laminate, I'd grind out the damaged area and replace it. It would be a good time to remove the chainplate and inspect it for crevice corrosion too. YMMV.
