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→Diesel-Electric
Diesel-electric propulsion systems are not new. They have been used in submarines, trains and buses. In a diesel-electric system, the engine drives a high output DC generator. The current from this drives one or more electric motors connected to the propeller shaft. The chief advantage of an electrical drive is increased performance throughout the speed range. This results because the engine can be run at its optimum RPM, while the speed of the boat is regulated by the electrical drive.
Much is said about the higher efficiency of mechanical drives, but electric drives are more or less as efficient as mechanical ones. The efficiency of an electric motor is measured in Watts-out/Watts-in, where Watts out are measured in hp x 746. Generally, modern electric motors (NEMA standard) have an efficiency between 87 and 97%, with larger motors being more efficient. Overall the efficiency will run around 81%, assuming the electrical generator converts 90% of the diesel engine's output into electrical energy and the traction motors convert 90% of this electrical energy back into mechanical energy on the propeller shaft.
Relevant standards are IEEE 112, JEC 37, and IEC 34-2. The main energy loss in electric drives is through energy transformed to heat.<ref>AC Motor Efficiency Guide, Rockwell Automation, http://www.reliance.com/mtr/b7087_5/b7087_5_3.htm</ref> <ref>Advanced Energy, http://www.advancedenergy.org/progressenergy/motor_efficiency.html</ref>