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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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