A lateral thruster fitted in an athwartships tunnel near the bow to improve manoeuvrability. When the bow thruster is used while the vessel is moving forward the thrust is partially counteracted by a vacuum created in the wake of the water jet emanating from the thrusters. The effect is worst when the vessel is moving forward at four to six knots. In such cases the vacuum on the hull can be relieved by the addition of an anti-suction tunnel.
Bow thruster should be located as far forward as possible. Parallel side walls have favorable influence. The suitable tunnel length: 2-3D. An attempt should be made to locate the propeller in the midship plane. In short tunnels the propeller is located eccentrically on the port side, in order to improve the thruster performance to starboard. The average bow thruster power in ferries is 0.54kW/m2 (total bow thruster power/projected windage area), varying up to 0.96 kW/m2. The tendency seems to be towards 0.6-0.8 kW/m2. Stern thrusters seem to be dimensioned at 0.2-0.25 kW/m2.