An icebreaker is a special-purpose ship or boat designed to move and navigate through ice-covered waters, and provide safe waterways for other boats and ships. 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 vise and causing damage. This vise-like action is caused by the force of winds and tides on ice formations. Although these wind and tidal forces may originate many miles away, the ice acts as a medium for this force.
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 most of the year, in the conditions of the Medieval Warm Period.
In the 11th century, in North-Russia started settling the coasts of the White Sea, named so for being ice-covered for over half of a year. The mixed ethnic group of the Karelians and the Russians in the North-Russia 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. The koch's hull was protected by a belt of ice-floe resistant flush skin-planking 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 like that of Pomor boats, for example the famous Fram, used by Roald Amundsen 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.
The first true modern sea-going icebreaker was built at the turn of the 20th century. Icebreaker Yermak, was built in 1897 at the Armstrong Whitworth naval yards in England under contract from the Russian Navy. The ship was able to run over and crush pack ice, weighing 5,000 tons, and its steam-reciprocating engines delivered 10,000 horsepower. The ship was so well built that it was only finally decommissioned and scrapped in 1963, making it one of the longest serving ice-breakers in the world.
At the beginning of the 20th century, several countries began to operate purpose-built icebreakers. Most were coastal icebreakers, but Canada, Russia, and later, the Soviet Union, also built several oceangoing icebreakers of around 10,000 ton displacement.
The world's first diesel-electric icebreaker was the 4,330-ton Swedish icebreaker Ymer in 1933. At 9,000 hp divided between two propellers in the stern and one propeller in the bow, she remained the most powerful Swedish icebreaker until the commissioning of Oden in 1957. Ymer was followed by the Finnish Sisu, the first diesel-electric icebreaker in Finland, in 1939. Both vessels were decommissioned in the 1970s and replaced by much larger icebreakers in both countries, the 1976-built Sisu in Finland and the 1977-built Ymer in Sweden.
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 1980s.
Canada's largest and most powerful icebreaker, the 120-metre (390 ft) CCGS Louis S. St. Laurent, was delivered in 1969. A multi-year mid-life refit project (1987–1993) saw the ship get a new bow, and a new propulsion system. The new power plant consists of 5 diesels/ 3 generators/ 3 electric motors giving about the same SHP. On 22 August 1994, Louis S. St-Laurent and USCGC Polar Sea became the first North American surface vessels to reach the North Pole. The vessel was originally scheduled to be decommissioned in 2000; however, a refit extended the decommissioning date to 2017.
Russia currently operates all existing and functioning nuclear-powered icebreakers. The first one, NS Lenin, was launched in 1957 and entered operation in 1959, before being officially decommissioned in 1989. It was both the world's first nuclear-powered surface ship and the first nuclear-powered civilian vessel.
The second Soviet nuclear icebreaker was NS Arktika, the lead ship of the Arktika class. In service since 1975, she was the first surface ship to reach the North Pole, on 17 August 1977.
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.
Today, most icebreakers are needed to keep trade routes open where there are either seasonal or permanent ice conditions. While the merchant vessels calling ports in these regions are strengthened for navigation in ice, they are usually not powerful enough to manage the ice by themselves. For this reason, in the Baltic Sea, the Great Lakes and the Saint Lawrence Seaway, and along the Northern Sea Route, the main function of icebreakers is to escort convoys of one or more ships safely through ice-filled waters. When a ship becomes immobilized by ice, the icebreaker has to free it by breaking the ice surrounding the ship and, if necessary, open a safe passage through the ice field. In difficult ice conditions, the icebreaker can also tow the weakest ships.
Some icebreakers are also used to support scientific research in the Arctic and Antarctic. In addition to icebreaking capability, the ships need to have reasonably good open water characteristics for transit to and from the polar regions, facilities and accommodation for the scientific personnel, and cargo capacity for supplying research stations on the shore. Countries such as Argentina and South Africa, which do not require icebreakers in domestic waters, have research icebreakers for carrying out studies in the polar regions.
As offshore drilling moves to the Arctic seas, icebreaking vessels are needed to supply cargo and equipment to the drilling sites and protect the drillships and oil platforms from ice by performing ice management, which includes for example breaking drifting ice into smaller floes and steering icebergs away from the protected object. In the past, such operations were carried out primarily in North America, but today Arctic offshore drilling and oil production is also going on in various parts of the Russian Arctic.
Icebreakers are often described as ships that drive their sloping bows onto the ice and break it under the weight of the ship. In reality, this only happens in very thick ice where the icebreaker will proceed at walking pace or may even have to repeatedly back down several ship lengths and ram the ice pack at full power. More commonly the ice, which has a relatively low flexural strength, is easily broken and submerged under the hull without a noticeable change in the icebreaker's trim while the vessel moves forward at a relatively high and constant speed.
When an icebreaker is designed, one of the main goals is to minimize the forces resulting from crushing and breaking the ice, and submerging the broken floes under the vessel. The average value of the longitudinal components of these instantaneous forces is called the ship's ice resistance. 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 calculating the velocity at which the thrust from the propellers equals the combined hydrodynamic and ice resistance of the vessel. An alternative means to determine the icebreaking capability of a vessel in different ice conditions such as pressure ridges is to perform model tests in an ice tank. Regardless of the method, the actual performance of new icebreakers is verified in full scale ice trials once the ship has been built.
In order to minimize the icebreaking forces, the hull lines of an icebreaker are usually 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 and a short parallel midship to improve maneuverability in ice. However, the spoon-shaped bow and round hull have poor hydrodynamic efficiency and seakeeping characteristics, and make the icebreaker susceptible to slamming. For this reason, the hull of an icebreaker is often a compromise between minimum ice resistance, maneuverability in ice, low hydrodynamic resistance, and adequate open water characteristics.
Icebreakers and other ships operating in ice-filled waters require additional structural strengthening against various global and local loads resulting from the contact between the hull of the vessel and the surrounding ice. As ice pressures vary between different regions of the hull, the most reinforced areas in the hull of an icegoing vessel are the bow, which experiences the highest ice loads, and around the waterline, with additional strengthening both above and below the waterline to form a continuous ice belt around the ship.