Why do I get stress cracks or crazing in my gelcoat? How can I prevent them?

Most stress cracking is purely cosmetic, moisture is unlikely to penetrate to the laminate in the topsides and deck, and can easily be detected with a moisture meter. It is nevertheless important to establish the origins of any crazing in the gel coat to try to prevent it happening. The appearance of the crazing is an important indicator and can indicate the type and location of impact and give an idea of how to minimise the chance of getting more.

Star crazing which appears as concentric circles or half moon shapes are usually the result of an impact on the outside of the hull. These can only be avoided by skill and care of the helmsman and crew.

Star crazing which appears as a star of radiating cracks from a central point are usually the result of an impact on the inside of the hull. This is often a result of poorly secured heavy items of equipment, anchors, outboard engines etc impacting on the inside of hull when the boat is slamming in a seaway. These can be avoided by stowing heavy items carefully, or fitting a cockpit locker or anchor locker with a sacrificial lining of plywood.

Crazing which is a series of near parallel lines occurs where the impact has been some way away from the apparent area of damage. This is because the design of the hull has created a hard point which locally prevents a lightly built laminate from flexing to absorb an impact. It is forced to crease along the hard point and crack the gel coat. This can occur where there is an internal bulkhead bonded to the hull or where the hull has a change of profile which is naturally stiffer than the surrounding area. At the manufacture stage these can be minimised by the use of crush bondings where a trapzoidal strip of closed cell foam is put between the bulkhead and hull and laminated over.