Hydraulics aboard: steering gears, winches and cranes

Hydraulics is power transmission using a fluid under pressure and is the preferred technology for shipboard tasks requiring great force in a relatively small space: turning the rudder, pulling a winch or lifting a heavy load with a crane. As a ship fitter you must be able to install the ship's hydraulic systems including steering gears and winches and you must independently be able to assemble and troubleshoot hydraulic systems using diagrams and other documentation.

A hydraulic system basically consists of a pump that puts oil under pressure valves that control where the oil flows and one or more cylinders or hydraulic motors that convert the pressure into movement. The greater the pressure and the larger the piston area the more force the system can deliver — that is why a relatively small hydraulic system can move a very heavy load.

The steering engine is the hydraulic system that turns the rudder after a command from the bridge. The system must work reliably in all conditions, as a ship without steering in bad weather is in serious danger. Therefore, steering engines are often built with redundancy — for example two independent pump sets — so a single component failure does not completely put the steering out of service.

• 01Pump unit(s) — supplies oil under pressure to the system
• 02Directional control valves — determine the direction and amount of oil flow
• 03Cylinders or rotor unit — converts pressure into rotor movement
• 04Feedback system — reports the rudder's actual position back to the bridge
• 05Reserve/emergency system — ensures the ship can be steered even in the event of a failure

Winches are used to haul wires or anchor chains in, while hydraulic cranes lift cargo aboard. Both require the system to deliver smooth force at low speed without jerking or vibrating — because a jerky winch can at worst break a wire or tip the load.

Troubleshooting on hydraulics starts with the diagram for that specific system. You follow the oil through the system: does the pump deliver the correct pressure and flow? Do the valves open and close correctly? Is there air in the system that makes movement uneven? Are there external leaks at joints and hoses? By working your way through the system in the order the oil flows, you quickly narrow down where the error is.

Common to all hydraulic troubleshooting is that you must know the system's normal operating values before you can determine if something is abnormal. The manual's pressure and flow values are therefore as important as the diagram itself when you encounter an error that needs to be found.