page_title: Icebreaker
section_title: Power and propulsion
hierarchical_section_title: Icebreaker / Characteristics / Power and propulsion
caption_attribution_description: Mastera approaching Port of Rotterdam, Holland 20-Jun-2007.
context_page_description: 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 frozen-over water or pack ice. The bending strength of sea ice is low enough that the ice breaks usually 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. 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.
context_section_description: Before the first diesel-electric icebreakers were built in the 1930s, icebreakers were either coal- or oil-fired steam ships. Reciprocating steam engines were preferred in icebreakers due to their reliability, robustness, good torque characteristics, and ability to reverse the direction of rotation quickly. 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).
  Since the Second 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 with direct current (DC) generators and propulsion motors, but over the years the technology advanced first to alternating current (AC) generators and finally to frequency-controlled AC-AC systems. 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. Since the mid-1970s, the most powerful diesel-electric icebreakers have been the formerly Soviet and later Russian icebreakers Ermak, Admiral Makarov and Krasin which have nine twelve-cylinder diesel generators producing electricity for three propulsion motors with a combined output of 26,500 kW (35,500 hp). In the 2020s, they will be surpassed by the new Canadian polar icebreaker, CCGS John G. Diefenbaker, which will have a combined propulsion power of 36,000 kW (48,000 hp).
  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 and controllable 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.
  The steam-powered icebreakers were resurrected in the late 1950s when the Soviet Union commissioned the first nuclear-powered icebreaker, Lenin, in 1959. It had a nuclear-turbo-electric powertrain in which the nuclear reactor was used to produce steam for turbogenerators, which in turn produced electricity for propulsion motors. Starting from 1975, the Russians commissioned six Arktika-class nuclear icebreakers of which the last, 2007-built 50 Let Pobedy, is the largest and most powerful icebreaker in the world as of 2013 at 52,800 kW (70,800 hp). In addition, two shallow-draft Taymyr-class nuclear icebreakers were built in Finland for the Soviet Union in the late 1980s. The 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 a new 60,000 kW (80,000 hp) icebreakers to replace the aging Arktika class. The first vessel of this type is expected to enter service in 2017.
  The 1969-built Canadian polar icebreaker CCGS Louis S. St-Laurent was 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.
  The most powerful conventional (non-nuclear) icebreakers in the world, two Polar-class icebreakers operated by the United States Coast Guard, have a combined diesel-electric and mechanical propulsion system that consists of six diesel engines and three gas turbines. While the diesel engines are coupled to genera
caption: Mastera, one of the first double acting tankers, in open water. In ice, the vessel will proceed stern first.
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