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Icebreakers and Ice Strengthened Ships
Such strengthening was designed to help the ship push through ice and also in case the ship was "nipped" by the ice. Nipped is an innocuous sounding word to describe a terrible and powerful event when ice floes around a ship driven by winds and tides (often many miles distant) push against the ship trapping it as if in a vice and causing damage - sometimes damage enough to reduce the ship to match-wood.
Such damage might be survivable, it might cause the loss of the ship when the ice finally relents - the ship now no longer being able to float as happened to Shackleton's Endurance or it might cause the loss of the ship in as little as 15 minutes from first pressure being exerted. In the days of wooden ships, the only vessel that could survive such treatment was the Fram, built for Fridtjof Nansen. The Fram was prodigiously strong, but it's chief defence was that when squeezed from the sides it would respond by rising up due to a rounded hull shape. Even the mighty Fram at one point looked to be be in danger when ice floes built up to such an extent that they might fall on it and prevent it rising when squeezed.
These days, ships that go to the polar regions are of course no longer made of wood, but of steel. They still need to be specially strengthened to work in ice conditions. An ordinary ship with no strengthening will not risk touching ice at all, no matter how gently. A modern ship weighing thousands of tonnes meeting an iceberg weighing perhaps as much again or up to thousands of times more can easily sustain enough damage to require major repairs or to sink her. Ice will easily hole a non-strengthened ship.
Ships therefore that have any chance of contacting ice are at least ice-strengthened if not being designed to plough through the ice as do ice-breakers.
Icebreakers are needed if there is a trade route to keep ice free, if there are military reasons for patrolling in areas with heavy sea ice or if you need to work in heavy ice conditions, particularly in winter. Icebreakers are expensive to build and very expensive in fuel to run (sometimes powered by gas turbines or a nuclear generator). They are uncomfortable to travel in on the open sea. All ships designed for the ice have rounded keels with no protuberances, these things provide stability in normal ships and result in ships that are designed to contact ice rolling heavily in a even a light sea.
Rounded keels and a lack of stabilizing fins means that progress is quicker and smoother through ice and that there aren't any parts to be ripped off. A further discomfort comes from breaking through continuous thick ice with constant vibration, noise and jarring against the ice.
Icebreakers are generally owned by those countries with an interest in the north-east and north-west passages in the Arctic or that have other shipping lanes and ports that need to be kept open during the winter months.
Ice strengthening on the other hand is found much more commonly in ships designed for Arctic or Antarctic work. There is no actual universal definition of what needs to be done to a ship to be "officially ice strengthened" and it can be applied to all manner of ships, whether supply ships, tankers, container ships, warships etc. Commonly ice-strengthened ships can cope with continuous one year old ice about 50cm - 100cm thick.Breaking ice by any ship is not a case of forcing the ice aside, but by the ship riding up and over the ice in front of it, with the weight of the ship then breaking the ice, this may be a continuous process or can result in a lot of back-and-forth in particular thick places.
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