There are many different types of ships all designed for specific purposes. When sailing through extremely cold oceans where ice is likely, an ice breaker ship is needed.
As the name itself suggests, ice-breakers have a special design which helps them to clear the navigation waterways for other ships. They are also sometimes research vessels designed to travel to polar regions. The design features of these ships include the following.
These ships have a double hull; this means that there are two layers of water-tight surface at the bottom and the sides of the vessel. You can see and understand this by seeing the picture below which shows the cut section of a double hull ship. The thickness of the hull is also greater than normal and the steel used for its construction is designed to not lose any strength at low temperatures.
The producing parts of the propulsion system, namely the rudder and the propeller, could be damaged by collision with ice, so they are designed for additional strength and placed in a position where they are protected by the hull to the maximum possible degree. Even so, they are exposed to open water and could get damaged. Because of this they are designed so they can be inspected and replaced while the ship is still in water.
Another feature is that modern ice breakers have are two propellers both at the fore and the aft and bow thrusters at the sides. All this is meant to help the ship in case of emergency situations if it is bound by ice on all sides.
The bow of most ships is usually pointed so as to cut through the waves, but in ice breakers the bow is usually round so that it can ride over the ice and make it break under its own weight. The images below show you some examples.
Limitations to the Design
These very features which help the ship to break the ice also makes it quite unsuitable for normal sailing. The shape of the hull and the bow make ice breakers very unstable; such a ship can roll quite a lot even in relatively calm seas. Moreover the bow shape makes it susceptible to capsize by waves since it is not designed to effectively deal with them.
Nevertheless these ships are used extensively for ice breaking purposes. Such ships are normally powered by gas turbines or marine diesel engines. Normally the fuel efficiency is not as good as conventional ships due to their bulky design.
Some ice breakers have anti roll tanks in order to counter-effect the tendency to roll. These tanks are semi-filled with water in order to stabilize the ship.