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Chapter One APPLICATIONS validity of this practice, due to possible rotting of the wood if the GRP encapsulation is porous, but this method of construction has been used successfully in commercial boats for years and offers sufficient advantages so that it is likely to continue. It is desirable to cover plywood floors and bottom girders with at least 0.25 inches (6mm) of GRP on both sides, so that sufficient reserve strength (bending and buckling) remain if the wood rots. Plywood is highly favored for the construction of bulkheads and flats. A facing of GRP is applied for water resistance, but the plywood provides strength and stiffness. Wood has also been used extensively for decks in conjunction with GRP sheathing. This extensive use of wood increases the trawler’s weight above the optimum values, but represents a significant cost saving. The space between the fish hold and the shell is usually foamed in place, which gives an excellent heat-insulation. Many GRP trawlers incorporate concrete in the skeg aft for ballast. This has been required in some cases to provide adequate submergence of the propeller and rudder in light load conditions. Thus, the potential weight savings afforded by GRP is often partially reduced by the requirement for ballast. A reinforced concrete beam may be encapsulated in the keel. The use of concrete can be minimized by proper selection of hull shape. GRP construction is generally credited with reducing the hull structural weight, sometimes as much as 50%. However, this saving has not been realized in these trawlers, since hull scantlings have tended to be heavier than theoretically required to increase hull ruggedness and resistance to damage. In addition, the extensive use of wood in the hull structure and non-integral steel fuel tanks has increased hull weight considerably. In general, it may be stated that when initial expense is of primary importance, wood might be preferred. However, when maintenance costs receive prime consideration, GRP should be chosen. The number of GRP Figure 1-22 Northwest in the Late 1970s [Johannsen, 1985] Typical Trawler Built in the Pacific Figure 1-23 Built in the 1970s in High Number and in Limited Production Today [Johannsen, 1985] 23 Typical Northeast Fishing VesselPDF Image | Marine Componsites
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