electric propulsion

electric propulsion is basically the driving of a ship's propeller shaft by an electric motor, with the electric power for it being produced in one or more generators driven by steam propulsion, diesel engines using residual fuel oil, or gas turbines. Early electric propulsion systems used direct current but modern systems use alternating current. With early alternating current systems the electric motor speed was changed by changing the speed of the alternator, and the direction was changed by means of switchgear. This meant that the propulsion electrical generation system had to be separate from the system serving the rest of the ship. With a modern alternating current system the same electrical generation plant generates power for the propulsion motors and the rest of the ship. The propulsion motor generally runs at a constant speed and the speed and direction of the ship are changed by means of a controllable pitch propeller.

The French ocean liner Normandie was fitted with electric propulsion, the generators being driven by steam turbines, a type of installation known as turbo-electric. The advantage of this is that there is no need for long intermediate shafts from the turbine, or diesel engine, to the propeller shaft, thus increasing the space available for cargo or other machinery. Also, unlike the conventional system, only the propulsion motor, not the machinery driving it, has to be in line with the propeller shaft. This means that the machinery can be located where convenient and, particularly in the case of passenger ships, it allows for large open space areas in the middle of the ship. The P&O liner Canberra had a turbo-electric propulsion system located at the after end of the ship with boilers and turbines above the propulsion motors.

The USA has had considerably more experience with electrical systems than Europe and has used electric propulsion for many years in many types of ship. The aircraft carriers Lexington and Saratoga, built in 1927, had electric propulsion as did many American-built merchant ships of the period, including the Second World War (1939–45) T2 tankers. Today, electric propulsion is used extensively in cruise ships, the installations being the diesel electric type where a number of medium-speed diesel engines, each driving an alternator, supply electrical power. This electrical power is then used for the propulsion motors and the ship's hotel services, a transformer reducing the voltage of the electrical supply to serve the hotel needs. Large cruise ships need a lot of power for services such as lighting and air conditioning. So some of them, including Queen Mary 2, also have gas turbines which drive alternators to supplement their diesel-driven alternators.

Denis Griffiths