On a ship, the engine room (ER) is the compartment where the machinery for marine propulsion is located. To increase a vessel's safety and chances of surviving damage, the machinery necessary for the ship's operation may be segregated into various spaces. The engine room is generally the largest physical compartment of the machinery space. It houses the vessel's prime mover, usually some variations of a heat engine (steam engine, diesel engine, gas or steam turbine). On some ships, there may be more than one engine room, such as forward and aft, or port or starboard engine rooms, or may be simply numbered. The engine room is usually located near the bottom, at the rear or aft end of the vessel, and comprises few compartments. This design maximizes the cargo carrying capacity of the vessel and situates the prime mover close to the propeller, minimizing equipment cost and problems posed from long shaft lines. On some ships, the engine room may be situated mid-ship, such as on vessels built from 1900 to the 1960s, or forward and even high, such as on diesel-electric vessels. Equipment Engines The engine room of a motor vessel typically contains several engines for different purposes. Main, or propulsion, engines are used to turn the ship's propeller and move the ship through the water. They typically burn diesel oil or heavy fuel oil, and may be able to switch between the two. There are many propulsion arrangements for motor vessels, some including multiple engines, propellers, and gearboxes. Smaller, but still large engines drive electrical generators that provide power for the ship's electrical systems. Large ships typically have three or more synchronized generators to ensure smooth operation. The combined output of a ship's generators is well above the actual power requirement to accommodate maintenance or the loss of one generator. On a steamship, power for both electricity and propulsion is provided by one or more large boilers giving rise to the alternate name boiler room. High pressure steam from the boiler is used to drive reciprocating engines or turbines for propulsion, and also turbo generators for electricity. Besides propulsion and auxiliary engines, a typical engine room contains many smaller engines, including generators, air compressors, feed pumps, and fuel pumps. Today, these machines are usually powered by small diesel engines or electric motors, but may also use low-pressure steam. Engine cooling The engine(s) get required cooling from liquid-to-liquid heat