marine engine

(naval architecture) An engine that propels a water-borne vessel.

An engine that propels a waterborne vessel. In all except the smallest boats, the engine is but part of an integrated power plant, which includes auxiliary machinery for propulsion engine support, ship services, and cargo, trade, or mission services. Marine engines in common use are diesel engines, steam turbines, and gas turbines. Gasoline engines are widely used in pleasure craft. See also Boat propulsion; Internal combustion engine; Marine machinery.

Diesel engines of all types and power outputs are in use for propulsion of most merchant ships, most service and utility craft, most naval auxiliary vessels, and most smaller surface warships and shorter-range submarines. The diesel engines most commonly used fall into either a low-speed category or the medium- and high-speed category. Low-speed engines are generally intended for the direct drive of propellers without any speed reduction, and therefore are restricted to a range of rotative speeds for which efficient propellers can be designed, generally below 300 revolutions per minute (rpm). The largest engines are rated for power output of over 5000 kW (almost 7500 horsepower) per cylinder at about 100 rpm. Because of their higher rotative speeds, medium- and high-speed engines drive propellers through speed-reduction gears, but they are directly connected for driving generators in diesel-electric installations. Large medium-speed engines are capable of over 1500 kW (2000 hp) per cylinder at about 400 rpm. The upper limit of the medium-speed category, and the start of the high-speed category, is generally placed in the range of 900–1200 rpm. See also Diesel engine.

While steam-turbine plants cannot achieve the thermal efficiency of diesel engines, steam turbines of moderately high power levels (above about 7500 kW or 10,000 hp) offer efficient energy conversion from steam, which can in turn be produced by combustion of low-quality fuel oil, coal, or natural gas in boilers, or from a nuclear reactor. For high efficiency, high turbine speeds are required, typically 3000–10,000 rpm, with reduction gearing or electric drive used to achieve low propeller rotative speeds. The combination of turbine and reduction gear or electric drive has usually proven robust and durable, so that most oil-fueled steamships currently in service are held over from an earlier era. Others, more recently built, are capitalizing on the availability, in their trade, of a fuel unsuitable for diesel engines. See also Steam turbine.

Aircraft-derivative gas turbines have become the dominant type of propulsion engine for medium-sized surface warships, including frigates, destroyers, cruisers, and small aircraft carriers. In all cases the turbines are multishaft, simple-cycle engines, with the power turbine geared to the propeller. In some installations, two to four turbines are the sole means of propulsion; in other cases, one or two turbines provide high-speed propulsion, while diesel engines or smaller gas turbines are used for cruising speeds. Factors favoring the aircraft-derivative gas turbine in this application are low weight, compact dimensions, high power, rapid start and response, standardization of components, and maintenance by replacement. See also Gas turbine.

In the electric drive arrangement, the engine is directly coupled to a generator, and the electricity produced drives an electric motor, which is most often of sufficiently low rotative speed to be directly connected to the propeller shaft. Any number of engine-generator sets may be connected to drive one or more propulsion motors. Electric drive has been used with engines of all types, including low-speed diesels. Advantages of electric drive includeflexibility of machinery arrangement, elimination of gear noise, high propellertorque at low speed, and inherent reversing capability. In ships with highelectric requirements for cargo, mission, or trade services—for example,passenger ships, tankers with electric-motor-driven cargo pumps, or warshipswith laser weapons—there is an advantage in integrating propulsion andship service support through a common electric distribution system. However,electric drive is usually heavier, higher in initial cost, and less efficientthan direct or geared drive.