Gelcoat is a thin, hard, brittle skin. It doesn't bend like the laminate underneath, so it cracks first and shows where the boat has been hit or where it flexes. Most cracks are only skin deep. Some are the first sign of a laminate that's moving.
Reading the pattern
Safe Skipper's guide gives the patterns. Star or starburst cracks come from impact, with the centre at the point of impact. Linear cracks, often in parallel lines, mean bending or flexing; it names deck edges and the deck to cabin side corner. Stress cracks don't crisscross.
So a star in a cockpit sole is usually a dropped tool. Parallel cracks fanning from a stanchion, a winch or a traveller mean the fitting is working the deck.
How a surveyor checks it
Look in low, raking light along the hull and deck. For each crack, note the pattern and what's nearby, and look at the other side if you can. Press round it and sound it: Safe Skipper separates scratches from cracks that go right through the gelcoat and indicate some structural failure beneath. A dull note or movement means the laminate needs a closer look.
Check the high-load places: round the keel, chainplates, winches, the traveller, the stem and anywhere the boat might have hit something.
What decides the grade
What's under the crack. Moving or soft laminate at a loaded area is an A. Cracks through the gelcoat at a loaded fitting on firm laminate are a B, to find the cause. Local crazing or a small impact star on firm laminate is a C.