An ice breaker ship creates wide passages through ice sheets that form around harbors during winter to allow uninterrupted activity. Lakes, deltas, bays, and inlets in Scandinavia, around Siberia, and on the Great Lakes of North America are susceptible to thick ice that reaches out from land into the body of water. This would prevent ships importing and exporting goods or carrying tourists from accessing seaboard cities for several months if the ice wasn't broken up.
An ice breaker ship is equipped with strong propellers, a steel body, forceful engines, and sensitive navigation tools. Its main duty is to crushes ice into small enough chunks that they will melt, bob out to sea, or at least not pose such a threat to smaller ships. Of course, the exterior front of the ship, the hull, must be resilient in sub-freezing temperatures without deforming or weakening. This is the area that reaches the stubborn ice first, so it is carefully engineered and reinforced.
Modern ice breakers have a stepped and sloped hull designed to help the ship rise up on top of an ice block. Once sitting partially out of the water, it smashes through vertically, using the power of gravity on its enormous mass. The propellers also help to push the front of the ship downward to crush the ice, churning large hunks beneath the water. Increasingly, icebreakers are used in scientific research expeditions to the Arctic and Antarctic regions, rather than merely to keep trade routes operational.
The engines of an ice breaker must be powerful enough to run the giant propellers at the rear and front of the ship that allow it to navigate with precision and confidence. These diesel engines are housed within an incredibly sturdy steel frame that won't collapse or rupture. Even a single hole would not sink the entire ship. In some instances, the vessel gets its power from a nuclear source, so as not to carry so much flammable fuel onboard.
There is more to ice breaking than ramming ice. The ice breaker must navigate the ocean without getting stuck, while blazing a trail that other ships can use. The ice, initially, doesn't have much place to go, so sometimes the nuclear reactor on the ship heats water to melt the ice during the first run. In later runs, weeks apart, the ice will drift out to open water and melt on its own.