After the rather experimental applications of battery driven electric propulsion at the end of the 19th century took place in Russia and Germany, the first generation electric propulsion was taken into use in the 1920’s as a result of the strong competence of reducing transatlantic crossing times for passenger liners. At that time, the high propulsion power demand could only be achieved by turbo-electric machinery. “S/S Normandie” was one of the most renowned.
Steam turbine generators provided electric power that was used to drive the 29MW synchronous electrical motors on each of the four screw shafts. The rotational speed was given by the electrical frequency of the generators. The generators would normally run one propulsion motor each, but there were also possibility for feeding two propulsion motors from each generator for cruising at lower speeds.
With the introduction of high efficient and economically favorable diesel engines in the middle of the 20th century, steam turbine technology and electric propulsion more or less disappeared from merchant marine vessels until the 1980’s.
The development of variable speed electric drives, first by the AC/DC rectifier (Silicon Controlled Rectifier –SCR) in the 1970’s and the AC/AC converters in the early 1980’s enabled the power plant based electric propulsion system, which is typical for the second generation electric propulsion.
A fixed voltage and frequency power plant consisting of a number of generator-sets feeding to the same network was supplying the propulsion as well as the hotel and auxiliary power. The propulsion control was done by speed control of the fixed pitch propellers (FPP).
These solutions were firstly used in special vessels like survey ships and icebreakers, but also in cruise vessels. “S/S Queen Elizabeth II” was converted to electric propulsion in the mid 1980’s, and later followed the Fantasy and Princess class cruise vessels, several DP vessels, and shuttle tankers.
Notice that in direct driven diesel propulsion the thrust is normally controlled by a hydraulic system varying the propeller pitch
angle. This is denoted as controllable pitch propellers (CPP).
Podded propulsion was introduced in early 1990’s where the electric motor is installed directly on the fixed pitch propeller shaft in a submerged, rotateable pod. While this concept was originally developed to enhance the performance of icebreakers, it was early found to have additional benefits on hydrodynamic efficiency and maneuverability.
After the fist application in a cruise liner, “M/S Elation”, the advantages were so convincing that podded propulsion almost over night became a standard on new cruise liners.