Tunnel thruster

A tunnel thruster is a transverse thruster set in an athwartships tunnel through the hull, driving a propeller that pushes water out either side for side thrust at low speed. Fitted at the bow, the stern, or both, it lets a ship move sideways, turn on the spot, and hold position alongside without tug help. Drive is electric or hydraulic through a right-angle gear in a submerged bulb, with a fixed or controllable-pitch propeller. Thrust drops sharply as ahead speed rises because cross-flow disturbs the tunnel inflow, so the device is effective mainly below about 3 to 4 knots. It is the building block of many dynamic-positioning fits.