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Creating the Mold

By Peter Frederiksen

Workers finish a hand-layup in an open mold.

Designs and building techniques have changed but the mold remains the key to a fiberglass boat’s construction, appearance, and performance. Manufacturing the mold begins by building the plug. A flying bridge plug, for example, can be made from wood that is shaped to resemble the actual bridge with its side wings, integral seating, and other features. By hand, this is pretty artistic work and requires a sharply trained eye and feel for the part. The plug’s shell is covered with polyester resin and 10-ounce fiberglass cloth. Five or six layers of polyester resin follow with a final coat of primer to give a good sanding base. When it is prepared for making the mold, the plug is covered with release wax, gelcoat, and resin-wetted fiberglass laminations. After sufficient curing time, the outer shell is removed to reveal the form of the female mold. The plug is only used for this application one time and then usually is destroyed as the female mold will now be the source of future parts. To make the actual fiberglass component, the mold then receives ample coats of release wax, gelcoat, which imparts the color to the part, and as many fiberglass laminates as the design parameters call for. After the part cures, it is pulled from the mold and is ready for the construction process. A well-constructed fiberglass mold can be used for many years with occasional refurbishing and polish. This provides an economy of scale, which is another reason why fiberglass boats can be built to accommodate various budgets and lifestyles. A hull is pulled from the mold, ready for the next stage in the build process.