With a mechanical propulsion system, if one engine goes out, the ship will immediately lose 50 percent of its propulsion power. Diesel electric drive systems have multiple diesel engine generators, but they do not all need to be used at once. They are turned on and off based on current electrical demand of the vessel. If one of the diesel engine generators went out on a ship that uses a marine electric propulsion system, the vessel would still have enough propulsion power to continue on its way.