Postby jimh » Wed Aug 28, 2019 12:29 pm
In areas of the hull molds were a rapid curving of the surface takes place, it is common for there to be a build up in the thickness of the gel coat layer. Gel coat resin is not particularly flexible, and in thick areas the resin has a tendency to crack. It is common to see on older Boston Whaler boats that some cracks appear in the gel coat layer in certain areas of the hull which may have some bending stresses on them (such as at the transom) and where the gel coat layer is thicker than elsewhere.
Gel coat applied to flat hull surfaces will generally not be particularly thick, perhaps only 0.020 (or "twenty mils") thick. But if the gel coat layer has been build up to be much thicker, it may exhibit stress cracks.
As long as the crack is confined to the gel coat layer and the underlying laminate and resin layers are not involved, a gel coat crack is mostly a cosmetic flaw. However, a gel coat crack was probably produced by some stress on that area of the hull, so it could be seen as an indicator of some slight flexing.
In the case of a crack that extends for several inches and runs vertically along the edge of the thicker portion of the transom where the hull has embedded reinforcement material, a careful examination should be made of the stiffness and rigidity of the transom. A simple test is to tilt the outboard engine lower unit to an 45-degree angle with the transom, and then use the gear case as a lever arm, pulling up and down on the gear case while observing closely the transom for any indication of movement. On a solid transom there won't be any movement.
