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See Tug (disambiguation) for alternative meanings of tug.

The Le Four maneuovering in Brest harbour

A tugboat, or tug, is a motor ship used to manoeuvre, primarily by towing, other vessels (see shipping) in harbours, over the open sea or through rivers and canals. They are also used to tow barges, disabled ships, or other equipment.

Tugboats in Vancouver, British Columbia

Tugboats are quite strong for their size. Early tugboats had steam engines; today diesel engines are used. Tugboat engines typically produce 750 to 3000 horsepower (500 to 2000 kW), but larger boats (used in deep waters) can have power ratings up to 25000 hp (20000 kW). The engines are often the same as those used in railroad engines, but typically drive the propellor mechanically instead of converting the engine output to power electric motors, as is common for railroad engines. For safety, tugboats engines feature two of each critical part for redundancy.

Tugboats are highly manoeuvrable due to their propulsion units. Instead of a normal propeller, often the so called Schottel propulsion system or the Voith-Schneider propulsion system are used on tugboats designed for tasks such as ship docking and marine construction. Conventional propeller/rudder configurations are more efficient for port-to-port towing. Thrust is sometimes enhanced by the installation of Kort nozzles.

The Kort nozzle is a sturdy cylindrical structure around a special propeller having minimum clearance between the propeller blades and the inner wall of the Kort nozzle. The thrust:power ratio is enhanced because the water approaches the propeller in a linear configuration and exits the nozzle the same way.

Types of tugboat

Tugboats placing the USS John F. Kennedy (CV 67) into port.

Seagoing tugboats are in three basic categories:

1- The standard seagoing tugboat with model bow that tows its "payload" on a hawser (long steel or soft fiber rope).

2- The "notch tug" which can be secured in a notch at the stern of a specially designed barge, effectively making the combination a ship. This configuration, however, is dangerous to use with a barge which is "in ballast" (no cargo) or in a head or following sea. Therefore, the "notch tugs" are usually built with a towing winch for use under such conditions, thereby entering the first category under certain conditions.

3- The "integral unit" or "integrated unit" which is comprised of specially designed vessels that lock together in such a rigid and strong method as to be certified as such by authorities (classification societies) such as the American Bureau of Shipping, Lloyd's Register of Shipping, or several others. These combinations stay combined under virtually any sea conditions and the "tugs" usually have poor seakeeping designs for navigation without their "barges" attached. Vessels in this category are legally considered to be ships rather than tugboats and barges,must be manned accordingly, and must show navigation lights compliant with those required of ships rather than those required of tugboats and vessels under tow.

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