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A tugboat, or tug, is a boat used to maneuver,
primarily by towing or pushing, other vessels (see
shipping) in harbors, over the open sea or through
rivers and canals. Tugboats are also used to tow barges,
disabled ships, or other equipment like oil platforms.
Tugboats are quite strong for their size. Early tugboats
had steam engines (see steamboat); today diesel engines
are used. 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) and usually have an extreme power: tonnage-ratio
(normal cargo and passenger ships have a P:T-ratio (in
kW: GRT, of 0.35-1.20, whereas large tugs typically are
2.20-4.50 and small harbor-tugs 4.0-9.5). 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 railroad
engines. For safety, tugboats' engines often feature two
of each critical part for redundancy.
A tugboat's power is typically stated by its engine's
horsepower and its overall Bollard pull.
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 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 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 Schottel system, many
brands exist: Schottel, Z-Peller, Duckpeller,
Thrustmaster, Ulstein, Wärtsilä, etc. The 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
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