An
icebreaker is a special-purpose
ship or
boat designed to move and
navigate through
ice-covered waters. Although
the term usually refers to icebreaking
ships,
it may also refer to smaller vessels (e.g., icebreaking boats that
were used on the
canals of Great
Britain in the days of commercial carrying).
For a ship to be considered an icebreaker, it requires three traits
most normal ships lack: a strengthened
hull, an ice-clearing shape, and the power to
push through ice-covered waters.
To pass through ice-covered water, an icebreaker uses its great
momentum and power to drive its
bow up
onto the ice, breaking the ice under the immense weight of the
ship. Because a buildup of broken ice in front of a ship can slow
it down much more than the breaking of the ice itself, the speed of
the ship is increased by having a specially designed hull to direct
the broken ice around or under the vessel. The external components
of the ship's propulsion system (
propellers, propeller shafts, etc.) are at even
greater risk of damage than the vessel's hull, so the ability for
an icebreaker to propel itself onto the ice, break it, and clear
the debris from its path successfully is essential for its safety.
History
Even in the earliest days of
polar
exploration, ice-strengthened ships were used. These were
originally wooden and based on existing designs, but reinforced,
particularly around the
waterline with
double planking to the hull and strengthening cross members inside
the ship. Bands of iron were wrapped around the outside. Sometimes
metal sheeting was placed at the bows, stern and along the keel.
Such strengthening was designed to help the ship push through ice
and also to protect the ship in case it was "nipped" by the ice.
Nipping occurs when ice floes around a ship are pushed against the
ship, trapping it as if in a vise and causing damage. This
vise-like action is caused by the force of winds and tides on ice
formations. Although such wind and tidal forces may be exerted many
miles away, the ice transmits the force.
The first steam-powered icebreaker was the
City Ice Boat
No. 1, built by the city of Philadelphia in 1837. She
was a wooden
paddle steamer intended
to break ice in the harbor. The first European steam-powered
icebreakers were the Russian
Pilot (1864) and the German
Eisbrecher I (1871).
At the beginning of the 20th century, several countries began to
operate purpose-built icebreakers.
Most were coastal icebreakers, but
Russia
, and later, the Soviet Union
, also built several oceangoing icebreakers of
around 10,000 tonnes displacement. Several technological
advances were introduced over the years, but it was not until the
introduction of
nuclear power in the
Soviet icebreaker
Lenin in 1959 that icebreakers developed their full
potential.
World's largest icebreaker
In May 2007, sea trials were completed for the
nuclear-powered Russian
ice-breaker NS 50 Years Since Victory (
, transliterated as
50 Let Pobedy). The vessel was put
into service by
Murmansk
Shipping Company, which manages all eight state-owned nuclear
icebreakers.
The keel was originally laid in 1989 by
Baltic Works of Leningrad
(now St Petersburg
), and the ship was launched in 1993 as the NS
Ural. This icebreaker was intended to be the sixth
and last of the class. In 1994, outfitting was suspended as a
consequence of Russia's temporary economic difficulties. The first
of the class, the NS
Arktika, entered service back in
1974—or in other words, a little more than thirty years separate
the first and last of what is known as the
Arktika class icebreaker.
Function of icebreakers
Icebreakers are needed to keep trade routes open where there are
either seasonal or permanent ice conditions. Icebreakers are
expensive to build and very expensive to run, whether the
icebreaker is powered by
gas turbines,
diesel-electric powerplant or
nuclear energy. They are
uncomfortable to travel in on the open sea: almost all of them have
thick, rounded keels, and with no protuberances for
stability, they can roll
even in light seas. They are also uncomfortable to travel in when
breaking through continuous thick ice due to constant motion,
noise, and vibration.
A modern icebreaker typically has shielded propellers both at the
bow and at the stern, as well as side thrusters; pumps to move
water ballast from side to side; and holes on the hull below the
waterline to eject air bubbles, all designed to allow an icebreaker
stuck amidst thick ice to break free. Many icebreakers also carry
aircraft (formerly
seaplanes but now
helicopters) to assist in reconnaissance
and liaison.
Design and construction
Icebreakers are constructed with a
double
hull and watertight compartments in case of a breach. The
ship's hull is thicker than normal, especially at the bow, stern,
and waterline, using special steel that has optimum performance at
low temperatures. The thicker steel at the waterline typically
extends about 1 m above and below the waterline and is
reinforced with extra internal ribbing, sometimes twice the ribbing
of a normal ship. The bow is rounded rather than pointed, allowing
the vessel to ride up over the ice, breaking it with the weight of
the vessel. The hull has no appendages likely to be damaged by the
ice, and the rudder and propeller are protected by the shape of the
hull. The propeller blades are strengthened, and the vessel has the
ability to inspect and replace blades while at sea.
Recent advances
The optimal shape for moving through ice makes icebreakers
uncomfortable in open water and gives them poor fuel
efficiency.
In open-water travel, icebreakers tend to roll side to side to the
discomfort of the crew. Some new icebreakers, such as the
USCGC Healy, make use of anti-roll
tanks. Anti-roll tanks are incompletely filled ballast tanks which
span the beam of the vessel. Ballast water in these tanks is
allowed to move side to side, or slosh, as a
free surface. Retarding baffles inside the
anti-roll tank slow the side-to-side flow of water. By varying the
water level inside the anti-roll tank, the natural frequency of the
slosh is used to counteract the rolling of the vessel. Anti-roll
tanks by their nature decrease a ship's stability and must always
be used with caution. Use of computer-controlled valves allow for
better control of these anti-roll tanks.
A greater concern is how well a ship cuts through waves. The
ability of a ship to cut through waves can greatly affect its fuel
efficiency and even its safety in a storm. Most ships use a sharp
or
bulbous bow to cut through waves and
help prevent waves from slamming the bow of the ship. However,
icebreakers have a round sled-like bow. They tend to slam into
waves, which can be risky in high seas.
Recent advances in ship propulsion have produced new experimental
icebreakers. Electrically driven propellers are mounted to
steerable pods under the ship. These
Azimuthing Podded Propulsors, or
Azi-pods, improve fuel efficiency, ship steering, and ship
docking and remove the need for rudders. Azipods also allow a ship
to travel backwards as easily as it travels forwards. The
double-acting icebreaker is unique because its stern is shaped like
an icebreaker's bow. Normally traveling forward, a double-acting
icebreaker uses a conventional ship bow for a more comfortable
ride. When ice is encountered, the ship turns around and travels
backwards through the ice. The
MT
Mastera and
MT
Tempera are two vessels using this new
technology.
In the 1980s,
hovercraft were shown to be
effective as icebreakers on rivers. Instead of displacing or
crushing the ice from above, they work by injecting a bubble of air
under the ice sheet, causing it to break off under its own weight
and be swept downstream by the current. The purpose is usually not
to provide navigation channels but rather to prevent ice dams from
forming and causing local flooding.
See also
References
External links
Wikis
Quiz
Facts
Map
Misc
Search
