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FINDINGS LIBRARY · HULL AND STRUCTURE

Gelcoat stress cracks and spider cracks: what they mean and how to write them up

UPDATED 12 NOVEMBER 2026 · 3 EXAMPLE FINDINGS · 3 SOURCES

Are spider cracks in gelcoat bad? How a surveyor reads stress cracks at fittings and corners, tells them from laminate damage and grades them A, B or C.

AT A GLANCE
Gelcoat stress cracks
USUAL GRADE
C for a single hairline crack or one at a screw hole. B for a group of cracks at a stanchion, cleat or corner that point to flexing. A where the laminate under them is fractured or the deck feels soft at a structural point
FOUND WITH
Looking along the deck and hull in raking light, pressing and sounding around the cracks, and a moisture meter where the deck is cored
JUDGED AGAINST
The cause. West System's technical adviser treats a concentrated group of cracks running the same way as a sign of a more severe problem than a single line
REPAIR COST
No published figure. A hairline crack is ground out and filled. Cracks over flexing or wet core mean structural or core repair, which Power & Motoryacht calls messy and pricey

A gelcoat crack is a symptom. The gelcoat is a hard, thin skin, and when the laminate or the hardware under it moves, it cracks. West System's technical adviser Terry Monville puts it plainly: repair the crack without the cause, and it cracks again. The surveyor's job is to say which kind it is.

Reading the pattern

Pattern What it usually means
A single line crack Thick gelcoat without fibre behind it, cracked in the mould or by later flexing. Monville calls it part of boat ownership
Hairline cracks from a screw hole A self-tapping screw, or no pilot hole or too small a one
Radiating cracks round a stanchion or cleat The fitting flexes. BoatTEST says the hardware should be tightened, with backing blocks used appropriately
A group of cracks running the same way at a corner or hard spot A more severe problem, in Monville's words: flexing against a bulkhead or another hard spot behind the laminate

Pike's two questions are the ones to ask on board: do the cracks spread from a stanchion base that flexes, or from a radius that twists in a seaway?

Cosmetic or structural

Pike draws the line at the laminate. Some cracks stay in the gelcoat; others go deeper, into a laminate that is often cored. A deck that feels a little spongy around the cracks suggests a core problem.

Monville's test needs the gelcoat off: remove it, wipe the area with a wet cloth, and cracked fibreglass shows as white lines in the laminate. A surveyor doesn't grind a client's boat, so the report recommends it where the pattern and the sounding point that way.

BoatTEST adds a winter warning: water that gets into open cracks and freezes widens them.

Gelcoat stress cracks: example findings

The same defect written up at each grade, the way Axsail writes it from what the surveyor says on board. The grade follows what was found, so use these as wording, then grade what you see.

HULL AND STRUCTUREA
Foredeck, around the port bow pulpit stanchion base

Radiating gelcoat cracks up to 60 mm long around the stanchion base. Deck soft underfoot within 150 mm of the base and moisture readings high in the same area. The stanchion moves under hand load.

Recommendation: Do not use the pulpit as a handhold. Remove the base, investigate the core and repair the deck and backing to the yard's specification before the boat is next used.

HULL AND STRUCTUREB
Cockpit coaming, starboard aft corner

A group of parallel stress cracks, 20 to 80 mm long, running the same way along the inside radius of the corner. Laminate sounds sound and readings are normal.

Recommendation: Have the cracks ground out to establish their depth and repaired with reinforcement behind the corner if the laminate is cracked. Within 12 months.

HULL AND STRUCTUREC
Deck, around the screws of the port genoa track

Hairline cracks running from two screw holes, in the gelcoat only. Track secure, deck firm, readings normal.

Recommendation: Open the cracks into a V, fill and gelcoat, and rebed the fastenings at the next maintenance period.

ASafety. Safety or compliance deficiency. Fix before the vessel is next underway.
B12 months. Deficiency that needs timely attention. Fix within about 12 months.
CRoutine. Note or observation, cosmetic or maintenance. Routine maintenance at the owner's convenience.

How it's repaired

  • Minor cracks: open each one into a V down to the fibreglass, fill with thickened epoxy, then sand and apply colour-matched gelcoat. Where flexing caused it, Monville adds glass reinforcement on the back.
  • Stress cracks in the laminate: grind out to a 12 to 1 taper from the deepest point, lay in glass patches, fair and finish.
  • Screw holes in cored decks: Monville seals the core around the hole with epoxy before refitting.
  • Cored decks: replacing core in an existing deck is, in Pike's words, a messy, pricey job best left to professionals.

Pike warns that brush-on crack fillers don't replace doing the job properly.

For crazing, the fine random cracking from temperature, see gelcoat crazing. With Axsail the surveyor says where the cracks are and what the deck feels like around them, and the finding lands with its photo and grade.

Frequently asked

Are spider cracks in gelcoat bad?

It depends how deep they go. Power & Motoryacht's Bill Pike says some don't go beyond the gelcoat, while others go deeper and threaten the laminate, which is often cored. Cracks where the deck feels spongy are the ones to deal with first.

What causes stress cracks in gelcoat?

Flexing. BoatTEST says stress cracks come from the fibreglass or hardware underneath flexing until the gelcoat cracks. West System's Terry Monville adds hard spots such as bulkheads, and screws driven without the right pilot hole.

How do you repair gelcoat stress cracks?

Find and fix the cause first, or the cracks come back. Then open each crack into a V and fill it before gelcoating. Where the laminate is cracked, West System's method grinds it out to a 12 to 1 taper and lays in new glass.

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