Stress cracks are a commonly occurring phenomenon with reinforced plastic boats.

A lot of boat owners get very upset when they see these cracks, sometimes needlessly, while at other times their fears are well founded.

Clients will often ask about cracks and the surveyor will be expected to provide answers.

This chapter is included not because stress cracks are unsightly and tend to upset people, but because they are often indicators of more serious, underlying problems.

After looking at many thousands of boats with stress cracks, I still have to say that evaluating the import of stress cracks, particularly in the bottom, is not always easy.

There are often times when the surveyor can find no basis or cause for their existence, and thus it is left without an explanation to give. It will be useful to have an understanding of how and why these cracks commonly occur.

The primary reason why stress cracks appear in gel coat is because gel coat is brittle and the laminate to which it is bonded is less so.

Gel coat is basically a resin to which a large amount of coloring pigment has been added, pigments that are oxides of metals like aluminum.

Because reinforced plastic and gel coat are two very different materials, they do neither expand and contract at the same rates when heating and cooling, nor do they have the same degree of elasticity and thus behave differently when flexing.

We can add to this another factor, and that is the intended cured hardness. Gel coats that are harder than others are more durable, but they also have the down side of being more brittle and prone to cracking.

Gel coat cracking is the result of some kind of induced stress. This stress can come from improper design, or it can be the result of imposed stresses from within, without, or both.

It can be the result of aging or faulty application such as excessive catalization or hardeners.

By far, the most common reason for stress cracking is due to improper structure design. Even the finish on a flat panel will crack when bent too much.

When it comes to large, flat panels, cracking usually results from excessive panel flexing or panting, such as on an unsupported hull side.

Cracking also occurs as a result of an engineering phenomenon called a stress riser. A stress riser occurs at point in a panel or hull skin where a normally somewhat flexible area is suddenly made rigid without a means of transferring the imposed stress gradually to the rest of the laminate.

A good example is a strut attachment to the bottom without any kind of doubler. Another example would be a bottom stringer butting hard against the laminate with no fillet to distribute the load. Yet another is a railing stanchion attached to a deck laminate with no fillet or doubler beneath. The end result is that the amount of stress suddenly rises at a certain point and so stress cracks are likely to appear.