These ships aren't running a little electric prop that you can just route power to: it's a massive diesel engine that needs to be capable of outputting thousands of pounds of torque constantly. It's also a ship, in water, dealing with tides, currents and wind. There is no turning it or altering course without main engine power to move the ship relative to the water to make the rudder effective. Some large ships have steering screws that they can use to slip the ship sideways, but they are meant for minor corrections and maneuvers in calm water and have limited uses.