A number of protection mechanisms must be incorporated as discussed above; however, complete prevention is impossible. Contact of ice with a propeller often places heavy loads on the propeller blades and generates a sudden violent increase in the shaft torque, called ice torque. In sum, the propulsion system of an icebreaking ship must have either the sufficient strength to endure all of the loads described above or effective mechanisms to damp them.
A typical solution that meets the abovementioned requirements is the adoption of an electric propulsion system. An electric propulsion system offers a significant advantage over other systems, in that it can easily and precisely control field current of the motor, enabling it to change the number of revolutions of the propeller and direction of motion quickly, responding rapidly to abrupt variations in shaft torque. This system is presently used in many icebreakers. The most common prime mover of the electric propulsion systems is the diesel engine; Russia possesses a number of nuclear-powered icebreakers, but mainly for safety reasons they are not commonly used in other countries. Direct diesel propulsion systems are also commonly used in icebreaking ships, particularly in commercial fleets. These designs can provide rapid changes in output, such as switching between forward and reverse motion, through the adoption of a controllable-pitch propeller (CPP). An example of a propulsion system used in an icebreaking ship is illustrated in Figure 4.1-5 (Kishi and others, 1999). Another solution with considerable merit is the duct propeller. Because they offer high efficiency under high load, duct propellers are well suited to navigation in ice. Unfortunately, the duct happens to be blocked by ice fragments in the inlet. This problem has been addressed in some cases by mounting fins or flow-liners on the stern slightly forward of the duct.
In recent years the Azimuth system has attracted attention as a worthy propulsion system for icebreaking ships. The system has been installed in some vessels serving in ice-infested waters and shown promising early results. Like an outboard-engine, the Azimuth system itself rotates around the vertical shaft, rendering a rudder unnecessary. The electric motor that drives the propeller is housed in a cylindrical container, and is registered as a product under the name Azipod (short for Azimuthing Podded Drive). The greatest advantage of azimuth propulsion systems is the ability to turn the propulsion device in any direction, providing excellent turning performance that delivers far better maneuverability in ice than systems that rely on fixed rudders. Taking maximum advantage of the Azimuth's capability to provide propulsion in any direction, the concept has been developed of the Double-acting Azipod Ship (DAS), which can reverse direction of advance in ice or open water.
