Bow tunnel thruster

A bow tunnel thruster is a transverse thruster mounted in a fore-and-aft athwartships tunnel near the bow, providing side thrust for low-speed maneuvering and station keeping. A fixed or controllable-pitch propeller in the tunnel pushes water out either side, generating a lateral force without forward way. Drive is electric or hydraulic, with the motor or right-angle gearbox set in a bulb above the tunnel. Thrust falls off as ship speed rises because flow across the tunnel mouth degrades inflow, so it works best at berthing speeds below about 3 knots. It cuts or removes tug assistance in port.