A transverse thruster is used as a manoeuvring aid on board ship at slow speeds, for example to enable the vessel to be independent of tugs. They are particularly useful for turning manoeuvres in narrow harbour basins, when travelling through canals, and when docking and casting off. See Fig. 1 Transverse thruster
Large vessels often have two transverse thrusters arranged one behind the other, because a single transverse thruster of the same performance would have too large a diameter. Some special ships may also be equipped with an additional stern thruster.
For manoeuvring, the propeller pumps a flow of water through the transverse tunnel either to port or starboard. This generates a reaction force which displaces the ship in the opposite direction to the flow of water.
The change of direction of the flow of water can be effected either by changing the direction of rotation of the propeller with the aid of reversing the direction of rotation of the motor or, while maintaining the direction of rotation, by using a variable pitch propeller (see Impeller blade pitch control).