Podded propulsion

Winter in the Gulf of Bothnia between Finland and Sweden is one of the most hostile maritime environments in the world. As temperatures plummet, the sea freezes, sealing off ports on the gulf from the rest of the world while winds whip the ice into jagged ridges that can reach several meters in height. Seldom is the ice offshore less than half a meter thick.

By Editorial services

When the Finnish Maritime Agency wanted to see what its new multipurpose icebreaker, the MSV Botnica, was capable of, it sent the ship straight to the Gulf of Bothnia just as winter was tightening its icy grip on the region.

At seemingly opposite ends of the world, a floating resort from Royal Caribbean International - called Voyager of the Seas - makes its way through tropical waters.

When launched, the 142,000-ton Voyager of the Seas was the world's biggest cruise ship. But she is much more than simply big, offering 3,100 passengers luxury, amenities like an onboard ice rink and large amounts of cabin space.

Opposites in every sense but one

In the extremely unlikely event that the Voyager of the Seas and the Botnica ever meet, the cruise ship with its 14 elevators and 15 levels would completely dwarf the little icebreaker.

It's hard to see what these two vessels have in common - until one looks below the waterline. Both ships are equipped with Azipod, a propulsion system first developed by ABB that is now revolutionizing ships of all types.

The 96-meter Botnica has two five-megawatt azimuthing thrusters supplied by ABB, propellers that can rotate in a horizontal plane to provide a level of maneuverability that until recently was only a captain's vague dream. Originally derived from astronomy and navigation, the term "azimuthing" refers to the Azipod's ability to turn through 360 degrees, an ability which makes steering (and especially docking) much easier than with conventional propeller systems. There is far less vibration and no rudder is necessary.

When he took command of the Botnica, captain Atso Uusiaho had no previous practical experience of Azipod and was looking forward to seeing how the system would perform in a variety of conditions. In ice fields 600 cms thick, in slush fields, going backward and forward, Azipod proved highly reliable.

"I would be very surprised if future ice-worthy vessels built in Finland were fitted with any propulsion system other than Azipod," he said.

Until recently Azipod was available in power ranges of five megawatts to 38 megawatts. Now, with Compact Azipod, all the benefits that the owners of larger vessels enjoy are available for uses in the 400 kilowatt to five-megawatt range.

Functionally, the propulsion drive can be divided into following parts: supply transformer, propulsion motor, frequency converter and propulsion control. In an AC drive, a frequency converter is used to precisely control the speed and torque of the electric motor. The speed of the AC electric motor can be controlled by varying the voltage and frequency of its supply.

A frequency converter works by changing the constant frequency mains electrical supply into a variable frequency output. The benefits of this kind of controlling system are obvious. Precise and smooth speed control means less equipment stress resulting in a lower maintenance costs. An electric motor itself is virtually maintenance-free.

Along with this, the Azipod system helps to free up space for cargo or passengers, since the traditional drive shaft, rudder, gearbox, and propeller are combined into one compact unit mounted outside the vessel.

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