There are two types of systems - 4 ram and rotary vane.
Both systems are required to have complete redundancy - Any failure of any one part should not affect the ship ability to steer.
The 4 ram system achieves this by doubling everything. The rotary vane system achieves this normally by having two identical units attached to the rudder.
The 4 ram system without any failures, uses hydraulic fluid driven by a pump and acting on all 4 rams to move a tiller attached to the rudder. The fluid is controlled normally by a pump that varies its delivery depending on the demand to the system.
For example, if the demand is straight ahead, and the rudder points straight ahead, then the pump move no fluid. The rudder stays fixed. However, if the demand was full starboard and the rudder was elsewhere, this difference would result in the pump moving fluid in the correct direction to move the rudder to full starboard. This is known in the world of control theory as 'P' control, and in case is achieved entirely mechanically.
more modern system use electronic control systems to control valves to move the rudder. The result is the same.
During a failure of a component, the 4 ram system will operate on 2 rams. This results in reduced performance.
the rotary vane system has 3 compartments separated by vanes and fixed to a central shaft. As hydraulic fluid acts on the vanes, the central shaft is moved. This shaft is fixed directly to the rudder. A similar 'P' control system to the 4 ram system controls the hydraulic fluid.
In case of loss of communication with the bridge, each type may be operated manually by a person in the steering compartment. However, there must always be some powered assistance. The forces involved are too large for a person to be able to steer a vessel unaided.
Here is a video of a ship steering rotary actuator operating:
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