What is the purpose of steering gear on the ship?

The steering gear on a ship turns the rudder(s) to keep them pointing in the required direction.

International law (the SOLAS regulations) requires most of the system to be duplicated so that, in the event of any reasonably foreseeable machinery breakdown, control of the ship is maintained.

Although there are many variants (with alternative approaches to almost every part of the design, and lots of scope for mixing and matching), a modern installation might look like:

Fitted in the steering gear compartment are at least two hydraulic power packs. Each of these contains an electric motor which drives a hydraulic pump and a valve block. In response to an electronic signal, the valve block switches the flow from the pump to either of two output pipes (one to move the rudder to port, the other to starboard) or just dumps it back to the hydraulic tank if no movement is demanded.

The power packs drive a hydraulic actuator which is attached to (and turns) the top of the rudder stock..

(so when an electric input is received at the valve blocks, hydraulic fluid flows from the pumps into the actuator, and makes the rudder move).

The electric signal comes from a control system. Again, to protect against failures, this has at least two channels.

In the most basic mode, the helmsman can press one of a pair of "non-follow-up" (NFU) buttons on the bridge. Each of these opens a valve on the valve block making the rudder move in the required direction. The rudder will keep moving until the button is released, and will then stay there until the other button is pressed to start it moving the other way.

Steering in NFU mode requires concentration, so the control system also provides a "follow up" (FU) mode. In FU steering, the helmsman sets the desired rudder angle using a small handwheel on the bridge. This sends an electrical signal proportional to the desired angle down to the steering gear compartment.

There is a small unit called a "rudder angle transmitter" attached to the top of the rudder stock which generates an electrical signal proportional to the actual rudder angle.

Both desired and actual rudder angle signals are sent to an electronic unit in the steering gear compartment called a "rudder servo amplifier" (RSA). The RSA calculates the difference between the desired and actual angles and uses this to generate signals to control the valves in the power packs. Consequently, in FU steering, the rudders automatically adopt the position set on the helmsman's wheel.

Finally, to provide additional protection against machinery breakdowns, there are usually additional NFU buttons and an alternative FU handwheel in the steering gear compartment, mechanical operating levers on the valve blocks on the power packs (to control the rudder if the electronics all get fried) and a hydraulic handpump. The handpump works too slowly to steer the ship, but can be used to centralise the rudder is a vessel that has lost all power, allowing her to be towed in a straight line.