a method of manufacturing boat parts submerged when in use comprises producing a metallic skeleton structure shaped for easiest production by conventional metal - forming methods , and covering this structure in a mold with an adhering layer or coating of a thermoplastic resin or a rubber - based vulcanizable substance . the inner walls of the mold have high finish and the desired streamlined outer shape of the finished product , such as a lower unit of a propulsion system , or a propeller and the like . the coating functionally neutralizes the technologically conditioned and generally not streamlined shapes of the skeleton structure and provides a protection of the skeleton structure against electroerosive corrosion .