Tugs & Tugboats
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 propeller 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.
A tugboat, or tug, is a boat used to maneuver,
primarily by towing or pushing, other vessels in harbors, 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 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) were developed.
A tugboat's power is typically
stated by its engine's Horsepower and its overall Bollard pull.
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
maneuvers 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 maneuver
freely and independently of the towed ship, and since the towing point
rotates towards the point nearest to the towed ship, the tugboat can
capsize only with difficulty. One prototype exists presently, but the
first new tugs are expected to sail in spring 2007.
There are two groups of
tugboats, either Inland or Oceangoing.
Inland tugboats come in two categories:
Harbor tugs are the most typical of the tugboats that people recognize.
They are used worldwide to move ships in and out of berth and to move
industrial barges around waterfront business complexes. Their job has
remained the same but their design and engineering has changed much
over the the decades.
Harbor tugboats have evolved
from paddle wheelers, to the conventional tug known by all, and now
to the Ship Docking Modules and tractor tugs in the modern industry.
River tugboats are also referred to as towboats or pushboats. They are
designed as large squared off vessels with flat bows for connecting
with the rectangular stern of the barges. They are large and powerful,
most commonly seen on the big rivers of the world.
Oceangoing tugboats come
in four categories: The conventional tugboats is the standard seagoing
tugboat with a model bow that tows its payload on a hawser. A hawser
is the nautical term for a long steel cable or large synthetic fiber
rope. It operates independently and is used to tow various loads, e.g.,
cargo barges, ships, oil rigs, and etc. This is the most versatile method
of towing since the conventional tugboat is able to move its load.
The notch tug is a conventional
tugboats which is assigned to tow and push a specific barge, usually
built to the shape and specifications of that tugboat. A notch tugboat
has a large towing winch on its stern but it gets its name from the
deep notch built into the stern of the barge. This notch is built in
the exact shape of the tug's forward hull and can be quite deep, up
to 90 feet, sometimes more.
The tugboat fits snugly
into the notch of the barge and with the use of various lines can be
secured firmly enough to push the barge at much higher speeds than it
would if it were towing. The towing hawser remains rigged during pushing.
In the event that the seas get too rough to push safely, the tug merely
releases any securing lines and backs out of the notch while extending
its towing hawser. Once in calmer waters, the tug can maneuver back
into the notch and resume pushing.
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