**2.1 FRP hull**

Hull construction using FRP is very common for speed boats, small to medium size (8–80 m) patrolling boats, research ships for acoustic and underwater mapping studies, coastal ships, corvettes, etc. [1–4]. A very comprehensive list of literature is given by Galanis [3] in his M.S. thesis. One very interesting naval ship is mine countermeasure vessel (MCMV) which is about 60 m long [1]. MCMV uses passive magnetic sensors to detect underwater mines, which use a magnetic sensor to trigger the mine. Therefore, the ship as such should not have any magnetic signature. Conventional hull material of MCMV is nonmagnetic steel. However, FRP is preferred because of lightweight and corrosion-free nature in addition to nonmagnetic character. Miller [4] reported one such MCMV of Royal Navy (U.K.) made of GFRP way back in 1973 and a bigger one (60 m long) in 1980s. FRP is being used since World War II by the United States in noncritical areas and small boats [2]. Till now, large commercial or Naval ships such as Frigates, etc., are not made with FRP. The FRP hull of ships of 60–80-m size is a sandwich construction with thick FRP skin and a foam core. Thickness of the FRP skin on both sides of the foam core can be 8–10 mm each, and the foam core can be 50–80 mm thick. Previously PVC foam core was very common. However, with the large variety of polyurethane foam available today, even fire-retardant type including polyisocyanurate-modified polyurethane [5], the scope and ease of foam filling in between two hull panels have facilitated production system, and also large seamless foam core is easily made by foam spray machine.

Generally, unsaturated polyester made with isophthalic acid and neopentyl glycol and epoxy-based vinyl ester resins is widely used for marine boats and ship hull. Both of these resins are cross-linked by styrene monomer to form a thermosetting polymer.

FRP hull of boats of maximum 8 meters contains six types of fiber and fabric layers arranged in a sequence and requires minimum 25 layers of reinforcing mats and fabrics. The different fiber-based layers are random chopped strand mat, 300–450 GSM, woven roving mat 400–600 GSM, fabric of different thicknesses (approximately 0.25 mm), core mat of 1 mm and 3 mm thickness, and a 1 mm skin layer of the resin with particulate fillers (titanium dioxide, aerosil, barytes, etc.). Aerosil (fumed silica) is used with the resin to make it more impermeable to water. However, aerosil makes the resin somewhat thixotropic. Due to large size, and to

make seamless hull, wooden mold is first made, and hand lay-up technique is used for fabrication. In an elaborate arrangement, vacuum bagging or vacuum-assisted resin infusion can be used for at least small boats. Vacuum-assisted molding can make composite with about 70% fiber and 30% resin, which is obviously advantageous for strength.
