Marine propulsion encompasses the technologies used to drive ships through the water, and sometimes also to steer them. In the basic cases, the requirement is to spin the propellers. The propellers may be turned by one or more types of combustion-powered engine, by electric motors, or combinations of the two. Generators that power the electric motors may be driven by combustion engines or by steam turbines heated by nuclear reactors; in special applications, electric motors may also be powered by batteries.
Not all vessels use propellers; they may use water jet pumps, possibly in combination with propellers. Other new types may put propellers or jets on a swiveling pod, so they may also be used for steering as well as linear propulsion.
Systems for boats are considered later in the article.
In modern ships, combustion-powered engines are either diesel ("D") or gas turbine ("G"). Diesels are optimal for lower cruising speeds while turbines are most effective at high speed, with the caveat that gas turbines can be designed for high or low power. Some older types use steam turbines "S") This leads to the combinations:
Combustion powered inboard propellers
- Pure diesel
- Pure turbine
- CODOG: Combined Diesel Or Gas, in which either the diesel or the gas turbine connects to the propellers
- CODAG: Combined Diesel And Gas, in which either the diesel, the gas turbine, or both connect to the propeller; an advanced, experimental version are on the Freedom-class Littoral Combat Ships and provide very high speed
- CODLAG: Combined Diesel-electric and gas: The diesel drives an electric motor that turns the propeller at low speed, but a gas turbine can be added mechanically for bursts of power
- CODAD: Combined Diesel and Diesel: a method that uses one diesel for low speed and two for high speed
- COSAG: Combined steam and gas: while an obsolete method, uses a steam turbine for low speed
- COGOG: Combined gas or gas: a system that uses two gas turbines, one optimized for low and one for high speed, but only one at a time
- COGAG: Combined gas and gas: like COGOG in that it has two types of gas turbines, but both can simultaneously contribute to the propeller; Burke-class destroyers have four turbines in a COGAG configuration
- COGAS: Combined gas and steam: an experimental method primarily driven by gas turbines, but that scavenges the waste heat from the gas turbines to drive a steam turbine
- CONAS: Nuclear steam turbine backed up with conventional steam turbines, all driving electric motors; unique to Kirov-class cruisers; see below for nuclear detail
Electric powered propellers
- COGES: Gas turbines drive the generators and electric motors turn the propellers, used on some cruise ships