A simple, efficient and reliable way of driving the rudder on vessels with mechanical steering systems. The unit is an assembly of a reversing pump and a marine hydraulic cylinder. It has a total of only 5 moving parts. The pump is a patented single radial piston design and the cylinder driving the rudder is a design proven in thousands of steering systems, and which gives minimal "drag" on the steering system. This assembly is mounted on the rudder quadrant and hull superstructure. When the autopilot is not in use (hand steering) the cylinder runs freely along with the rudder. When the autopilot is in use, a solenoid valve on the actuator closes and locks the cylinder. Steering signals from the autopilot are now fed to the dc motor which drives the pump to move the cylinder back and forth. De-energizing the solenoid valve immediately disconnects the automatic pilot from the rudder. The unit has a small translucent oil reservoir making it easy to check the fluid level. It is also shipped filled with oil and requires no additional hydraulic plumbing or purging.