gas-turbine engines

gas-turbine engines are derived from the aircraft engine and burn expensive high-quality fuel such as kerosene or gas oil, not the residual oil which the modern marine diesel uses. A ship's main turbine unit generates high-temperature gas which drives a number of turbine wheels, and these act as its propulsion. The wheels are connected to a propeller shaft by a gearbox which reduces the shaft speed, while the ship's speed and direction is changed by a controllable pitch propeller. This is the type of arrangement often used in gas-turbine-driven naval vessels, and most warships in the Royal Navy and US Navy which are not nuclear powered are driven by them because of the gas-turbine plant's high power to weight/size. The high cost of the fuel is not the prime concern with a naval vessel but the saving in space for the use of armaments is critical. The aircraft carriers on order for the French and British navies will be powered by gas turbines.

For commercial ships expense does matter. The first one to be fitted with a gas-turbine propulsion plant was the American Liberty ship John Sergeant, which was re-engined with one in 1956. This comprised a separate free piston gas generation unit and a gas turbine, connected via gearing to the propeller shaft. Although the gas turbine operated effectively, it could not compete with steam propulsion or the diesel engine. An attempt was also made in the 1970s to use the gas turbine commercially when a series of fast transatlantic container ships were constructed and fitted with aero-engine type gas turbines. But they also proved too expensive to operate and they were soon converted to diesel propulsion.

Nevertheless, gas-turbine plants are fitted in some modern cruise ships where they are used as generators to supply power to the ship's electrical propulsion system, as well as electricity for the ship's hotel services. The expense of the fuel is offset by the fact that the plant can be located at the funnel which frees up space elsewhere within the ship for use by passengers. They have other major advantages, too. Compared with the diesel engine, a gas-turbine plant has a low level of exhaust emissions such as oxides of nitrogen and sulphur—kerosene and gas oil contain no sulphur. This makes it an ideal power generator for cruise ships operating in environmentally sensitive areas such as Alaska, and its efficient insulation system means that it does not cause a noise problem. It is also well balanced so there is no vibration, and it can be started and put on load without any long preparation period.

Denis Griffiths