marine engine

marine engine, machine for the propulsion of watercraft. The earliest marine power plants, reciprocating steam engines, were used almost exclusively until the early 1900s. In later ship construction these were largely replaced by the steam turbine and the internal-combustion engine (see also diesel engine). For some applications, notably ferries, electric motors are used to allow greater maneuverability. Steam turbines having 1,000 shaft horsepower and more are used for the most powerful ships. Diesel engines may supply power for vessels ranging in size from small boats to medium-size ships requiring as much as 40,000 total horsepower. Gas turbines and fast diesels usually have a reduction-gear drive making it possible to run them at high speeds (for maximum economy) while the propeller turns at low speeds (for maximum efficiency). Gas turbines have been used experimentally in merchant ships and naval patrol boats. Some submarines, merchant ships, and icebreakers have nuclear power plants in which a nuclear reactor replaces the boiler of a stream turbine plant. Conventional submarines have a diesel-electric drive and run on batteries when submerged. Small boats usually have gasoline outboard engines that clamp on the stern or inboard engines to drive propeller shafts. Shallow-draft boats for use in swamps have aircraft engines and air propellers. A few small boats are propelled by a pumped jet of water. The inboard-outboard motor for small vessels incorporates features of both types: the engine, the reduction gearing, and the vertical propeller shaft compose a self-contained unit that is mounted with the engine inboard, usually just forward of the transom; the gear housing projects through an opening in the transom and the propeller shaft extends down from it. This arrangement makes possible the combination of a relatively large power plant with the convenience and maneuverability of an outboard installation; e.g., the propeller may be tilted up in order to beach the boat.

Bibliography

See C. Miller, Small Boat Engines (1961); J. E. Flack et al., Marine Combustion Practice (1969); K. T. Rowland, Steam at Sea (1971).

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marine engine [mə′rēn ′en·jən]

(naval architecture)

An engine that propels a water-borne vessel.

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Marine engine

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 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 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 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 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 include flexibility of machinery arrangement, elimination of gear noise, high propeller torque at low speed, and inherent reversing capability. In ships with high electric requirements for cargo, mission, or trade services—for example, passenger ships, tankers with electric-motor-driven cargo pumps, or warships with laser weapons—there is an advantage in integrating propulsion and ship service support through a common electric distribution system. However, electric drive is usually heavier, higher in initial cost, and less efficient than direct or geared drive.