Electric propulsion with diesels used as prime movers. Diesel-electric drive takes less space than the equivalent direct-drive two-stroke engine allowing the aft section to be slimmer and giving better flow over the propeller. Not only is the diesel-electric drive train lighter than a two-stroke engine, but also its weight can be distributed more evenly. Also there is no need for auxiliary generators.
Diesel-electric (DE) propulsion emerged early in the 1900s but for most of the century was confined to specialist niches.
A much wider application is now enjoyed thanks to the developments in AC drive technology, the central power station concept for propulsion and ship services, and increasing interest in low emissions and propulsion plant redundancy.
More and more vessels are fitted with electric propulsion plant that have succeeded in cruise ships, icebreakers, ferries, shuttle tankers, chemical carriers, and research vessels. The recent Gaz de France LNG tanker project marks another ship type reference for this transmission system.
For offshore vessels it may be selected for flexibility. Where the vessel has to be efficient under numerous operating conditions, it may be that propulsion is only one of several requirements, such as drill ships where the main propulsion system is used in transit, otherwise the power is split between dynamic positioning (DP) and deck power consumption for equipment. Icebreakers are a special case. Here DE propulsion in various forms is a favourite one since it meets the requirements for maximum output at very low speeds. For vessels such as FPSO, the choice of diesel-electric drive is almost automatic, since about 80% of time is spent on production at low power. The remaining time is split between transit, offloading, standby and production at variable loads.