Thruster

In addition to this array of
equipment is the ships thruster,
typically operated by electric
motors controlled from the bridge.

These thrusters are laterally
mounled propellers that can suck
or blow water from port to star
board (i.e. left to right) or vice
versa.

They are normally used only in
maneuvering, e.g. docking operations,
and are often banned in tight confines,
e.g. dry dock.

Thrusters, like main propellers,
are reversible by hydraulic operation.
Small embedded hydraulic motors rotate
the blades up to 180 degrees to reverse
the direction of the thrust.


Taken from wikipedia