Tug

A tug is a motorship, which is used to pull other ships (see shipping) in harbours, over the open sea or through rivers and canals.

Tugs are quite strong for their size. Early tugs had been steam engines , today mostly diesel engines are used. They have mostly a high manoeuvrability 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 tugs 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 / horsepower ratio is enhanced because the water approaches the propeller in a linear configuration and exits the nozzle the same way.

Seagoing tugs are in three basic categories:

1- The standard seagoing tug with model bow that tows it's "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 American Bureau of Shipping, Lloyd's Register, 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 tugs and barges,must be manned accordingly, and must show navigation lights compliant with those required of ships rather than those required of tugs and vessels under tow.

Various Facts

• Tugs have a top speed of about 10 knots (12mph).
• The engine room typically has two of everything (for safety).
• The engines are typically the same that are used in a railroad engine.