Need some advice on how to fix this...

[Bushwacker]

I agree that's a job I'd only want to do once! I think Strick had to fix a lot of spider cracks on his 20, so you could probably follow his lead. Not sure he used epoxy (maybe vinylester?), but I'd stick with epoxy due to higher bond strength.

Saw my neighbor this morning, and he said those cracks running across the side decks could be caused by a couple of things, one of them being an impact to the gunnel from dock walloping. Deck would try to bulge upward locally adjacent to the impact, so you'd get a membrane type stress that would produce those cracks. He confirmed that max stress direction is always perpendicular to the crack. There does appear to be some impact marks along the rub rail in the pictures, but don't know if that pattern repeats at every place there is a side deck crack. If cracks are due to impact loads, he said what you've proposed would fix it, provided the new glass is bonded direct to the old glass, not the old brittle gel coat. Might want to wrap the glass up into the coaming if the cracks go up there, and definitely around the toe rail. Sounds like lots of sanding involved!

The other possibility could be a buckling failure, since the side deck is probably not nearly as stiff as the coaming and the overlapped toe rail/hull joint. This is what happens to an I-beam where the vertical section or web of the beam is much thinner than the top and bottom plates. As you increase load on a beam like that, the web tends to buckle well before you reach max load capability of top & bottom plates. Same thing can happen to stringers if they're too thin or are not braced by enough bulkheads. In this case, the I-beam is shaped more like a Z laying on it's side, but the basic issue is the same. The fix for that would be to laminate a core underneath the side decks; he said it's important to taper the thickness where the core stops, otherwise it'll just crack where the core stops because of the sudden change is stiffness.