power ratings up to 4.5MW. ABB points out that development of the latest products was carried out with high levels of customer involvement, in order that specific areas for product improvement and optimisation could be identified. Improvements from earlier Azipod versions now include: • a smaller propeller hub and modified strut for increased hydrodynamic efficiency • a revised vertical turning axis, reducing steering torque, wear and energy use • improved shaft sealing system increasing integrity and reliability • new hybrid thrust bearing technology with increased ease of servicing • improved and safer pod access • advanced condition monitoring features • a new, fully electric steering system • an intelligent bridge control interface, to assist in optimal operation. The XO range continues to provide the high levels of flexibility associated with the Azipod system, including tight turning circles and good in-harbour manoeuvrability. The system also performs well in terms of passenger comfort, producing low noise and vibration levels. In November 2011, ABB announced further design improvements, delivering an additional 2 per cent increase in hydrodynamic efficiency. A revised fin shape and pod cap structure, referred to by ABB as the X-tail, has resulted in straightened water flow at the exit of the propulsor and has minimised swirl.
The first installation of the new design was completed in May 2011 on the Royal Caribbean International
cruise ship Radiance of the Seas.
Efficiency benefits were identified through comparison with data from previous operation and against equivalent data from a sister vessel, checked over a four month operating period. Based on the positive results of the trials, ABB will now include the revised design features in its new Azipod XO and will also be offering retrofits to operators with older VO and XO units. By the end of 2011, over 100 vessels had been
fitted with Azipod propulsion units, accumulating in the order of seven million reliable operating hours in demanding marine applications. Cruise ship operators have been particularly impressed and Anders Aasen, associate vice president of
www.passengership.info
technical services for Royal Caribbean International and Azamara Cruises, said, “The fuel efficiency improvements measured on Radiance of the Seas are a welcomed reprieve from today’s higher fuel prices. Any improvements we can make on fuel efficiency will serve us well for future operational costs.” Despite the advantages of Azipod technology,
there are other attractive options available for passenger vessels requiring high manoeuvrability in operation. Bow and stern thrusters are often favoured and are a practical solution for ship designers. Spirit of Britain, which entered service in 2011, has been built with twin controllable pitch propellers and three bow thruster units. Operated by P&O, the ferry is 213m long and 30.8m wide and is the largest that can be operated with current UK Channel port restrictions. It is also the first ferry to comply with the 2010 IMO Safe Return to Port regulations. The bow thrusters selected were CT300M units, made by Wärtsilä of Finland. Designed for maximum power output within class limits, the Wärtsilä CT range of products features controllable pitch propeller blades actuated by a hydraulic cylinder yoke. A mechanically linked feedback system delivers smooth and accurate pitch control. Blades are of the skew back design and a pitch-to- zero system is incorporated to minimise thruster trailing under normal sailing conditions. The CT300M unit is the biggest in the Wärtsilä
bow thruster range, which starts at power levels of about 500kW. The high power thruster units fitted to Spirit of Britain are rated at a maximum of 3,551kW. The design is simple and robust, offering high efficiency and long service lives of key components including gears and bearings. Wärtsilä bow and stern thrusters have also been selected for the new Viking Line ferry scheduled for delivery in 2013. As yet un-named, the vessel is currently referred to as the NB1376 project. Deliveries of the first thrusters, build at the Wärtsilä plant in Wuxi, China, took place in December 2011. Scheduled for operation in the Baltic Sea area, the main interest in the project is environmental, with LNG being the chosen fuel. The area also demands high manoeuvrability and ability to operate
in
localised shallow water. The new ferry will be certified for Ice Class 1AS.
For smaller vessels, particularly roro and local Passenger Ship Technology I Spring 2012 I 69
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