An icebreaker is a special purpose ship or boat designed to move and navigate through ice-covered waters. Although this article mainly discusses icebreaking ships, the term can refer to smaller vessels (eg 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 components: a strengthened hull, an ice-clearing shape, and the power to push through, all of which most normal ships do not possess.
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 successfully clear the debris from its path is essential for its safety.
Function of icebreakers
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 water bubbles, all designed to allow an icebreaker stuck amidst thick ice to break free. Many icebreakers also carry aircraft (formerly seaplanes and now helicopters) to assist in reconnaissance and liaison.
The shape which allows icebreakers to move through ice also makes them uncomfortable in open water and means they have poor fuel efficiency.
Icebreakers tend to roll side to side causing discomfort to the crew. Some new icebreakers such as the USCGC Healy make use of anti-roll tanks. Anti-roll tanks use computer controlled pumps to rapidly shift ballast water side to side to keep the vessel upright. This same technology can be very hazardous if used improperly.
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 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 a risk in high seas.
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, 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 travelling 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 and be swept downstream by the current. The purpose is usually not to provide navigation channels, rather, to prevent ice dams from forming on bridge structures, thus damaging them and causing local flooding.
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 vice-like action is caused by the force of winds and tides transmitted through ice formations that, although many miles distant, transmit the pressure.
It is supposed that the first icebreaker equipped by steam engine was built in Kronshtadt in 1864. The steam-powered icebreaker was known as Pilot. 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 tonnes 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.