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J. Mar. Sci. Eng. 2020, 8, 26 6 of 28 structure manufactured by redesigning the shape to reduce electromagnetic interference, with an additional 10/20% weight reduction compared to the steel masts. The ITM improved the stealth capability of the warship and provided a better environmental and electromagnetic shielding with respect to conventional steel antennas. ITM technology has been installed on ships since 2011 [13]. In Germany, hollow shafts are made by winding multiple layers of continuous glass and carbon filaments immersed in epoxy resin on wood/aluminum spindles [13]. Propellers in ships and submarines are usually made of nickel-aluminum-bronze (NAB) alloy, due to the outstanding corrosion resistance and high yield strength. On the other hand, the manufacturing of propeller blades in NAB alloy is expensive due to their complex shape and some issues related to the poor acoustic damping properties leading to excessive vibration noise are unsolved. These limitations pushed the naval engineers to evaluate the feasibility of alternative materials to NAB alloys to fabricate propeller blades, being stainless steel, titanium alloys and FRP the most notable candidates [4,14]. Composite propeller blades are usually made with glass or carbon fibers laminate, whereas a thin layer of polyurethane or NAB or stainless steel can be adopted to reduce impact damages to the blade. The FRP blades are generally glued or fastened to the metal hub of the propeller, although composite based hubs have also been realized. Early composite propellers, up to 2 m in diameter, were tested in Soviet fishing boats during the 1960s, then they have been introduced in large commercial ships in the early 1970s reaching diameters of approximately 6 m. In the same years, prototype applications of such components were tested in hovercrafts. Composite propellers reported similar performances as metal ones in terms of navigation speed, fuel consumption, engine workload, and life cycle. Additionally, they reduced the amplitude of the resonance vibrations in the engine and transmission shaft by about 25%, with consequent reduction of hull vibrations and noise. On the other hand, composite propellers present higher manufacturing cost, large deviations of the blade tip and lower impact resistance than those made of NAB [5]. Apart from masts and propellers, some secondary elements, especially in the warships, are nowadays produced replacing metal with polymer composites. Some examples are rudders, bulkheads, decks and hatches, discharge funnels, protection systems and shields, engine components, and heat exchangers. 2.3. Materials for Naval Applications 2.3.1. Resins for Matrices Boatbuilders require resins that have good thermal and mechanical properties and, at the same time, can be easily processed during the manufacturing of composite parts. Suitable resins should be easy to laminate, with room temperature curing, and compatibility with the reinforcing materials. Vinyl esters have mechanical properties and prices that are positioned halfway between polyesters and epoxies. They show satisfactory affinity with polyesters due to the presence of ester groups and, similarly to polyesters, vinyl esters required a catalyst and accelerator for curing at room temperature [8,15]. Vinyl esters satisfactory withstand chemical corrosion due to less reactive sites and exhibit an acceptable behavior with respect to absorption and hydrolytic attack. Due to these reasons, they represent an attractive choice to be used in boat hulls [1–3,11,15]. A vinyl ester-based skin on a glass/polyester laminate is able to prevent the osmotic blistering induced by hydrolysis. Boatbuilders or owners can adopt this protection instead of the more expensive epoxies, even if these are a more effective barrier against humidity. Vinyl esters are more flexible and have higher hardness than polyesters. Therefore, if used as a matrix in an FRP laminate, they better withstand the effects of fatigue causing failure in hulls and decks. The vinyl esters have a relative higher working temperature, up to approximately 200 ◦C. British Columbian West Bay SonShip first promoted this transition replacing polyester resins with vinyl esters for the production of GRP components in their motor yachts. Vinyl ester is increasingly used for the construction of largerPDF Image | Marine Application of Fiber Reinforced Composites
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