A tug (tugboat or towboat) is a type of vessel that maneuvers other vessels by pushing or pulling them either by direct contact or by means of a tow line. Tugs typically move vessels that either are restricted in their ability to maneuver on their own, such as ships in a crowded harbor or a narrow canal, or those that cannot move by themselves, such as barges, disabled ships, log rafts, or oil platforms. Tugboats are powerful for their size and strongly built, and some are ocean-going. Some tugboats serve as icebreakers or salvage boats. Early tugboats had steam engines, but today most have diesel engines. Many tugboats have firefighting monitors, allowing them to assist in firefighting, especially in harbors.
Seagoing tugs (deep-sea tugs or ocean tugboats) fall into four basic categories:
Compared to seagoing tugboats, harbour tugboats are generally smaller and their width-to-length ratio is often higher, due to the need for a lower draught. In smaller harbours these are often also termed lunch bucket boats, because they are only manned when needed and only at a minimum (captain and deckhand), thus the crew will bring their own lunch with them. The number of tugboats in a harbour varies with the harbour infrastructure and the types of tugboats. Things to take into consideration includes ships with/without bow thrusters and forces like wind, current and waves and types of ship (e.g. in some countries there is a requirement for certain numbers and sizes of tugboats for port operations with gas tankers).
River tugs are also referred to as towboats or pushboats. Their hull designs would make open ocean operation dangerous. River tugs usually 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, often with large pushing knees.
Tugboat engines typically produce 500 to 2,500 kW (~ 680 to 3,400 hp), but larger boats (used in deep waters) can have power ratings up to 20,000 kW (~ 27,200 hp). Tugboats usually have an extreme power:tonnage-ratio; normal cargo and passenger ships have a P:T-ratio (in kW:GRT) of 0.35 to 1.20, whereas large tugs typically are 2.20 to 4.50 and small harbour-tugs 4.0 to 9.5. The engines are often the same as those used in railroad locomotives, but typically drive the propeller mechanically instead of converting the engine output to power electric motors, as is common for diesel-electric locomotives. For safety, tugboats' engines often feature two of each critical part for redundancy.
A tugboat is typically rated by its engine's power output and its overall bollard pull. The largest commercial harbour tugboats in the 2000s-2010s, used for towing container ships or similar, had around 60 to 65 short tons-force (530–580 kN) of bollard pull, which is described as 15 short tons-force (130 kN) above "normal" tugboats.
Tugboats are highly maneuverable, and various propulsion systems have been developed to increase maneuverability and increase safety. The earliest tugs were fitted with paddle wheels, but these were soon replaced by propeller-driven tugs. Kort nozzles (see below) 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 (see below) was developed prior to World War II and was occasionally used in tugs because of its maneuverability. 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 1950s, the Z-drive or (azimuth thruster) was developed. Although sometimes referred to as the Aquamaster or Schottel system, many brands exist: Steerprop, Wärtsilä, Berg Propulsion, etc. These propulsion systems 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-to-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.
The cycloidal propeller is a circular plate mounted on the underside of the hull, rotating around a vertical axis with a circular array of vertical blades (in the shape of hydrofoils) that protrude out of the bottom of the ship. Each blade can rotate itself around a vertical axis. The internal mechanism changes the angle of attack of the blades in sync with the rotation of the plate, so that each blade can provide thrust in any direction, similar to the collective pitch control and cyclic in a helicopter.
A recent Dutch innovation is the Carousel Tug, winner of the Maritime Innovation Award at the Dutch Maritime Innovation Awards Gala in 2006. The Carousel Tug adds a pair of interlocking rings to the body of the tug, the inner ring attached to the boat, with the outer ring attached to the towed ship by winch or towing hook. Since the towing point rotates freely, the tug is very difficult to capsize.
Tugboat Annie was the subject of a series of Saturday Evening Post magazine stories featuring the female captain of the tugboat Narcissus in Puget Sound, later featured in the films Tugboat Annie (1933), Tugboat Annie Sails Again (1940) and Captain Tugboat Annie (1945). The Canadian television series The Adventures of Tugboat Annie was filmed in 1957.
To date, there have been four children's shows revolving around anthropomorphic tugboats.
"Tugger" is a tugboat in the animated series South Park. He appears in the episode "The New Terrance and Phillip Movie Trailer" as a sidekick for Russell Crowe in a fictitious television series entitled Fightin' Round The World with Russell Crowe. Tugger follows Crowe as he engages various people in physical conflicts, providing emotional support and comic relief. At one point Tugger even attempts to commit suicide, upon being forced to hear Russell Crowe's new musical composition.
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Vintage Tugboat races have been held annually in Olympia, Washington since 1974 during the Olympia Harbor Days Maritime Festival (www.HarborDays.com). Tugboat races are held annually on Elliott Bay in Seattle, on the Hudson River at the New York Tugboat Race, the Detroit River, and the Great Tugboat Race and Parade (2012 event was on June 29–30) on the St. Mary's River.
Since 1980, an annual tugboat ballet has been held in Hamburg harbour on the occasion of the festival commemorating the anniversary of the establishment of a port in Hamburg. On a weekend in May, eight tugboats perform choreographed movements for about an hour to the tunes of waltz and other sorts of dance music.
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