Fibre-Reinforced Plastic (FRP or fiberglass reinforced plastic) is a composite material used in boat building. Composite boats can range in size from small speedboats and skiffs to large megayachts up to around 165 feet in length. The composite material is comprised of glass fiber reinforcements (strength elements) and resin (acts as the matrix). By varying resin types and by using different types of reinforcement oriented in different directions the engineer is able to better design a part to better suit its mission.
The stranded fibers used in FRP exhibit excellent tensile strength, particularly for their weight. As the strands only individually work under tension, resin is needed to support and locate the strands. The resin also allows for the colouring of components (as in a gelcoat), and for the protection of the fibers from chemical or abrasive damage.
FRP's closest competitor for building materials is aluminum, which has similar weight characteristics and often cheaper single-unit construction costs.
FRP is an engineered material, allowing for the precision strengthening or lightening of different areas of a vessel by changing the laminate schedule (by thickness, material, or directionality). It's inert nature means it is resistant to corrosive marine environments, i.e., it does not rust or rot (though it does not inhibit fouling). The ability to mould large pieces and ease of bonding via adhesives create watertight vessels, without leak or weep-prone seams.
FRP lacks stiffness compared to alternative construction techniques. It may be prone to vibration, or deflection under load (this can drive perceptions as well; for example, a deck may have a higher load capacity than a comparable aluminum example, but exhibit 'springiness' when walked on - a subjective measure of weakness). If continually loaded, FRP can creep & will suffer more from fatigue than steel or aluminum.