A tugboat, or tug, is a boat 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 like towboats.
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 maneuverable and various propulsion systems have been developed to increase manoeuvrability and increase safety. The earliest tugs were fitted with paddle wheels but these were soon replaced by propeller-driven tugs. Kort nozzles have been added to increase thrust per kW / hp. This was followed by the nozzle-rudder which omitted the need for a conventional rudder. The cycloidal propeller was developed prior to World War II but was only occasionally used in tugs because of its manoeuvrability. After World War II it was also linked to safety due to the development of the Voith Water Tractor; a tugboat configuration which could not be pulled over by its tow. In the late 1950's the Z-drive or (azimuth thruster) were developed. Although sometimes referred to as the Schottel system many brands exist: Schottel, Z-Peller, Duckpeller, Thrustmaster, Ulstein, Wartsila etc. The 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.
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. The Kort nozzle is named after its inventor, but many brands exist.
A new type of tugboat has been invented in the Netherlands. The so-called carousel tug consists 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 can therefore manoeuvre freely and independently of the towed ship, and since the towing point rotates towards the point nearest to the towed ship, the tug can capsize only with difficulty. One prototype exists presently, but the first new tugs are expected to sail in spring 2007.
Types of tugboat
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. With this configuration, the barge being pushed might approach the size of a small ship, the the interaction of the water flow allows a higher speed with a minimal increase in power required or fuel consumption.
3.The "integral unit," "integrated tug and barge," or "ITB," 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, Indian Register of Shipping, Det Norske Veritas or several others. These units stay combined under virtually any sea conditions and the "tugs" usually have poor sea keeping designs for navigation without their "barges" attached. Vessels in this category are legally considered to be ships rather than tugboats and barges must be staffed accordingly. Such vessels must show navigation lights compliant with those required of ships rather than those required of tugboats and vessels under tow. Articulated tug and barge units also utilize mechanical means to connect to their barges. ATB's generally utilize Intercon and Bludworth connection systems. ATB's are generally staffed as a large tugboat, with between 7-9 crew members. The typical American ATB operating on the east coast, per custom, displays navigational lights of a towing vessel pushing ahead, as described in the '72 COLREGS.
Historically tugboats were the first seagoing vessels to receive steam propulsion, freedom from the restraint of the wind, and capability of going in any direction. As such, they were employed in harbors to assist ships in docking, tying up to the piers, and for departure.
River tugs are also referred to as towboats or pushboats. Their hull designs would make open ocean operations dangerous. River tugs usuallly do not have any significant hawser or winch. Their hulls feature a flat front or bow to line up with the rectangular stern of the barge.