Steering gear is the hydraulic machine a ship can least afford to lose. Its job sounds simple: turn the rudder to the angle the bridge orders and hold it there, whatever the sea does. In the animation, drag the orange pointer on the rudder angle indicator (or use the helm slider), then run the SOLAS timing test. Try it at full speed with a worn pump, and again with the ship stopped.
In simple terms
The rudder sits under the stern on a vertical shaft, the rudder stock. On top of the stock, in the steering gear room, is an arm called the tiller. Push the end of the tiller sideways and the rudder turns.
The pushing is done by rams: thick steel plungers in cylinders full of oil. On a two-ram gear there is one pair, lined up across the ship, one ram each side of the tiller. Pump oil into one cylinder and its ram pushes the tiller across, while the ram on the other side is pushed back into its cylinder and its oil goes back to the tank.
The ram can’t push the tiller directly because the tiller swings in an arc while the ram moves in a straight line. So the ram drives a block, the crosshead, that slides along the tiller. This is the Rapson slide. Watch it in the animation: as the rudder angle grows, the crosshead slides further out along the tiller.
Two independent pump units supply the oil. Each one alone must be able to steer the ship. When the helm is moved, the follow-up control compares the order with the actual rudder angle and opens the valve until they match. Then the locking valves shut and trap the oil, so the rudder holds its angle while the pump idles.
The water is the load. At speed, the flow over the rudder tries to push it back to midships, and the further it is turned, the harder the water pushes. That is why the ram pressure rises with speed and angle, and why a gear that is quick alongside in harbour can be slow at sea.
The engineer's explanation, the deep dive, key numbers and the quiz open with a free account.
For engineers
Rapson slide torque. The rams push across the ship at a fixed distance r₀ from the stock. Because the crosshead slides along the tiller, the force on the tiller is F / cos θ and its lever arm is r₀ / cos θ, so:
T = F × r₀ / cos²θ, with F = p × A(rams pushing)
At 35° the cos² term is 0.67, so the same pressure gives about 49% more torque than at midships. That suits the load, which also grows with angle, and is the main reason the Rapson slide has lasted so long.
| Quantity | Formula | Example in the animation |
|---|---|---|
| Ram force | F = p × A | 100 bar × 25,447 mm² (180 mm ram) = 254 kN |
| Torque on the stock | T = F × r₀ / cos²θ | 254 kN × 0.65 m / 0.67 = 247 kNm at 35° |
| Ram travel for an angle | x = r₀ × tan θ | 0.65 × tan 35° = 455 mm |
| Rudder rate | dθ/dt = Q × cos²θ / (A × r₀) | 60 L/min → 3.5°/s at midships |
| Ram travel, 35° to 30° | r₀ × (tan 35° + tan 30°) | 0.83 m |
The timing test. SOLAS chapter II-1, regulation 29, requires the main steering gear to move the rudder from 35° on one side to 30° on the other in 28 seconds or less at the deepest seagoing draught and full ahead service speed. The animation’s example gear does it in about 22 s with one pump unit. With both units it halves. Run the test with the worn pump at 16 knots and it fails; run it again at 0 knots and it passes, because the worn pump’s leakage grows with pressure. This is why a test alongside proves little about performance at sea.
Two-ram and four-ram. A four-ram gear uses two pairs of rams, one pair on each arm of a double tiller. At any moment one ram of each pair pushes, so the pushing rams are diagonally opposite and piped together (the animation’s A and B sides). With the same total ram area, a four-ram gear makes the same torque at the same pressure as a two-ram gear. Its advantage is redundancy: if a ram seal fails, that pair can be isolated and the gear keeps steering on the other pair, with half the area. Half the area means twice the pressure for the same torque, and the same pump flow now moves the rudder twice as fast. At full speed and large angles, the needed pressure may pass the relief setting, so the rudder stalls short of the order. That is the condition the regulations expect the crew to handle by reducing speed.
Holding. With the rudder at an angle, the trapped oil on the loaded side carries the water torque. On the animation’s gear at 14 knots and 20°, that is about 55 bar sitting behind the locking valves. Any leak path (the locking valve seats, the directional valve spool, a ram gland) lets the water push the rudder back. The follow-up then corrects it, so the symptom is a rudder that wanders and a pump that keeps cutting in.
Deep dive
Follow-up and hunting. Follow-up control is a closed loop: helm order in, rudder angle feedback, valve or pump stroke out. Older gears do this mechanically with a floating lever (the “hunting gear”) that moves the pump’s stroke control. Newer ones use an electric feedback unit and solenoid or proportional valves. A deadband, typically around half a degree, stops the gear chasing tiny errors. Too small a deadband, a sticky valve or a worn feedback linkage makes the gear oscillate around the order.
Air in the system. Air makes the oil column springy. The rams move, but the rudder lags, then overshoots, and the follow-up corrects it the other way. That is hunting, usually with noise and jerky movement. Air comes in at suction leaks, through ram glands on the return stroke, or after maintenance that wasn’t bled properly. Bleed points are at the top of each cylinder for a reason.
Pumps. Many ram gears use variable-displacement axial or radial piston pumps whose stroke is set by the follow-up, so the pump itself is the control valve and the circuit is closed. Others use fixed pumps with solenoid directional valves and locking (pilot-operated check) valves, as in the animation. Either way the worn-pump symptom is the same: flow falls as pressure rises, so the gear slows down most when it is most loaded. On piston pumps, case drain flow measured against the maker’s limit is the quick check.
Relief and shock valves. Each side has a relief valve to protect against overload, and many gears have shock (or buffer) valves that let a wave strike on the rudder push oil from one side to the other instead of bursting a pipe. A shock valve passing at a low pressure gives the same symptom as a leaking locking valve.
Heat in the Gulf. Steering gear rooms on ships trading in the Gulf get very hot in summer, and oil at 60 to 70 °C is thinner, so internal leakage in pumps and valves rises. A gear that times well in winter can slow down in August. Check the oil temperature when you record test times.
Rotary vane gear does the same job with vanes on a rotor inside a housing keyed to the stock. There is no Rapson slide: torque is pressure × vane area × mean radius, constant with angle. Its typical wear points are the vane and stopper seals.
Key numbers
| What | Typical value |
|---|---|
| SOLAS main gear timing | 35° one side to 30° other side in 28 s or less |
| SOLAS auxiliary gear timing | 15° to 15° in 60 s or less |
| Pre-departure check | within 12 hours before departure |
| Emergency steering drill | at least every three months |
| Rapson slide torque gain at 35° | 1 / cos²35° = 1.49 |
| Follow-up deadband | around 0.5° |
| Ram gear working pressure | commonly 70 to 200 bar at full load |
