The early "electric vessels" were not 100 percent electric, but rather diesel /electric hybrids. A diesel engine turns a generator to produce electricity to power the electric motors. The electric motors themselves are connected to the propellers. This three-step process of power transference from diesel to electric may seem overly complicated, but a good reason exists for this. An electric motor produces a great deal of torque at low rpm, and this is needed in a ship. According to Richard O. Aichele, reporting in "Professional Mariner" magazine, two large cargo ships, the MV Midnight Sun and MV North Star, are diesel / electric hybrids. The ships were built from 2002 to 2003.
Diesel power is not the only source of power to produce electricity to drive electric motors. For example, many modern submarines use nuclear power to produce steam. The steam drives turbines, which in turn drive generators. Other sources of power produce electricity as well. Breanne Wagner of "National Defense" magazine reported in 2007 that the United States Navy's Office of Naval Research (ONR) is investigating fuel cells to produce electricity. Fuel cells produce electricity by a chemical process. When oxygen combines with hydrogen, the chemical process produces electricity. Fuel cells draw off the electricity to charge batteries.
In 2011, ships can use superconducting electric motors. A superconducting motor is cooled almost to absolute zero, which is minus 459 degrees Fahrenheit. At this temperature, all motors lose internal resistance. The end result is that a very small motor can produce a great deal of torque. Bichlien Hoang and Ashley Caudill of the International Institute of Electrical and Electronics Engineers (IEEE) reported that a superconducting motor is 70 percent smaller than a conventional motor producing the same amount of torque.
Because modern electric motors are compact, ship designers are "freed up" to place the motors in different areas of the ship. In a conventional diesel-powered ship, the engine must be placed near the back and connected to a transmission, which in turn drives a propeller shaft. With an electric ship, the motors can be placed in nacelles, completely outside of the ship. "Nacelle" is another word for a waterproof pod. Hoang and Caudill further reported that with small superconducting motors, extra room is freed up for cargo or extra passenger compartments.