A tugboat, or tug, is a ship used to manoeuvre, primarily by towing or pushing 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 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 25 000 hp (20 000 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 (azimuth thruster) 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.

A revolutionary new type of tugboat has been invented in the Netherlands. The so-called carrousel tug comprises of a design wherein the flexibility and effectiveness of the tugboat's manoeuvres is determined not by the propulsion system, but by a steel construction on deck, consisting of two steel rings. The inner ring is fixed to the ship, and the second ring rotates freely and carries a hook or winch. The ship cannot only manoeuvre now 360 degrees independent from the seegoing ship, but due to the fact that the towing point rotates towards the point nearest to the seagoing ship (the towline force always stays over the ship's center of Gravity), the tug cannot capsize anymore. This is considered to be the safest tugboat at the moment. Only one prototype exists, but the first new builds are expected to sail in 2006.

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.