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{{short description|Transverse or steerable propulsion device in a watercraft}}
{{refimprove|date=April 2014}}
[[File:2012 09 03 Innovation 3-Querstrahler im-Bug 2kk DSCI0786.JPG|thumb|300px|A ship equipped with tunnel thrusters, indicated by the circled "X" markings above the water line]]

'''Manoeuvering thrusters''' ('''bow thrusters''' and '''stern thrusters''') are transversal propulsion devices built into or mounted to either the [[Bow (watercraft)|bow]] or [[stern]] (front or back, respectively) of a [[ship]] or [[boat]] to make it more manoeuvrable. Bow thrusters make docking easier, since they allow the captain to turn the vessel to [[port (nautical)|port]] or [[starboard]] side, without using the main propulsion mechanism which requires some [[Steering#Watercraft|forward motion for turning]];<ref>[http://www.osmotech.co.uk/boat-building/bow-thrusters/39/ Osmotech UK on Uses and advantages of bow thrusters] {{webarchive |url=https://web.archive.org/web/20130726031611/http://www.osmotech.co.uk/boat-building/bow-thrusters/39/ |date=July 26, 2013 }}</ref> The effectiveness of a thruster is curtailed by any forward motion due to the [[Coandă effect#Problems caused|Coandă effect]].<ref>{{cite book |last1=Faltinsen |first=O. |url=https://books.google.com/books?id=qZq4Rs2DZXoC&q=bow+thruster+coanda+effect&pg=PA272 |title=Sea Loads on Ships and Offshore Structures |date=Sep 23, 1993 |publisher=Cambridge University Press |isbn=9780521458702 |page=272}}</ref> A stern thruster is of the same principle, fitted at the stern. Sufficiently large vessels often have multiple bow thrusters and stern thrusters.

==Tunnel thrusters==
[[File:Anchoring-3.JPG|thumb|left|A bow thruster in action. Note the transversal water jet.]]

Large vessels usually have one or more tunnel thrusters built into the bow, below the [[waterline]]. An [[impeller]] in the tunnel can create thrust in either direction that 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 [[tugboat]]s, saving the costs of such service. Ships equipped with tunnel thrusters typically have a sign marked above the waterline over each thruster on both sides, as a big cross in a red circle: '''{{color|red|(}}⨯{{color|red|)}}'''.

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. 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 thruster and the hull around it.{{cn|date=October 2014}}

==Externally mounted bow thrusters==
[[File:Propulseur d'étrave de voilier de plaisance.JPG|thumb|A retractable bow thruster lowered under the front of a sailboat]]

Instead of a tunnel thruster, [[boat]]s from {{convert|30|to|80|ft|0}} in length may have an '''externally mounted bow thruster'''. As its name suggests, an external bow thruster is attached to the [[Bow (watercraft)|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 [[propeller]]s driven by a small reversible electric motor which provides thrust in either direction. The added control provided by a bow thruster helps the captain to avoid accidents while docking.

==Waterjet bow thrusters==

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.

==See also==
*[[Azimuth thruster]]
*[[Azipod]]
*[[Z-drive]]
*[[Cyclorotor]]
*[[Rim-driven thruster]]
*{{annotated link|Reaction control system}}

==References==
{{Reflist}}
*[http://www.nauticexpo.com/boat-manufacturer/bow-thruster-2706.html Nautic Expo on Bow Thrusters]
*[http://www.marineinsight.com/marine/marine-news/headline/how-bow-thruster-is-used-for-maneuvering-a-ship/ How Bow Thruster is Used for Maneuvering a Ship? by Marine Insight, September 13, 2012, By Amitava Chakrabarty]
*[https://web.archive.org/web/20130125062513/http://sailmagazine.com/boatworks/upgrade-installing-bow-thruster Sail Magazine, on Upgrade: Installing a Bow Thruster, By Roger Marshall, Dec. 3, 2012]

{{DEFAULTSORT:Bow Thruster}}
[[Category:Propellers]]

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