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ice breaker 
Boat that opens up a navigable passage through ice.
rear propeller
Screw driven by a powerful engine to propel the ice breaker.
stem propeller
Screw that pulls up water from under the ice sheet to weaken its support thus making it easier to break and move.
stem
Reinforced part of the boat’s prow that crushes the ice with its weight and then pushes it aside to open a channel.
