Today, the Navy generally identifies and classifies ships by their purpose or mission, such as destroyer, tanker, or aircraft carrier. But for centuries, ships more often were known by their means of propulsion, with terms such as galley, caravel, carrack, galleon, and brig used to describe their configurations of oars and/or sails.
Eventually, manpower and wind were replaced by thermodynamic systems as the means of marine propulsion. First relying on paddle wheels, then propellers, and today, water-jets, ships harnessed fossil fuels and nuclear technology to drive vessels.
For the better part of a century, steam was the primary method of marine propulsion for sizable ships, and still is used in many U.S. Navy vessels. Steam plants convert thermal energy into mechanical energy by using boilers to transform fresh water into steam and turbines to convert that pressurized steam into usable power to turn a ship’s propellers. Because turbines run at speeds too high for efficient propeller use, a system of reduction gears lowers the rpms to a practical speed. Condensers cool the spent steam to revert it to fresh water, which then is returned to the boilers to begin the cycle again. This “steam cycle” is repeated over and over to provide the energy to propel a ship through the water.
Even though this is a closed cycle—meaning the water and steam theoretically are contained in the system and cannot escape—a certain amount of the water is used up, so a continuous supply of feedwater is required for sustained operations. Saltwater was found to be too corrosive for the systems, so it became necessary to convert it to fresh. This is accomplished by on board distilling plants.
Boilers need fuel to provide heat. Wood proved impractical, so coal became the primary fuel in the early days of steam propulsion. Coal was extremely labor intensive, however, because it had to be manhandled on board, then shoveled into the boilers, and then the ash had to be removed. Oil was a welcome substitute as it uses pumps to move it about and feed it to the boilers and does not leave the same ash residue.
This is a very specialized form of steam propulsion. Instead of using oil-fired boilers, nuclear-powered ships have a reactor that produces the heat to convert fresh water to steam. Nuclear powerplants give a ship the advantage of great endurance at high speed. Instead of refueling every few thousand miles like an oil-burning ship, a nuclear-powered ship can operate for years on one reactor core. Because there is no need to replenish oil, they can steam almost indefinitely, limited only by the need to replenish food, spare parts, and, in wartime, ammunition.
Another favorable feature of nuclear power is that, unlike in conventional oil-fired systems, generating it does not require oxygen. This makes it particularly useful as a means of submarine propulsion, allowing the vessel to operate submerged for extended periods.
Although nuclear power has been used for a few merchant ships, icebreakers, and surface warships, today it primarily is used to propel aircraft carriers and submarines.
Gas turbines are very similar to aircraft jet engines, except that they have been adapted for use on ships. The burning fuel produces gases that spin turbines in the engines, converting the energy from the burning fuel into the power that turns the ship’s propellers.
Although certain principles are the same, some of the primary differences between these propulsion plants and those that use steam are that gas turbines combine the functions of the boiler and the turbines into one element, and gas turbines do not need feedwater. This means that they are smaller, more efficient, and easier to maintain. They also are more quickly “brought on the line” (turned on)—a steam-powered vessel requires hours to prepare to get under way, while gas turbine–powered ships can be ready in minutes.
The obvious advantages of gas-turbine technology have led the U.S. Navy to build more and more of these ships. Whereas steam was once the main means of naval propulsion, today more ships in the Navy use gas-turbines than any other kind of propulsion.
For relatively small ships that need no more than 5,000 to 6,000 horsepower, diesel engines are frequently used. Diesels take up less space and are more efficient than steam turbines. The diesel can be coupled directly to the shaft through reduction gears and perhaps a clutch, or it can drive a generator that produces current for the main drive.
Diesel engines are preferred over gasoline engines because highly volatile gasoline fumes are heavier than air and tend to collect in low places in a ship, making them very dangerous. Diesel fuel, which does not vaporize as readily, is much safer.
The future of marine propulsion is not clear. Hydrogen power, biofuels, liquid natural gas, solar power, and even some attempts at returning to wind are all on design tables, but for now, steam, gas-turbine, and diesel remain the primary means of moving vessels through the water.
LCDR Cutler is the Gordon England Chair of Professional Naval Literature at the U.S. Naval Institute. His many books include The Bluejacket’s Manual (Naval Institute Press), a copy of which every U.S. Navy enlistee receives.