Electrical propulsion was used extensively during the days of Second World War, when the ships had steam turbines as their prime movers, along with electrical drives and large gearboxes. However, nowadays most electrical drives have medium or high speed diesel engines as their prime movers.The only disadvantage of the electrical drive is that it is extremely expensive in the first cost when compared with geared drive. This is mainly because of lower mechanical efficiency, which leads to more fuel consumption and cost.
In earlier times, DC motors were used with the electrical drives and the ships used to have completely separate electrical system for propulsion and other purposes. But after the invention of marine type thyristor converters the ships are able to connect all their machineries to one single electrical system, just as in a power station. Thus the power for the propulsion system, marine auxiliary machineries, and ship’s hotel load all comes from a common energy pool. Generators are also used with the main engine to get the near peak efficiency.
Also, combining all the electrical power sources into one system has drastically helped in reducing the extra costs and method is chosen for almost all the ships now. The system is suitable especially for ships with high power requirements such as large cruise liners and specialist ships such as research vessels, ice breakers, fish factory ships, oil production and drilling vessels, cable laying ships etc. An electrical drive is thus advantageous for a ship that has a large non propulsion electrical load or for a ship wherein the number of propulsion devices are installed throughout the ship e.g. dynamically positioned offshore vessel.
Apart for this, advantages of electrical propulsion also include drastic reduction in noise and vibration levels, power for the occasional use of bow thrusters without the help of any other power source, and smooth operate on very low speeds with high level of reliability. Moreover, an electrical propulsion system can be easily maintained and repaired. It provides the facility of using all machineries on one fuel type. The requirement of spares is also absolutely low. The system can work on low grade fuels, increasing the cost efficiency of the whole operation.
