Bow thruster

A bow thruster is a transverse propulsor mounted forward, usually a reversible propeller in an athwartships tunnel through the hull near the bow, that produces side thrust for low-speed maneuvering and berthing without tugs. Its turning moment is the thrust times the long lever arm from the bow to the pivot point, so a modest thrust gives a strong yaw moment at zero or low headway. Effectiveness falls off sharply above about 3 to 4 knots as the tunnel inflow stalls and the developing hull side force opposes it, so it is a docking and station-keeping aid, not a steering device underway. Tunnel placement, immersion, and a faired grid balance thrust against the drag the opening adds at sea.