Strength and Weight
But even the best gelcoats remain permeable to moisture, and that can cause blistering, Reeves says. Boatbuilders address that problem with a viable skin coat that often employs chopped strand — fiberglass cut into strands — and vinylester resin, which features better water resistance. Workers usually shoot the strands and resin through a "chop gun" into the mold.

Some builders use a chopped-strand mat, a cloth made with fiberglass strands, and apply it by hand with a vinylester resin, says Joe Hartley, lamination superintendent for Four Winns, owned by Genmar, which makes more than a dozen boat brands.

"Fiberglass itself wants to wick," Maverick's Johnson says. "It's made to wick because you want it to absorb all the resin [in the various hull layers]. But at the same time, it wants to wick more water. The laminators must try to achieve the perfect balance of resin to glass while ensuring no trapped air remains in the laminate."

Using chopped strand rather than a more tightly woven or stitched  fiberglass mat, which requires more resin, creates thickness and stiffness with less weight.

On top of the skin coat lie bulk layers of fiberglass and resin. Fiberglass is just what it sounds like — fibers of glass. Most glass used in boats is called E glass, which has specific technical properties. Fiberglass companies take E glass and stitch or weave it into various patterns called biaxial, triaxial, quad-axial and woven roving. To make the first three, companies stitch the glass fibers in two, three or four directions, providing radial strength. To make woven roving, they put the fiberglass strands together in basket-weave fashion.

Some glass companies now combine various types of glass, binding a layer of chopped strand mat to a woven roving, for instance. Builders can choose those combinations or use a chop gun to add a strand layer where needed.

Fiberglass comes in rolls or mats of varying weights. Adding the resin increases weight; woven roving generally requires more resin than the stitched-glass mats, thus it often becomes the heaviest material. Kevlar and carbon fiber materials may also be used. They generally weigh less than fiberglass and have superior strength characteristics but are more expensive.

With interior layers, companies may use resin blends or polyester resin where water permeability may not be an issue.

Depending on the size of the boat and its intended use, the hull may feature three to eight "steps," a step being either a combination layer or single layer of glass and resin, Reeves says.

As each layer is added, workers must either use a roller to squeeze out air or use a vacuum system to suck out the air, Hartley says. "Vacuum bagging or closed molding processes are increasing," he says. "But that's a more expensive way of manufacturing a part."

A boat's bottom usually contains more fiberglass than its sides because the area that impacts the water absorbs tremendous punishment. In specific stress areas, builders use knitted tape made from biaxial or triaxial mat that comes in 6- to 8-inch-wide rolls, Johnson says. Such areas include corners, strakes (ridges in the hull design that create lift), stringers, transom and places where the bunks of a trailer support the boat.

While the builder is constructing the hull inside a mold, a similar mold process is occurring for the deck, which later fits into the hull. While the deck needs strength — to hold the anglers' weight — it usually doesn't need as much rigidity as the hull. Also, areas above the boat's center of gravity need to weigh less than those below. Builders often use a skin coat, followed by a lightweight PVC foam core material encapsulated in a layer of fiberglass. 

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As the first step, a worker sprays gelcoat into a waxed mold. The outer gelcoat layer usually measures about a half millimeter.
At the Cobia factory, several workers roll out the skin coat on the underside of a deck. Skin coats help form a water barrier between the gelcoat and the interior fiberglass.
After the application of a skin coat made from chopped fiberglass and resin, a worker rolls the mixture to saturate the fiberglass so no air is left.
Longitudinal stringers plus cross members form a stiff grid inside the hull. Fiberglass tape — dark bars in the bow — reinforces the hull where it contacts the trailer.