On a ship, the engine room or ER is the propulsion machinery spaces of the vessel. To increase a vessel's safety and chances of surviving damage, the machinery necessary for operations 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 - diesel engine, gas or steam turbine, or some combination of these (such as CODAG; see Marine propulsion). On some ships, the machinery space may comprise more than one engine room, such as forward and aft, or port or starboard engine rooms, or may be simply numbered. On a ship, the engine room or ER is the propulsion machinery spaces of the vessel. To increase a vessel's safety and chances of surviving damage, the machinery necessary for operations 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 - diesel engine, gas or steam turbine, or some combination of these (such as CODAG; see Marine propulsion). On some ships, the machinery space may comprise more than one engine room, such as forward and aft, or port or starboard engine rooms, or may be simply numbered.