How does a ship's steering gear work?

Steering gear has to do a deceptively hard job: move a rudder stock carrying enormous hydrodynamic loads, smoothly and quickly enough to meet SOLAS's rudder-to-rudder time requirement, roughly 35 degrees one side to 30 degrees the other within 28 seconds at full sea speed for most cargo ships, while remaining reliable enough that a single failure cannot leave the ship unable to steer.

Hydraulic power is universal on large ships because it delivers high force in a compact space. In a ram-type system, hydraulic rams connected to a tiller or rotary crosshead on top of the rudder stock push and pull to rotate it; in a rotary-vane gear, the rudder stock itself carries vanes inside a fixed housing, and admitting oil to one side of each vane while releasing it from the other turns the stock directly, a compact arrangement popular on smaller and medium-sized ships. Variable-delivery pumps control the flow and hence the speed and torque delivered, commanded by the steering control system, whether from the wheel, the non-follow-up lever, or the autopilot, with a feedback unit on the rudder stock reporting the actual angle back so the system stops the rudder exactly where ordered.

Because loss of steering is one of the most dangerous failures a ship can suffer, particularly in a channel, near other traffic, or in heavy weather, SOLAS requires duplication at every level that matters: a main steering gear capable of the full rudder movement in the required time, plus an auxiliary steering gear that can move the rudder at a slower but still adequate rate if the main system fails, each normally with its own independent power unit, and arrangements so a single failure of piping, a pump, or a power supply cannot disable both. Larger ships often run two or more power units on the main gear simultaneously in congested waters or restricted visibility specifically to build in that redundancy from the outset.

Before departure the steering gear is tested, moving the rudder hard over each way and checking the response time, the alarm systems, the changeover between control positions, and the emergency steering arrangements that let the gear be run in manual, local control directly from the steering flat if bridge control fails. Watchkeeping engineers check hydraulic oil level and condition, look for external leaks at glands and pipework, and listen for unusual noise from the pumps, since a slow oil leak or a failing pump bearing is usually the first sign of trouble long before an actual steering failure occurs.