tunnel thruster · thrusters system

Wärtsilä LJX

The Wärtsilä LJX is a transverse tunnel thruster, a manoeuvring device fitted athwartships in a tunnel through the hull, usually near the bow, that produces sideways thrust to help a ship move laterally and turn. A tunnel thruster consists of a propeller mounted inside a transverse tube that opens on both sides of the hull; when the propeller turns, it draws water in one side and expels it the other, pushing that part of the ship sideways. Fitted at the bow it is commonly called a bow thruster, and some ships also have a stern thruster. The purpose of a tunnel thruster is manoeuvrability at low speed, particularly during berthing, unberthing and other close-quarters operations. By pushing the bow to port or starboard, a bow thruster lets a ship turn or move its head without relying solely on the main propeller and rudder, which are ineffective at very low speed, and it can reduce or remove the need for tug assistance in some conditions. The thrust is reversible, so the operator can push the bow either way from the bridge controls, and the unit is most effective when the ship has little or no headway. Tunnel thrusters may have fixed-pitch propellers whose thrust is varied by changing the motor speed and direction, or controllable-pitch propellers that run at constant speed with thrust set by blade angle. They are driven by electric motors or by a hydraulic or mechanical drive from the engine room. On vessels equipped for dynamic positioning, tunnel thrusters work together with azimuth or main propulsion units under automatic control to hold the ship on station. Well suited to the frequent manoeuvring of ferries, cruise ships, offshore vessels and many cargo ships, the tunnel thruster is a standard part of the manoeuvring equipment that makes a large ship far more manageable in port and confined waters.