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An icebreaker is a special-purpose ship or boat designed to move and navigate through ice-covered waters. Although the term usually refers to ice-breaking ships, it may also refer to smaller vessels, such as the icebreaking boats that were once used on the canals of the United Kingdom.
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 sea ice.
Icebreakers clear paths by pushing straight into ice pockets. The bending strength of sea ice is so low that usually the ice breaks without noticeable change in the vessel's trim. In cases of very thick ice, an icebreaker can drive its bow onto the ice to break it under the 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, icebreakers have 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 of an icebreaker to propel itself onto the ice, break it, and clear the debris from its path successfully is essential for its safety.
Sailing ships in the polar waters 
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 vice and causing damage. This vice-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 boats to be used in the polar waters were those of the indigenous Arctic people. Their kayaks are small human-powered boats with a covered deck, and one or more cockpits, each seating one paddler who strokes a single or double-bladed paddle. Such boats, of course, have no icebreaking capabilities, but they are light and well fit to carry over the ice.
In the 9th and 10th centuries, the Viking expansion reached the North Atlantic, and eventually Greenland and Svalbard in the Arctic. Vikings, however, operated their ships in the waters that were ice-free for the most part of the year, in the conditions of the Medieval Warm Period.
In the 11th century, Russians started settling the coasts of the White Sea, named so for being ice-covered for over half of a year. The ethnic subgroup of Russians that lived on the shores of the Arctic Ocean became known as Pomors ("seaside settlers"). Gradually they developed a special type of small one- or two-mast wooden sailing ships, used for voyages in the ice conditions of the Arctic seas and later on Siberian rivers. These earliest icebreakers were called kochi. Koch's hull was protected by a belt of ice-floe resistant flush skin-planking (made of oak or larch) along the variable water-line, and had a false keel for on-ice portage. If a koch became squeezed by the ice-fields, its rounded bodylines below the water-line would allow for the ship to be pushed up out of the water and onto the ice with no damage.
In the 19th century, similar protective measures were adopted to modern steam-powered icebreakers. Some notable sailing ships in the end of the Age of Sail also featured the egg-shaped form alike that of Pomor boats, for example the famous Fram, used by Fridtjof Nansen and other great Norwegian Polar explorers. Fram is said to be the wooden ship to have sailed farthest north (85°57'N) and farthest south (78°41'S), and perhaps the strongest wooden ship ever built.
Steam-powered icebreakers 
The first ship designed and built for icebreaking purposes was a 51-metre (167 ft) wooden paddle steamer, City Ice Boat No. 1, that was built for the city of Philadelphia by Vandusen & Birelyn in 1837. The ship's wooden paddles, powered by two 250-horsepower steam engines, were reinforced with iron coverings.
The first European steam-powered icebreaker, as well as the first ever metal-hull icebreaker was the Russian Pilot, built in 1864 on orders of merchant and shipbuilder Mikhail Britnev. It had the bow altered to achieve an ice-clearing capability (20° raise from keel line). This allowed the Pilot to push itself on the top of the ice and consequently break it. Britnev fashioned the bow of his ship after the shape of old Pomor boats, which had been navigating icy waters of the White Sea and Barents Sea for centuries. Pilot was used between 1864-1890 for navigation in the Gulf of Finland between Kronstadt and Oranienbaum thus extending the summer navigation season by several weeks. Inspired by the success of the Pilot, Mikhail Britnev built a second similar vessel Boy ("Breakage" in Russian) in 1875 and a third Booy ("Buoy" in Russian) in 1889.
The cold winter of 1870-1871 led to the international recognition of Britnev's design. That year the Elbe River and the port of Hamburg froze, which caused a prolonged halt of navigation and huge commercial losses. In such circumstances, Germans purchased the Pilot's design from Britnev for some 300 rubles. Thus the German Eisbrecher I appeared in 1871, and other European countries soon followed suit.
With its rounded shape and strong metal hull, Pilot had all the main features present in the modern icebreakers, of which is why it is often considered the first true icebreaker. Another contender for this title is icebreaker Yermak, built in England for Russia according to the design of Admiral Stepan Makarov and under his supervision. Makarov borrowed the main principles from Pilot and applied them for creation of the first polar icebreaker, which was able to run over and crush pack ice. Between 1899-1911 Yermak sailed in heavy ice conditions for more than 1000 days.
At the beginning of the 20th century, several other 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, eventually converting to diesel-electric propulsion.
In 1941 the United States started building the Wind class. Research in Scandinavia and the Soviet Union led to a design that had a very strongly built short and wide hull, with a cut away forefoot and a rounded bottom. Powerful diesel-electric machinery drove two stern and one auxiliary bow propeller. These features would become the standard for postwar icebreakers until the 1990s.
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.
Nuclear icebreakers 
Several technological advances were introduced into icebreaking technology over the years, but it was not until the introduction of nuclear power in the Soviet icebreaker Lenin in 1959 that icebreakers developed to their full potential. NS Lenin was launched in 1957. It was both the world's first nuclear-powered surface ship and the first nuclear-powered civilian vessel. Lenin was put into operation in 1959 and officially decommissioned in 1989.
In May 2007, sea trials were completed for the nuclear-powered Russian ice-breaker NS 50 Let Pobedy. The vessel was put into service by Murmansk Shipping Company, which manages all eight Russian 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 Arktika class, and currently is the world's largest icebreaker.
Function of icebreakers 
Although sometimes used for polar research, most icebreakers today are needed to keep trade routes open where there are either seasonal or permanent ice conditions as well as escort merchant ships in ice-infested waters. Two major examples are in the Baltic Sea, where Finland has become the prime builder, and to Siberia, where the Soviets then Russians have operated their nuclear powered vessels. Icebreakers are expensive to build and very expensive to run, whether the icebreaker is powered by diesel engines, gas turbines 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.
Modern icebreakers may have one or more of the following special features designed to prevent the ship from getting stuck in ice or help it to break free if it does so: propellers in both bow and stern; azimuth thrusters; pumps to move water ballast from side to side; and holes on the hull below the waterline to eject air bubbles. Many icebreakers also carry aircraft (formerly seaplanes but now helicopters) to assist in reconnaissance and liaison.
Design and construction 
Ice resistance and hull form 
It is often incorrectly believed that an icebreaker drives its bow onto the ice and breaks it under the weight of the ship. While this may hold true in very thick ice, in reality the bending strength of sea ice is so low that the ice fails due to vertical loading before any noticeable change in the ship's trim is detected.
When an icebreaker is designed, one of the main goals is to minimize the ice resistance of the vessel. Although a number of different analytical, numerical, semi-empirical and empirical models have been developed over the years to estimate the forces from the interaction of the ship's hull and the surrounding ice, these forces are generally divided into breaking, submergence and frictional forces. The breaking forces are caused by crushing and bending of the ice as well as turning of the broken ice floes against the ship's hull. Once broken, the ice floes are pushed down along the hull, resulting in submergence and frictional forces. In unbroken level ice, the breaking component is generally dominating, but in brash ice and ice ridges the submergence and frictional components become more important. The average value of the longitudinal components of these instantaneous forces is called the ship's ice resistance. In addition to this, the hydrodynamic (open water) resistance of the vessel must also be taken into account.
Naval architects who design icebreakers use the so-called h-v-curve to determine the icebreaking capability of the vessel. It shows the speed (v) that the ship is able to achieve as a function of ice thickness (h). This is done by solving the velocity at which the thrust from the propellers equals to the total resistance of the vessel. An alternative means to determine the icebreaking capability of a vessel in different ice conditions is to perform model tests in an ice tank. Regardless of the method, the actual performance of new icebreakers is verified in ice trials once the ship has been built.
In order to minimize the icebreaking forces, the hull lines of an icebreaker are designed so that the flare at the waterline is as small as possible. As a result, icebreaking ships are characterized by a sloping or rounded stem as well as sloping sides. However, a spoon-shaped bow results in poor seakeeping characteristics and makes the foreship susceptible to slamming. Some icebreakers have a hull that is wider in the bow than in the stern. These so-called "reamers" increase the width of the ice channel and thus reduce frictional resistance in the aftship as well as improve the ship's maneuverability in ice. In addition to low friction paint, some icebreakers utilize an explosion-welded abrasion-resistant stainless steel ice belt that further reduces friction and protects the ship's hull from corrosion. In addition, several novel designs such as the flat Thyssen-Waas bow have been tried over the years.
Structural design 
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 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.
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, 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.
If built according to the rules set by a classification society such as the American Bureau of Shipping, icebreakers may be assigned an ice class based on the level of ice strengthening in the ship's hull. It is usually determined by the maximum ice thickness where the ship is expected to operate and other requirements such as possible limitations on ramming. In the 2000s, International Association of Classification Societies (IACS) has proposed adopting an unified system known as the Polar class to replace classification society specific ice class notations. It should be noted that the ice class is only an indication of the level of ice strengthening, not the actual icebreaking capability of an icebreaker.
Power and propulsion 
|This section requires expansion. (October 2012)|
The majority of icebreakers built since 1940 have a diesel-electric propulsion system consisting of a power plant, usually a number of medium-speed diesel engines coupled to generators, and either direct current (DC) or alternating current (AC) propulsion motors turning the propellers. Such propulsion arrangement has several advantages in icebreaking operations mainly due to the low-speed torque characteristics of electric motors. However, a number of icebreakers have also been built with controllable pitch propellers mechanically coupled to the main engines.
The United States has used diesel-electric powerplants in all icebreakers. Early ships had up to ten Fairbanks-Morse diesel engines, the same engine as used in submarines. The Polar class, with six Alco engines, have less diesel power for their displacement than previous ships but utilize gas turbines for "boost" power. The newest USCG icebreaker, Healy, has returned to full diesel power, with four Sulzer engines.
The Soviet Union and later Russia has operated multiple engine diesel-electric drive icebreakers, many built in Finland, since the 1950s. The Krasin has nine Sulzer engines, the same type found in the U.S. Healy. The Soviets have also developed nuclear-steam turbine-electric drive icebreakers, see separate section.
Until the 1980s, icebreakers operating regularly in ridged ice fields were fitted with first one and later two bow propellers to create a powerful flush along the hull of the vessel. This considerably increased the icebreaking capability of the vessels by reducing the friction between the hull and the ice, and allowed the icebreakers to penetrate thick ice ridges without ramming. However, the bow propellers are not suitable for polar icebreakers operating in the presence of harder multi-year ice and thus have not been used in the Arctic. Alternative methods used in newer icebreakers have included blowing air bubbles from nozzles under the waterline or flushing the ice with a powerful water deluge system from above, and operating a vessel fitted with azimuth thrusters astern (backwards).
Recent advances 
Recent advances in ship propulsion have produced new experimental icebreakers. Azimuth thrusters remove the need of traditional propellers and rudders by having the propellers in steerable gondolas that can rotate 360 degrees around a vertical axis. These thrusters improve propulsion efficiency, icebreaking capability and steering. The use of azimuth thrusters also allows a ship to move astern in ice without losing maneuverability, which has led to the development of double acting ships, vessels with the stern shaped like an icebreaker's bow. This allows the bow to be designed for open water performance without compromising the ship's ability to operate in difficult ice conditions. Azimuth thrusters have also made it possible to develop icebreakers that operate sideways to open a wide channel though ice.
There has also been some research done[by whom?] to make a bulbous bow for ferries and cargo ships that can make the initial break in the ice, not by sledding up on top of the ice, but by pushing it up from underneath, so as to spare the rest of the hull from the impact that would otherwise occur, thus keeping the speed up. This also has marked advantages in wavebreaking and fuel efficiency, but breaking thicker ice may be limited, since it has to be able to reach under the ice in the first place. It is mostly used in conditions where ice thickness is limited.
See also 
- List of icebreakers
- Nuclear-powered icebreaker
- Double acting ship
- Ice class
- Polar class
- River icebreaker
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- "Sulzer Z40". Jastra Co. Ltd. Retrieved 2013-01-09.
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|Wikimedia Commons has media related to: Icebreakers|
- Ice breaker by Picture
- Gallery of Russian icebreakers
- "Ice heroes": Read a Q&A with Canadian Coast Guard acting commanding officer.
- Canadian Geographic: View a Canadian Coast Guard slideshow.
- Pushing the Limits Short history of Russian icebreakers by Roderick Eime
- Icebreaker at the North Pole: Video of nuclear icebreaker Yamal visiting the North Pole in 2001
- Book Polar Icebreakers in a Changing World: An Assessment of U.S. Needs (2007)