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Icebreaker

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For other uses, seeIcebreaker (disambiguation).
USCGCHealy(WAGB-20)at right breaks ice around the Russian-flagged tankerRenda,250 miles (400 km) south ofNome, Alaska.

Anicebreakeris a special-purposeshiporboatdesigned to move andnavigatethroughice-covered waters, and provide safewaterwaysfor other boats and ships. Although the term usually refers to ice-breakingships,it may also refer to smaller vessels, such as the icebreaking boats that were once used on thecanals of the United Kingdom.

For a ship to be considered an icebreaker, it requires three traits most normal ships lack: a strengthenedhull,an ice-clearing shape, and the power to push throughsea ice.[1]

Icebreakers clear paths by pushing straight into frozen-over water orpack ice.The bending strength of sea ice is low enough that the ice breaks usually without noticeable change in the vessel'strim.In cases of very thick ice, an icebreaker can drive itsbowonto the ice to break it under the weight of the ship. A buildup of broken ice in front of a ship can slow it down much more than the breaking of the ice itself, so 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 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.[2]

History[edit]

Earliest icebreakers[edit]

Prior to ocean-going ships, ice breaking technology was developed on inland canals and rivers using laborers with axes and hooks. The first recorded primitive icebreaker ship was a barge used by the Belgian town ofBrugesin 1383 to help clear the town moat.[3][4]The efforts of the ice-breaking barge were successful enough to warrant the town purchasing four such ships.

Ice breaking barges continued to see use during the colder winters of theLittle Ice Agewith growing use in theLow Countrywhere significant amounts of trade and transport of people and goods took place. In the 15th century the use of ice breakers inFlanders(Oudenaarde,Kortrijk,Ieper,Veurne,DiksmuideandHulst) was already well established. The use of the ice breaking barges expanded in the 17th century where every town of some importance in the Low Country used some form of icebreaker to keep their waterways clear.

Before the 17th century the specifications of icebreakers are unknown. The specifications for ice breaking vessels show that they were dragged by teams of horses and the heavy weight of the ship pushed down on the ice breaking it. They were used in conjunction with teams of men with axes and saws and the technology behind them didn't change much until the industrial revolution.

Two teams of horses and a team of workers drag an ice breaker through the canals ofAmsterdamin 1733.

Sailing ships in the polar waters[edit]

Ice-strengthened ships were used in the earliest days of polar exploration. These were originally wooden and based on existing designs, but reinforced, particularly around thewaterlinewith 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, at the 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.

A 17th-century Russiankochin a museum

The first boats to be used in the polar waters were those of theEskimos.Theirkayaksare small human-powered boats with a covered deck, and one or more cockpits, each seating onepaddlerwho strokes a single or double-bladedpaddle.Such boats have no icebreaking capabilities, but they are light and well fit to carry over the ice.

In the 9th and 10th centuries, theViking expansionreached theNorth Atlantic,and eventuallyGreenlandandSvalbardin the Arctic.Vikings,however, operated their ships in the waters that were ice-free for most of the year, in the conditions of theMedieval Warm Period.

In the 11th century, in North Russia the coasts of theWhite Sea,named so for being ice-covered for over half of a year, started being settled. The mixed ethnic group of the Karelians and the Russians in the North-Russia that lived on the shores of theArctic Oceanbecame known asPomors( "seaside settlers" ). Gradually they developed a special type of small one- or two-mast woodensailing ships,used for voyages in the ice conditions of the Arctic seas and later onSiberianrivers. These earliest icebreakers were calledkochi.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-iceportage.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.[5]

In the 19th century, similar protective measures were adopted to modern steam-powered icebreakers. Some notablesailing shipsin the end of theAge of Sailalso featured the egg-shaped form like that ofPomorboats, for example theFram,used byFridtjof Nansenand other greatNorwegianPolar explorers.Framwas the wooden ship to have sailed farthest north (85°57'N) and farthest south (78°41'S), and one of the strongest wooden ships ever built.

Steam-powered icebreakers[edit]

City Ice Boat No. 1at theDelaware River.Thepaddle steamerwas built in 1837.

An early ship designed to operate in icy conditions[6]was a 51-metre (167 ft) woodenpaddle steamer,City Ice Boat No. 1,that was built for the city ofPhiladelphiaby Vandusen & Birelyn in 1837. The ship was powered by two 250-horsepower(190 kW) steam engines and her wooden paddles were reinforced with iron coverings.[7]

With a rounded shape and strong metal hull, the RussianPilotof 1864 was an important predecessor of modern icebreakers with propellers. The ship was built on the orders of merchant and shipbuilderMikhail Britnev.She had the bow altered to achieve an ice-clearing capability (20° raise from keel line). This allowedPilotto push herself 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 andBarents Seafor centuries.Pilotwas used between 1864 and 1890 for navigation in theGulf of FinlandbetweenKronstadtandOranienbaumthus extending the summer navigation season by several weeks. Inspired by the success ofPilot,Mikhail Britnev built a second similar vesselBoy( "Breakage" in Russian) in 1875 and a thirdBooy( "Buoy" in Russian) in 1889.

The cold winter of 1870–1871 caused theElbe Riverand the port ofHamburgto freeze over, causing a prolonged halt to navigation and huge commercial losses. Carl Ferdinand Steinhaus reused the altered bowPilot's design from Britnev to make his own icebreaker,[8]Eisbrecher I.[9]

Yermakis considered the first true modern sea-going icebreaker.

The first true modern sea-going icebreaker[10]was built at the turn of the 20th century. IcebreakerYermak,was built in 1899 at theArmstrong Whitworthnaval yard inEnglandunder contract from theImperial Russian Navy.The ship borrowed the main principles fromPilotand applied them to the creation of the first polar icebreaker, which was able to run over and crushpack ice.The ship displaced 5,000 tons, and her steam-reciprocating enginesdelivered 10,000 horsepower (7,500 kW). The ship wasdecommissionedin 1963 andscrappedin 1964, making her one of the longest serving icebreakers in the world.

In Canada, the government needed to provide a way to prevent flooding due to ice jam on theSt. Lawrence River.Icebreakers were built in order to maintain the river free of ice jam, east ofMontréal.In about the same time, Canada had to fill its obligations in the Canadian Arctic. Large steam icebreakers, like the 80-metre (260 ft)CGSN.B. McLean(1930) andCGSD'Iberville(1952), were built for this dual use (St. Lawrence flood prevention and Arctic replenishment).

At the beginning of the 20th century, several other countries began to operate purpose-built icebreakers. Most were coastal icebreakers, but Canada, Russia, and later, theSoviet Union,also built several oceangoing icebreakers up to 11,000 tons in displacement.

Diesel-powered icebreakers[edit]

Before the firstdiesel-electricicebreakers were built in the 1930s, icebreakers were either coal- or oil-firedsteam ships.[11]Reciprocating steam engines were preferred in icebreakers due to their reliability, robustness, good torque characteristics, and ability to reverse the direction of rotation quickly.[12]During the steam era, the most powerful pre-war steam-powered icebreakers had a propulsion power of about 10,000 shaft horsepower (7,500 kW).[11]

Ymeris the world's first diesel-electric icebreaker, built in 1933.

The world's firstdiesel-electricicebreaker was the 4,330-ton Swedish icebreakerYmerin 1933. At 9,000 hp (6,700 kW) divided between two propellers in the stern and one propeller in the bow, she remained the most powerful Swedish icebreaker until the commissioning ofOdenin 1957.Ymerwas followed by the FinnishSisu,the first diesel-electric icebreaker in Finland, in 1939.[13][14]Both vessels were decommissioned in the 1970s and replaced by much larger icebreakers in both countries, the 1976-builtSisuin Finland and the 1977-builtYmerin Sweden.

In 1941, the United States started building theWind 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.[15][16][17]These features would become the standard for postwar icebreakers until the 1980s.

Since the mid-1970s, the most powerful diesel-electric icebreakers have been the formerly Soviet and later Russian icebreakersErmak,Admiral MakarovandKrasinwhich have ninetwelve-cylinderdiesel generators producing electricity for three propulsion motors with a combined output of 26,500 kW (35,500 hp).[11]In the 2020s, they will be surpassed by the new Canadian polar icebreaker,CCGSJohn G. Diefenbaker,which will have a combined propulsion power of 36,000 kW (48,000 hp).

Canada[edit]

TheCCGSLouis S. St-Laurentis aCanadian Coast Guardicebreaker.

In Canada, diesel-electric icebreakers started to be built in 1952, first withHMCSLabrador(was transferred later to the Canadian Coast Guard), using theUSCG Wind-class design but without the bow propeller. Then in 1960, the next step in the Canadian development of large icebreakers came whenCCGSJohn A. Macdonaldwas completed at Lauzon, Quebec. A considerably bigger and more powerful ship thanLabrador,John A.Macdonaldwas an ocean-going icebreaker able to meet the most rigorous polar conditions. Her diesel-electric machinery of 15,000 horsepower (11,000 kW) was arranged in three units transmitting power equally to each of three shafts.

Canada's largest and most powerful icebreaker, the 120-metre (390 ft)CCGSLouis S. St-Laurent,was delivered in 1969. Her original three steam turbine, nine generator, and three electric motor system produces 27,000shaft horsepower(20,000 kW). 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 five diesels, three generators, and three electric motors, giving about the same propulsion power.

On 22 August 1994Louis S. St-LaurentandUSCGCPolar Seabecame 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. It is now planned to be kept in service through the 2020s pending the introduction of a new class ofpolar icebreakerfor the Coast Guard.[18]

Nuclear-powered icebreakers[edit]

Main article:Nuclear-powered icebreaker

Russiacurrently operates all existing and functioningnuclear-poweredicebreakers.[19]The first one,NSLenin,was launched in 1957 and entered operation in 1959, before being officially decommissioned in 1989. It was both the world's firstnuclear-powered surface shipand the firstnuclear-powered civilian vessel.

A Soviet stamp ofLenin,the world's first nuclear-powered icebreaker.

The second Soviet nuclear icebreaker was NSArktika,the lead ship of theArktikaclass.In service since 1975, she was the first surface ship to reach theNorth Pole,on August 17, 1977. Several nuclear-powered icebreakers were also built outside the Soviet Union. Two shallow-draftTaymyr-class nuclear icebreakerswere built in Finland for the Soviet Union in the late 1980s.[11]

In May 2007,sea trialswere completed for the nuclear-powered Russian icebreaker NS50 Let Pobedy.The vessel was put into service byMurmanskShipping Company, which manages all eight Russian state-owned nuclear icebreakers. The keel was originally laid in 1989 byBaltic WorksofLeningrad,and the ship was launched in 1993 as NSUral.This icebreaker is intended to be the sixth and last of theArktikaclass.[20]

Function[edit]

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 fornavigation in ice,they are usually not powerful enough to manage the ice by themselves. For this reason, in theBaltic Sea,theGreat Lakesand theSaint Lawrence Seaway,and along theNorthern 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.[11]

Finnish icebreakerOtsoescorting a merchant ship in the Baltic Sea

Some icebreakers are also used to support scientific research in theArcticand 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.[11]Countries such asArgentinaandSouth Africa,which do not require icebreakers in domestic waters, have research icebreakers for carrying out studies in the polar regions.

Asoffshore drillingmoves to the Arctic seas, icebreaking vessels are needed to supply cargo and equipment to the drilling sites and protect thedrillshipsandoil platformsfrom ice by performing ice management, which includes for example breaking drifting ice into smaller floes and steeringicebergsaway from the protected object.[11]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.

AUnited States Coast Guardicebreaker inMcMurdo Soundin support ofOperation Deep Freeze.

TheUnited States Coast Guarduses icebreakers to help conduct search and rescue missions in the icy, polar oceans. United States icebreakers serve to defend economic interests and maintain the nation's presence in the Arctic and Antarctic regions. As the icecaps in the Arctic continue to melt, there are more passageways being discovered. These possible navigation routes cause an increase of interests in the polar hemispheres from nations worldwide. The United States polar icebreakers must continue to support scientific research in the expanding Arctic andAntarcticoceans.[21]Every year, a heavy icebreaker must performOperation Deep Freeze,clearing a safe path for resupply ships to theNational Science Foundation’s facility McMurdo in Antarctica. The most recent multi-month excursion was led by thePolar Starwhich escorted a container and fuel ship through treacherous conditions before maintaining the channel free of ice.[22]

Characteristics[edit]

Ice resistance and hull form[edit]

The Estonian icebreakerBotnicahas a typical round icebreaker bow with small stem and flare angles. Theexplosion-weldedice belt and "reamers" are also visible.

Icebreakers are often described as ships that drive their sloping bows onto the ice and break it under the weight of the ship.[23]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 lowflexural 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.[24]

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 architectswho design icebreakers use the so-calledh-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 thethrustfrom the propellers equals the combined hydrodynamic and ice resistance of the vessel.[1]An alternative means to determine the icebreaking capability of a vessel in different ice conditions such aspressure ridgesis to performmodel testsin anice 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 theflareat the waterline is as small as possible. As a result, icebreaking ships are characterized by a sloping or roundedstemas well as sloping sides and a short parallel midship to improve maneuverability in ice.[24]However, thespoon-shaped bowand round hull have poor hydrodynamic efficiency andseakeepingcharacteristics, and make the icebreaker susceptible toslamming,or the impacting of the bottom structure of the ship onto the sea surface.[1]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.[11]

The Swedish icebreakerOdenis built with a flat bow and a water deluge system designed to reduce friction between the hull and ice.

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 anexplosion-weldedabrasion-resistant stainless steel ice belt that further reduces friction and protects the ship's hull from corrosion. Auxiliary systems such as powerful water deluges and air bubbling systems are used to reduce friction by forming a lubricating layer between the hull and the ice. Pumping water between tanks on both sides of the vessel results in continuous rolling that reduces friction and makes progress through the ice easier. Experimental bow designs such as the flat Thyssen-Waas bow and a cylindrical bow have been tried over the years to further reduce the ice resistance and create an ice-free channel.[11]

Structural design[edit]

Icebreakers and other ships operating in ice-filled waters require additional structural strengthening against various 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.[2]

Bow of theRVPolarstern,a research icebreaker. The bow of an icebreaker is usually the most reinforced area of the ship.

Short and stubby icebreakers are generally built using transverse framing in which the shell plating is stiffened with frames placed about 400 to 1,000 millimetres (1 to 3 ft) apart as opposed tolongitudinal framingused in longer ships. Near the waterline, the frames running in vertical direction distribute the locally concentrated ice loads on the shell plating to longitudinal girders called stringers, which in turn are supported by web frames and bulkheads that carry the more spread-out hull loads.[2]While the shell plating, which is in direct contact with the ice, can be up to 50 millimetres (2.0 in) thick in older polar icebreakers, the use of high strength steel withyield strengthup to 500 MPa (73,000 psi) in modern icebreakers results in the same structural strength with smaller material thicknesses and lower steel weight. Regardless of the strength, the steel used in the hull structures of an icebreaker must be capable of resisting brittle fracture in low ambient temperatures and high loading conditions, both of which are typical for operations in ice-filled waters.[2][25]

If built according to the rules set by aclassification societysuch asAmerican Bureau of Shipping,Det Norske VeritasorLloyd's Register,icebreakers may be assigned anice classbased 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. While the ice class is generally an indication of the level of ice strengthening, not the actual icebreaking capability of an icebreaker, some classification societies such as theRussian Maritime Register of Shippinghave operational capability requirements for certain ice classes. Since the 2000s,International Association of Classification Societies(IACS) has proposed adopting an unified system known as thePolar Class(PC) to replace classification society specific ice class notations.

Power and propulsion[edit]

Since theSecond World War,most icebreakers have been built with diesel-electric propulsion in which diesel engines coupled to generators produce electricity for propulsion motors that turn the fixed pitch propellers. The first diesel-electric icebreakers were built withdirect current(DC) generators and propulsion motors, but over the years the technology advanced first toalternating current(AC) generators and finally to frequency-controlled AC-AC systems.[11]In modern diesel-electric icebreakers, the propulsion system is built according to the power plant principle in which the main generators supply electricity for all onboard consumers and no auxiliary engines are needed.

Although the diesel-electric powertrain is the preferred choice for icebreakers due to the good low-speed torque characteristics of the electric propulsion motors, icebreakers have also been built with diesel engines mechanically coupled to reduction gearboxes andcontrollable pitch propellers.The mechanical powertrain has several advantages over diesel-electric propulsion systems, such as lower weight and better fuel efficiency. However, diesel engines are sensitive to sudden changes in propeller revolutions, and to counter this mechanical powertrains are usually fitted with large flywheels or hydrodynamic couplings to absorb the torque variations resulting from propeller-ice interaction.[11]

The 1969-built Canadian polar icebreakerCCGSLouis S. St-Laurentwas one of the few icebreakers fitted with steam boilers and turbogenerators that produced power for three electric propulsion motors. It was later refitted with five diesel engines, which provide better fuel economy than steam turbines. Later Canadian icebreakers were built with diesel-electric powertrain.[11]

TwoPolar-class icebreakersoperated by theUnited States Coast Guard,have a combined diesel-electric and mechanical propulsion system that consists of six diesel engines and threegas turbines.While the diesel engines are coupled to generators that produce power for three propulsion motors, the gas turbines are directly coupled to the propeller shafts driving controllable pitch propellers.[11]The diesel-electric power plant can produce up to 13,000 kW (18,000 hp) while the gas turbines have a continuous combined rating of 45,000 kW (60,000 hp).[26]

View of the underside ofUSCGCHealyhull and propellers.

The number, type and location of the propellers depends on the power, draft and intended purpose of the vessel. Smaller icebreakers and icebreaking special purpose ships may be able to do with just one propeller while large polar icebreakers typically need up to three large propellers to absorb all power and deliver enough thrust. Some shallow draughtriver icebreakershave been built with four propellers in the stern.Nozzlesmay be used to increase the thrust at lower speeds, but they may become clogged by ice.[11]Until the 1980s, icebreakers operating regularly inridged ice fieldsin the Baltic Sea 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.[27]

Azimuth thrustersremove 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 maneuverability of the vessel. The use of azimuth thrusters also allows a ship to move astern in ice without losing manoeuvrability. This has led to the development ofdouble acting ships,vessels with the stern shaped like an icebreaker's bow and the bow designed for open water performance. In this way, the ship remains economical to operate in open water without compromising its ability to operate in difficult ice conditions. Azimuth thrusters have also made it possible to develop new experimentalicebreakers that operate sidewaysto open a wide channel through ice.

Nuclear-powered[edit]

Reactor head for theYamal,a nuclear-powered icebreaker.

The steam-powered icebreakers were resurrected in the late 1950s when the Soviet Union commissioned the firstnuclear-powered icebreaker,Lenin,in 1959. It had a nuclear-turbo-electric powertrain in which the nuclear reactor was used to produce steam forturbogenerators,which in turn produced electricity for propulsion motors. Starting from 1975, the Russians commissioned sixArktika-class nuclear icebreakers.Soviets also built a nuclear-powered icebreaking cargo ship,Sevmorput,which had a single nuclear reactor and a steam turbine directly coupled to the propeller shaft. Russia, which remains the sole operator of nuclear-powered icebreakers, is currently building 60,000 kW (80,000 hp) icebreakers to replace the agingArktikaclass. The first vessel of this type entered service in 2020.

Resonance method[edit]

Main article:Resonance method of ice destruction

Ahovercraftcan break ice by the resonance method. This causes the ice and water to oscillate up and down until the ice suffers sufficient mechanical fatigue to cause a fracture.[28]

See also[edit]

References[edit]

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  2. ^abcdChapter 5 Ship Design and Construction for Ice Operations.Canadian Coast Guard.Retrieved 2013-08-20.
  3. ^de Kraker, Adrian M.J. (6 June 2016)."Ice and water. The removal of ice on waterways in the Low Countries, 1330–1800".Water History.9(2): 109–128.doi:10.1007/s12685-016-0152-3.
  4. ^Degroot, Dagomar (19 February 2019)."Some places flourished in the Little Ice Age. There are lessons for us now".The Washington Post.Retrieved11 November2019.
  5. ^Marchenko, Nataly (21 November 2007)."Navigation in ice conditions. Experience of Russian sailors"(PDF).Norsk Polarinstitutt (Svalbard Science Forum).Archived fromthe original(PDF)on 2012-07-23.
  6. ^Madrigal, Alexis C. (17 January 2012)."7 Things You Should Know About (Nuclear-Powered, Drone-Guided) Icebreakers".The Atlantic.Retrieved22 December2018.
  7. ^"AMSA Background Research Documents: History and Development of Arctic Marine Technology er"(PDF).The Protection of the Arctic Marine Environment Working Group (PAME).Archived fromthe original(PDF)on 2021-02-24.Retrieved2011-07-03.
  8. ^Veselov, Pavel (1993)."Prolonging the navigation"(PDF)(in Russian). pp. 36–37. Archived fromthe original(PDF)on 2011-08-16.Retrieved2018-12-27.
  9. ^Bruun, P. (1989).Port Engineering, Volume 1: Harbor Planning, Breakwaters, and Marine Terminals(4th ed.). Gulf Publishing Company. p. 1375.ISBN0-87201-843-1.
  10. ^Farhall, David (13 November 2011)."U.S. Arctic Prospects Ride on New Icebreakers".Bloomberg.Archived fromthe originalon 23 October 2013.Retrieved22 December2018.
  11. ^abcdefghijklmnSegercrantz, H. (1989): Icebreakers — Their Historical and Technical Development.Interdisciplinary Science Reviews,Vol 14, No. 1.
  12. ^Laurell, Seppo (1992).Höyrymurtajien aika.Jyväskylä: Gummerus Kirjapaino Oy.ISBN951-47-6775-6.
  13. ^""Ymer": The first diesel-electric icebreaker in the world ".Scandinavian Shipping Gazette.Archived fromthe originalon 2013-08-09.Retrieved2013-08-09.
  14. ^"Photo from building of Icebreaker Ymer".Passagen. Archived fromthe originalon 7 January 2005.Retrieved5 September2013.
  15. ^Canney, Donald L."Icebreakers and the U.S. Coast Guard".US Coast Guard Historian's Office.United States Coast Guard.Retrieved2013-01-09.
  16. ^Jane's Fighting Ships of World War II.Crescent Books (Random House). 1998. p.308.ISBN0517-67963-9.
  17. ^Silverstone, Paul H (1966).U.S. Warships of World War II.Doubleday and Company. p. 378.
  18. ^"Canada's largest icebreaker to undergo life extension upgrade".4 November 2020.
  19. ^Henderson, Isaiah (July 18, 2019)."Cold Ambition: The New Geopolitical Faultline".The California Review.RetrievedJuly 18,2019.
  20. ^"World's largest icebreaker".Ships Monthly.May 2007. Archived fromthe originalon 2009-02-27.
  21. ^Scott, Nathan (2010).Coast Guard Polar Icebreakers.New York: Nova Science Publishers. pp. 1–20.ISBN978-1-60692-987-2.
  22. ^Atkinson, Peter (July 2018)."Keeping it Working".Sea Power Magazine.61(6): 26–28.
  23. ^Turunen, Ari; Partanen, Petja (2011).Raakaa voimaa—Suomalaisen jäänmurtamisen tarina[Brute force - the Finnish Icebreaking Story] (in Finnish). Jyväskylä: Atena Kustannus Oy.ISBN978-951-796-762-4.
  24. ^abNational Research Council (2007):Polar Icebreakers in a Changing World: An Assessment of U.S. Needs.The National Academies Press, Washington D.C.
  25. ^Norden, R. (1989): Extra high strength structural steels for ice breakers. Proceedings of the 10th International Conference on Port and Ocean Engineering under Arctic Conditions (POAC'89), Volume 2, page 839.
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  27. ^"Arctia Fleet".Arctia Shipping.Archived fromthe originalon 2013-10-23.Retrieved2013-01-11.
  28. ^"Canadian Coast Guard Ice Breaking hovercraft".griffonhoverwork.Archived fromthe originalon 2017-02-07.Retrieved2017-02-06.

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