Marine hydraulicsLesson 19 of 20
How ship steering gear works: rams, the Rapson slide and two pump units
Pumps push oil into rams, the rams push a crosshead along the tiller, and the tiller turns the rudder stock. Follow-up control stops the rudder at the helm order, and locking valves hold it there against the water.
9 minutes · For signed-up learners, free
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 sign-up.
- Play with it: Steer from the helm, change ship speed and gear type, and run the SOLAS timing test against the clock.
- Break it: a worn pump, air in the oil, a leaking locking valve and a failed ram seal.
Continue this lesson free
The rest of this lesson, the interactive Ram steering gear with a Rapson slide and the quiz are open to signed-up learners. It's free, takes a minute, and opens every lesson in all 14 courses.
Free awareness training. Al Safwan Marine is not an approved training provider, so no certificate is given, and these courses are not a qualification. They don't replace STCW training, manufacturer training or the certification technicians need for this work. The manufacturer's manual and your flag's rules always come first.
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This lesson is also published by our sister site hydraulicsuae.com.
