Rotary Vane Steering Gear

Hydraulic power unit

Twin pump sets, usually variable-displacement axial piston pumps on the larger sizes or fixed-displacement pumps with directional solenoid valves on smaller ones, supply the working chambers. SOLAS requires the main and auxiliary supplies to be independent so that a single pump, motor or pipe failure cannot disable steering.

Non-return, relief and buffer valves

Relief valves protect the housing from pressure spikes when the rudder meets green water or an unexpected load; buffer valves cushion the rotor as it approaches the mechanical stops so the vanes do not hammer against them at full helm.

Angle transmitter

A feedback unit mounted on the rotor or stock reports actual rudder angle back to the bridge indicator and to the autopilot. Misalignment here shows up as an indicated angle that does not match the rudder actually seen from aft.

Sizing and selection

The gear is selected on stall torque, not on rudder area alone: torque depends on rudder area, the ship's maximum ahead service speed squared, and the type of rudder, since a spade rudder with high lift needs more torque than a simple flat-plate rudder of the same area. Working pressure and vane count are then set to deliver that torque within the housing diameter the trunk allows.

• Rated torque at maximum working pressure, with margin for helm reversal under way
• Rudder angle range, normally 35 degrees each side with mechanical stops slightly beyond
• Response time from hard-over to hard-over at rated pump output
• Housing diameter versus the space available in the steering flat and rudder trunk

Regulations and class

SOLAS Chapter II-1 requires the main steering gear to put the rudder from 35 degrees on one side to 30 degrees on the other in not more than 28 seconds at maximum ahead service speed and deepest draught, and requires an independent auxiliary means capable of steering the ship on its own. Steering gear must be tested before departure, and class societies survey the gear periodically, including a tightness and function test and a check of the rudder stock coupling and seals.

Typical faults

What to look for in a supplier

• A type test certificate from a recognised class society matching the rated torque to the actual rudder torque calculation, not just the nameplate figure
• Seal kits and vane tip seals held as stock spares, since these are wear items with a defined service life
• An angle transmitter output compatible with the existing autopilot and bridge indicator wiring
• A service network able to reach the vessel for the hydraulic power unit, not only the vane assembly

A slow, steady rudder creep with the helm amidships and no alarm is usually vane seal wear, not a control system fault - run the pre-departure test and watch the drift rate before blaming the autopilot.