The main lightweight materialsThe main lightweight materials used in ships are fibre reinforced plastic (FRP) composites and aluminium alloys. FRP is used in both single-skin and sandwich configurations; in single-skin applications there is usually a system of stiffeners as illustrated in Fig. 1, but unstiffened monocoque solutions are also to be found. (Here we use the term “FRP” to include the special case of glass- reinforced plastics – GRP.) Aluminium alloys are commonly found in welded, stiffened plate configurations and in the form of extruded sections (both open and closed), but sandwich arrangements are also possible. Although not normally considered to be lightweight materials, high strength steels may also be used to reduce weight; these are to be found in stiffened plate and, recently, some sandwich configurations. There is increasing use of mixed solutions in which various materials are combined in one ship or superstructure.Current and potential applications of lightweight materials in ships are mainly related to high speed passenger and car ferries, patrol and rescue craft, smaller naval ships (e.g. mine countermeasure vessels), pleasure craft and sailing yachts. However, they are also used in superstructures of cruise ships and of larger naval ships (e.g. frigates). Furthermore they are used extensively in secondary structures and components for all types of ships, from masts and casings to moveable vehicle ramps and decks.As a rough guide, in the main hull structure, FRP is used for craft with length up to about 50 m, and aluminium for vessels up to about 120 m, while high-strength steel is mainly used for larger vessels. In recent years composites have been rarely used in hulls of new ferries; their use has been mainly confined to patrol/rescue craft, pleasure craft, yachts and naval vessels. The main reason for this is the severe restriction on the use of combustible materials that was introduced in the IMO Code of Safety for High Speed Craft in the 1990s. With new, approved fire protection systems now available, FRP has once more become a viable and safe alternative to aluminium for ferry applications.References 1 and 2 provide a good introduction to marine FRP composites and their applications. Much useful information about marine applications of aluminium alloys is provided in refs. 3 and 4.