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German icebreaker Polarstern

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.

An icebreaker in the Ross Sea

Contents

History

An early 20th-century icebreaker—Sankt Erik launched in 1915. The shape of the bow is designed to ride up over the ice. The Ship is part of the Vasa Museum exhibit in Stockholm.

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).[1]

City Ice Boat No. 1 at the Delaware River

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 ton 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.

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World's largest icebreaker

In May 2007, sea trials were completed for the nuclear-powered Russian ice-breaker NS 50 Years Since Victory (Russian: 50 лет Победы, 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.[2]

Function of icebreakers

Icebreaker Kapitan Khlebnikov in the Ross Sea, Antarctica from helicopter.OGG
Icebreaker Kapitan Khlebnikov in the Ross Sea

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

Single hull, Double bottom, and Double hull ship cross sections. Green lines are watertight; black structure is not watertight.

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.[3]

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.

MT Mastera outside the Port of Rotterdam

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

  1. ^ Bruun P (1989). Port Engineering, Volume 1: Harbor Planning, Breakwaters, and Marine Terminals (4th ed. ed.). Gulf Publishing Company. pp. 1375. ISBN 0872018431.  
  2. ^ "World's largest icebreaker," Ships Monthly. May 2007.
  3. ^ "Icebreakers and ice strengthened ships". http://www.coolantarctica.com/Antarctica%20fact%20file/ships/icebreaker.htm. Retrieved 2007-11-25.  

External links


Simple English

An icebreaker is a special type of ship. Its special features include extra powerful motors and a very strong hull designed to cut through thick sheets of ice floating on oceans and lakes.


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