Fiberglass consists of glass fibers embedded in a resin. This combination makes a boat light without compromising strength. Sheet metal screws use a self-drilling tip to create a hole in a material. The threads of the screw cut into the material to hold the screw in place. The self-drilling tip of a sheet metal screw dulls rapidly when it contacts the glass fibers contained in fiberglass. This leads to cracks forming in both the fiberglass and the gel coat covering the fiberglass. Using the proper installation techniques will keep cracks from forming, while allowing you to use sheet metal screws in the hull of your boat.
Match a drill bit from a drill bit set to the shaft size of the sheet metal screw. Secure a drill bit matching the size of the screw shaft that leaves the threads around the screw exposed.
Cover the hull of the fiberglass boat with masking tape where the sheet metal screw will be placed. Mark the location of the hole on the masking tape with a pencil.
Set the drill motor direction to "Reverse." Drill a hole at the pencil-marked location with the drill motor. Remove the drill bit. Secure a drill bit 1/16-inch larger than the drilled hole. Drill a 1/32-inch deep hole at the entry point of the hole drilled through the fiberglass. This relieves stress in the thin gel coat covering the hull of your fiberglass boat.
Coat the threads of a sheet metal screw with silicone caulk.
Thread the screw into the drilled hole by turning it clockwise with a nut runner. Stop turning the screw when the screw's head sits tight to the boat hull and caulk squeezes out around the screw. Wipe excess caulk from the surface of your boat with a rag.
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