The twin propellers and rudders had protection from grounding. The ship was propelled by two 21-megawatt electric motors coupled to fixed-pitch propellers. She used four Westinghouse geared turbines connected to four diameter propellers. The engines were attached directly to long shafts which drove the propellers. Blade balancers are used on parts such as fans, propellers, and turbines. She had two shafts and two propellers. These craft depend entirely on the variable speed of the motors driving the propellers. The tips of rotating surfaces (such as propellers and rotors) sometimes produce visible contrails. Cargo ships generate high levels of noise due to propellers and diesel engines. Bronze is still commonly used in ship propellers and submerged bearings. Dreadnoughts were propelled by two to four screw propellers. Some types also use steerable propellers or ducted fans. The simplest example is a boat with two separate engines driving two separate propellers. During World War II it manufactured "Pregwood" plastic-impregnated wooden aeroplane propellers and bomb parts. The drums connected via a differential gear to twin contra-rotating propellers.