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 ice-covered waters.
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 clear the debris from its path successfully is essential for its safety.
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.
Icebreakers are constructed with a double hull and watertight compartments in case of a breach. The ship's hull is thicker than normal, especially at the bow, stern, and waterline, using special steel that has optimum performance at low temperatures. The thicker steel at the waterline typically extends about 1 m above and below the waterline and is reinforced with extra internal ribbing, sometimes twice the ribbing of a normal ship. The bow is rounded rather than pointed, allowing the vessel to ride up over the ice, breaking it with the weight of the vessel. The hull has no appendages likely to be damaged by the ice, and the rudder and propeller are protected by the shape of the hull. The propeller blades are strengthened, and the vessel has the ability to inspect and replace blades while at sea.
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 air bubbles, all designed to allow an icebreaker stuck amidst thick ice to break free. Many icebreakers also carry aircraft (formerly seaplanes but now helicopters) to assist in reconnaissance and liaison.