Rotary vane steering gear

The fixed vanes are cast or keyed into the cylinder (housing) wall and project radially inwards; the moving (rotor) vanes are cast/attached to the rotating rotor (the rudder-driven element) and project radially outwards, interleaving with the fixed vanes to form radial sealing rings. Oil is pumped into the space between fixed and moving vanes on one side at high pressure; the moving vanes are pushed towards the low-pressure side, rotating the rotor and hence the rudder stock (a ram-shoe on the quill shaft or the vertical stock).

Typically three (or four) sets of vanes are provided - i.e. sealing rings at different heights. The number is limited by the available length (height) of the rotor/cylinder and the need to keep the moving vane rings within the stroke, and by the requirement to balance the thrust; each set carries a share of the torque. The limiting factor to the number of vanes is the geometric/axial space available and the capacity of the actuator/gearing.

The moving vanes are made substantially strong/heavy - thick and of high-strength material - and they engage solidly against stops (or the cylinder ends / fixed structure) so that when the rudder is at the hard-over position the moving vane acts as a mechanical stop, limiting further travel. Their strength prevents damage when the rudder is driven hard against the stop by wave/flow loads.

The vanes are made of gunmetal/bronze (brass/gunmetal), which is strong, corrosion resistant, and gives good wearing/sealing properties in the oil. Sealing at the vane tips is achieved by a sealing gasket/ring (e.g. a spring-loaded or rubber-impregnated sealing strip that is kept in contact with the curved vane tip by a spring ring), so each cell is hydraulically sealed and the high-pressure oil does not leak past between vane tips and cylinder/rotor.

Rudder uplift (the vertical load tending to lift the rudder) is taken by supporting the moving/rotor element or the rudder stock on a thrust bearing, or by the design of the vanes such that axial/vertical movement is restricted - the rotor is axially located and the uplift is carried by an axial thrust surface (often the actuator base or a special thrust pad), preventing vertical displacement of the moving vanes within the cylinder while allowing free rotation. Hydraulic pressure and the sealing rings keep the rotor axially in place.

(For the nearly identical question variant, the same principles apply with the wording about "limitation factor" and "drift accommodated".)