Main Engine Types

There are a few main types of marine propulsion engines that are commonly used aboard ships and boats. The most widespread are diesel engines and steam turbines.

Diesel Engines

Diesel engines provide good fuel efficiency even at partial loads, making them economical to run. They are also reliable, durable, and require less maintenance than other engines. Most large merchant ships and tankers use low or medium speed two-stroke diesel engines as their main engines. These can be either crosshead-type or trunk piston-type engines.

Steam Turbines

Steam turbines were historically common aboard naval vessels and ocean liners. They have very high power-to-weight ratios, making them suitable for applications requiring significant power. However, they are less fuel efficient than diesel engines. Steam turbines work by using steam generated by coal, gas, or oil-fired boilers to spin turbines which drive the propeller shaft. They require more complex machinery and staff to operate compared to diesel engines. Today they are mainly found on military and specialty vessels.

Gas Turbines

Marine Propulsion Engines  gas turbines work on the same principle as aircraft jet engines. They have high power density but poor fuel efficiency, especially at low loads. They are sometimes used as auxiliary engines aboard ships and oil rigs. Gas turbines can reach full power faster than other engine types, making them suitable for applications requiring an ability to change speed rapidly, such as naval vessels.

Propulsion System Components

In addition to the main propulsion engine, a marine propulsion system incorporates several additional components.

Reduction Gears

Due to the typically low rotational speeds of Marine Propulsion Engine  main engines, reduction gears are required to achieve the high shaft rotations needed to effectively turn the ship's propeller. Marine reduction gears can reduce the engine RPM by factors of 20-30 times. Different gearbox types include fixed ratio gears, controllable pitch gears, and reversing gears.

Propeller and Shafting

The propeller converts the rotational force of the propeller shaft into forward thrust. Propeller designs factor in the ship's size, intended service, and design speed. The propeller shaft transmits torque from the reduction gears or turbine through water-lubricated bearings to the propeller. It is supported by struts, bearings and seals.

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