Large vessels usually have one or more tunnels built into the bow below the waterline. An impeller in the tunnel can create thrust in either direction which makes the ship turn. Most tunnel thrusters are driven by electric motors, but some are hydraulically powered. These bow thrusters, also known as tunnel thrusters, may allow the ship to dock without the assistance of tugboats, saving the costs of such service. Ships equipped with tunnel thrusters typically have a sign above the waterline over each thruster on both sides, a big white cross in a white circle.
Tunnel thrusters increase the vessel's resistance to forward motion through the water, but this can be mitigated through proper fairing aft of the tunnel aperture (see below photo). Ship operators should take care to prevent fouling of the tunnel and impeller, either through use of a protective grate or by cleaning. During vessel design, it is important to determine whether tunnel emergence above the water surface is commonplace in heavy seas. Tunnel emergence hurts thruster performance, and may damage the device and the hull around it.
Instead of a tunnel thruster, boats from 30 to 80 feet in length may have an externally mounted bow thruster. As its name suggests, an externally mounted bow thruster attaches to the bow making it suitable for boats where it is impossible or undesirable to install a tunnel thruster due to hull shape or outfitting. Externally mounted bow thrusters have one or more propellers driven by a small reversible electric motor which provides thrust in either direction. The added control provided by a bow thruster helps the Captain avoid accidents while docking.
A Waterjet thruster is a special type of bow thruster that utilizes a pumping device instead of a conventional propeller. The water is discharged through specially designed nozzles which increase the velocity of the exiting jet. Waterjets generally have the advantage of smaller hull penetrations for an equivalent size thruster. Additionally, the higher exit velocity of the discharged water increases the relative efficiency as speeds of advance, or currents, increase, as compared to standard tunnel thrusters. Some waterjet bow thrusters can be configured to provide forward and aft auxiliary propulsion, or even full 360 degree thrust.