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January 2011 www.sname.org/sname/mt (mt notes )The past twenty years have seen rapid develop- ment in the propulsion of ships operating in ice-covered waters. e emergence of podded propulsors with high ice class has widened the traditional area covered by icebreakers. Before this time, rigid shaftlines and propellers, with or without nozzles, were the only options available. All over the world, the predominant choice of propulsion for state-owned icebreakers was electric drive with xed pitch, open propellers. Prime movers ranged from nuclear power in Russia, to steam and gas turbines in Canada and the U.S., to diesel engines. State-owned icebreakers had large crews, typically from 50 to 150 persons, and they were expensive to operate. e oil discoveries in the Beaufort Sea changed this picture. Privately owned and operated ships started to appear in the late 1970s. ese vessels were built to sup- port the drilling eet during the summer and to act as research platforms. e machinery layouts were typi- cally geared medium-speed diesels and controllable pitch propellers with and without nozzles. The main benefits of these ships, compared to electric drive, were price and simplicity. The main drawbacks were poor capability to handle ice torque and clogging of the nozzles by fragments of ice. In addi- tion, all of these vessels, with the exception of the bow propeller-equipped Baltic icebreakers, suered from maneuverability problems when performing icebreaker and ice management duties. To remedy this maneuverability problem, a couple of projects were started in Finland. Two small icebreaking waterway service vessels were equipped with steer- able thrusters, one with a mechanical thruster and the other with an Azipod propulsion unit. In this context, a mechanical thruster means a thruster in which the motor drives the propeller via a number of bevel gears, normally two; and the Azipod is a propulsion unit where the motor is located inside the pod and drives the propeller directly. Both concepts were thoroughly tested and found acceptable. As a result, the next generation Baltic ice- breakers were equipped with azimuthing propulsion units: Fennica and Nordica were tted with mechanical thrusters, and Botnica with Azipod units. All three are multiservice vessels, meaning that beside their main tasks as icebreakers during the win- ter, they serve worldwide during the summer season. ey have performed icebreaker duties in the Beaufort Sea and in the Arctic Ocean o eastern Greenland dur- ing the summer season. During the past twenty years a great number of vessels intended for operation in ice-covered waters have entered service. Based on personal communication with peo- ple from Rolls-Royce and Steerprop, it is estimated that approximately 200 mechanical thruster units with an ice class higher than 1A have been delivered, and of them, 14 units have an ice class higher than 1ASuper. Some 42 Azipod units have been delivered, and 29 have an ice class higher than 1ASuper. A summary conclusion from this is that the mechanical thrusters reign in ice classes up to 1ASuper and Azipods in higher ice classes. Over the years, ABB has measured ice loads on the Azipods on several ships in dierent geographical regions ranging from the Kara Sea to the Sea of Okhotsk to the Baltic. Propulsion in Ice The development of power solutions for operations in ice-covered waters BY TORSTEN HEIDEMAN AND SAMULI HÄNNINEN