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A to Z of azipods: revolutionary new propulsion systems drive 21st century cruise liners
Cruise Travel, May-June, 2002 by Phil Shapiro
In the wake of the introduction of azipod propulsion systems on cruise liners lies the wave of the future. Combining the propulsion and steering of conventional ships, an azipod system replaces traditional propellers and lengthy drive shafts and rudders used for more than a century on oceangoing vessels.
Eight cruise lines--Carnival, Celebrity, Crystal, Cunard, Holland America, Mediterranean Shipping, Norwegian, and Royal Caribbean--have a total of 12 azipod-propelled ships in service as of 2001; 17 others are being planned or under construction.
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The reliability and effectiveness of azipod propulsion has been proven over the last decade in the toughest ocean-going ship applications--Arctic tankers and ice breakers. In those non-passenger-carrying vessels, propulsion was provided by only one azipod; cruise ships have two for additional maneuverability and redundancy.
Each of the two azipods on passenger ships has a fixed-pitch propeller at the front to move the ship through the water. The azipods are suspended beneath the water line at the aft end of the ship. There's one azipod on the port and another on the starboard side. Each azipod is mounted on a shaft perpendicular to the center line of the ship's hull.
The shaft can be rotated to any position in 360 degrees. The angular position of rotation can change the direction of the ship's movement or keep it sailing straight ahead. Thus, ships with azipods are steered without a rudder, similarly as small boats use an outboard motor for power and steering (but with their rear-mounted propellers, outboards "push" their vessels, while azipods, with forward-mounted propellers, "pull" their vessels). The positioning of the azipods and the speed of their motors are determined by officers on the bridge.
This system integrating power and rotation into one unit was begging for a shorter, simpler descriptor. The resultant acronym "azipod" incorporates azimuth (distance in angular degrees measured clockwise from a standard direction) and pod (a streamlined compartment). Thus, an azimuthing, podded unit was shortened to azipod.
Cruise passengers needn't have concerns about safety when sailing on the newest ships with azipod propulsion systems. Azipods offer safer cruising in harsh weather and greater control in restricted passages. For instance, azipods are capable of moving the ship backwards during docking. The pods can be rotated to the 180-degree position, or they can be positioned straight ahead (O-degree position) while the motors are reversed. Each azipod can be operated independently of the other allowing subtle maneuvering. There's a lot of flexibility.
Also, during docking procedures the azipods can be rotated to the 90-degree or 270-degree position to move the ship to the side. In order for ships constructed in the latter part of the last century to move sideways, they needed thrusters built into the bow and the stern--or tugboats. A thruster is a separate motor system with propellers concealed within the hull that moves water to the side of the ship, thus moving the ship in the opposite direction. (Remembering our basic physics, "every action has an equal and opposite reaction.") Ships with azipods do not need a separate stern-thruster system; however, bow thrusters are still required.
Royal Caribbean's Voyager Class ships have azipods port and starboard as previously described, plus a third, central stationary pod for additional power when cruising. To date, the biggest installation planned is for Cunard Line's Queen Mary 2, which is scheduled to debut in late 2003. She will be the largest ocean liner ever built and the first with a four-pod installation--two fixed pods and two azipods. The Queen Mary 2 plans to have a maximum speed of 30 knots, which will be helpful in establishing a regular trans-Atlantic schedule of crossings, even in midwinter's stormy weather.
By looking at the photos accompanying this article, it's hard to know exactly what's inside the azipods. The azipods do not have engines inside, but instead have huge variable-frequency electric motors. The power plant providing the electricity to the motors is located in the hull of the ship. This power-generation system also provides electricity to support all the ship's other electrical needs.
Thus azipod ships are, basically, electrical vessels, from lighting and air-conditioning to cooking, dish-washing, laundry, and all the other creature comforts we expect. From the generation of freshwater to the processing of waste materials, the ship is an electrically operated, self-contained, ecologically correct community. The power needed just to drive the azipods on Carnival's 77,367-gross-register-ton Elation is as much as 28 megawatts--that's equal to 280,000 100-watt light bulbs!
The power plant's engines that drive the electric generators on today's azipod-propelled cruise liners are of two basic types: diesel and turbine. Diesel engines operate on the same principles used in cars and trucks. Turbine engines are similar in design to those on jet airplanes.
