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Marine Propulsion Azipull: Pulling Propulsion Ahead Advances in marine propulsion technology over the past decade has increasingly opened the choices for vessel owners, as innovative systems make their way through the R&D and real world test trials. One technology that has made dramatic technical strides is Rolls-Royce's Azipull azimuth thruster technology, which has been adopted in a wide range of vessel sectors. Initially incorporated into ferry designs, the Azipull solution has increasingly become popular in a widening range of vessels, from OSVs to product tankers and small LNG carriers. The Ulstein Aqumaster Azipull thruster takes the mechanical elements of a gear-driven azimuth thruster and puts them into a quite different hydrodynamic package. A pulling propeller - controllable pitch or fixed pitch depending on the drive system chosen - is mounted ahead of the leg, which is a hydrodynamically optimized streamlined unit incorporating the gear house and a lower fin. The leg/housing/fin combination recovers swirl energy from the propeller slipstream that would normally be wasted and converts it into additional forward thrust. At the same time the underwater unit has more rudder effect than a conventional azimuth thruster, improving the steering abilities of many hullforms. This can also save energy since less thrust vectoring is needed to keep a steady course. A double-ended catamaran ferry - Stavanger - was the first application of Azipull propulsors; four AZP085 units each rated at 1,340kW and placed one at each corner of the symmetrical ferry provided the required service speed of 22 knots. Operating on a demanding Norwegian fjord route, Stavanger has now accumulated almost 20,000 running hours on each of its thrusters. Other vessels of the same Fjellstrand type with AZP085 units are also in service in Turkish waters. Soon, the Azipull size range was extended to give thrusters suitable for other markets, and now the range comprises four sizes AZP085, 100, 120 and 150, covering unit powers up to 5,000 kW. These thrusters have caught the interest of the offshore industry, a vessel sector that in recent years has served as a proving ground for a host of new marine technologies, from propulsion solutions to innovative hull designs. Comparative trials on Bourbon Tampen against good reference conventional azimuth thrusters showed a reduction in fuel consumption of at least 16 percent in normal service, with much lower vibration levels, according to Rolls-Royce. To date more than 50 shipsets of Azipull thrusters have been sold for offshore vessel propulsion. Smaller merchant vessels are also a relevant application for Azipull thrusters, and the first installations in product tankers are now at sea. Traditionally product tankers have been conventional single screw vessels, but increased focus on maneuverability in confined waters and propulsion redundancy for safety's sake is causing owners to seek new technical solutions. Azipull thrusters are attractive not only for their effi- ciency but because their rudder effect can provide good steering on hullforms which are in themselves not particularly stable on course. Real world examples of such applications are Bro Deliverer and Bro Designer, the first two ships in a series of four 14,500dwt BrostrÝm tankers, the D-class, to go into service. Jinling shipyard in China is the builder and each of the vessels is fitted with two 2,380kW AZP120 thrusters for main propulsion plus a 1,000kW Ulstein Aquamaster tunnel bow thruster. Since each Azipull unit is driven by its own engine there is full propulsion redundancy, and these 146.8m long ships has RP and Clean Design class. A product tanker built in Turkey and recently delivered to Bergen Tankers features Azipull thrusters in an innovative solution developed by NVC-Design in Rolls-Royce. Here the thrusters are coupled to the main engines mechanically with shaft generators in the same short shaftline, giving propulsion redundancy and the ability to supply power for cargo pumps and ship services from one engine in port. Recently, the application of Azipull thrusters as the main propulsion system for merchant extended further, with the ordering of a pair of thrusters for a 7,500 cu. m. gas carrier to be built in Poland for the Netherlands shipowner Anthony Veder. The vessel is unusual in being designed to carry LNG, LPG or ethylene. On completion in 2008 it will go on long term charter to Gasnor, carrying gas along the coast of Norway. Rolls-Royce is supplying the complete power and propulsion package. Two AZP120 thrusters with fixed pitch propellers and BV ice class 1B will be driven by frequency controlled motors in a gas/diesel electric system. Two gas fueled gensets and two diesel gensets, all from Bergen, provide the power. The Marine Electrical Systems business is responsible for the electrical side, and the main thrusters will be assisted by a bow tunnel thruster. The latest application is in passenger vessels where two AZP120 units, each taking 2,310kW, will propel a new ship for the Hurtigruten Norwegian coastal route which is under construction at Fincantieri in Italy. The vessel will be named Fram, and is to operate the normal Bergen/Kirkenes service in winter, but in summer will cruise to Greenland. For this reason ice class has been specified for the Azipull thrusters. Another market that is starting to show interest in the Azipull concept is the superyacht industry. The superyacht industry is currently enjoying unprecedented growth in the size range of 50 m and higher. Particularly for faster displacement yachts with service speeds up to 24 knots, Azipull-based propulsion systems can prove to be an interesting option. The thrusters can be used in a variety of mechanical and diesel electric transmissions, giving freedom in positioning the machinery and opening the way for different accommodation layouts. Maritime Reporter & Engineering News 42