The greatest number of boats today are molded from composite plastic materials. The mold is a cavity having the exact shape of the boat. Construction of the boat involves (1) coating the interior of the cavity with a liquid plastic; (2) laying on reinforcing material in the form of a cloth or mat of fibers; (3) saturating the reinforcing material with liquid plastic; (4) possibly applying a layer of lightweight core material; and (5) applying more reinforcing and plastic. The plastic hardens under the action of a catalyst that has been mixed with it; heat may be applied to hasten the hardening action. The result is a strong and relatively lightweight hull that has been built quickly and without the necessity for great skill on the part of builders.
In many cases, interior structure, such as bulkheads and foundations for an engine, is inserted while the hull is in the mold. The interior structure is then bonded to the hull by the plastic and its reinforcement.
The plastic used is generally a resin of the polyester type. Epoxy and vinyl ester resins are also used. The reinforcing material is often a glass fiber, giving rise to the generic term fiberglass boat. Structures of successively greater strength and stiffness for a given weight can be produced with Kevlar or carbon fiber reinforcement.
A core is required to give the composite structure the thickness needed for adequate stiffness and to make it resistant to puncture. If the part being molded is a flat sheet, such as an interior bulkhead, the core is likely to be a sheet of plywood. If a low-weight bulkhead is required, aluminum formed into a honeycomb is a likely core. Sheets of balsa wood or PVC (polyvinylchloride) plastic can also be used where lightness is important. For the sharp curvature common to boat hulls, the core sheets are provided with the material cut into small squares and attached to a cloth sheet. With balsa, they are arranged with the grain of the wood perpendicular to the sheet; the final structure then has a high resistance to impacts and crushing loads because of the natural strength of wood in the direction of its fibers.