A fibreglass boat is typically moulded in two sections: the hull and the deck. Most of the furniture and machinery is installed inside the open hull before the deck goes on-like filling a box before putting on the lid.
If you stay off the rocks and don’t smash into the dock, the hull has a pretty good life… comfortable in the water and always half in the shade. The deck, on the other hand, is born to a life of abuse. It sits out in the sun like a piece of Nevada desert. It is assaulted by rain, pollution, and feet. It is eviscerated by openings, pierced by hardware, pried by cleat and stanchion.
You might think that to stand up to such treatment, decks are as strongly built as the hull they cover. You’d be wrong. Weight carried low in a boat has little detrimental impact. Boatbuilders can make the hull as thick as they want, but weight carried high reduces stability. A deck must first be light; strength is defined by “strong enough.” As a result, the need for deck repair is far more common than the need for repairs to the hull.
Deck repair can also be more complicated than hull repair (but not necessarily “harder”). While the surface of a hull is flat or uniformly curved and relatively featureless, a deck is a landscape of corners, angles, curvatures, and textures. Damage often extends under deck-mounted hardware. Backside access may be inhibited by a moulded headliner. And to provide stiffness without weight, deck construction generally involves a core.
Stress cracks can be identified by their shape. Typically they run parallel or fan out in starburst pattern. They appear in moulded corners, such as around the perimeter of the cockpit sole or where the deck intersects the cabin sides. There is weakness in the corner. Parallel cracks also show up on either side of bulkheads or other stiffening components attached to the inside surface of the hull or deck. The flexing stresses are concentrated at such hard spots causing the gelcoat, and sometimes the underlying laminate, to crack.
Starburst cracking can also be caused by impact, dropping an anchor or a heavy winch handle on deck. (Exterior impact may instead result in concentric cracks like the pattern of a target.)
Starburst cracks caused by flexing when the movement centres at a point rather than along an edge. The most common starburst cracking extends from beneath stanchion mounts, where literally the deck is levered up around the socket mounting holes.
Starburst cracking can usually be stopped by installing generous backing plates on the underside of the deck beneath the offending hardware to spread the load. Wooden plates are the easiest to fabricate, but stainless steel or bronze are better because of their resistance to crushing. Bevel the edges of the backing plate to avoid causing a hard spot. Polished stainless steel plates with threaded holes can look good.
Hull repair of hard spots.
Hard spots are more common on the hull than the deck, and usually appear where bulkheads attach. Stress cracks around hard spots are likely to return unless you eliminate the hard spot. This typically involves detaching the offending fixture, shaving some material from the edge, then reattaching it mounted on a foam spacer. Realistically, the work required may exceed the benefit, but anytime a bulkhead is detached or a new bulkhead is installed, it should always be mounted with a foam spacer.
Stress cracks related to general laminate weakness, such as those that too often appear around cockpit soles, can be prevented by stiffening the area with additional laminates. In this case you are trying to add stiffness, not strength, which translates into laminate thickness; use fiberglass mat to quickly build additional thickness.
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