Marine Log - January 2008 - (Page 15) BY OSKAR LEVANDER* FERRIES A NEW TWIST FOR ROPAX FERRIES F erry operations, like other sectors of shipping, are under increasing pressure to reduce greenhouse gas emissions. Simultaneously, high fuel prices necessitate a clear reduction in fuel consumption. In recent years, Wärtsilä has proposed a number of innovative propulsion concepts for RoPax (Roll-on/Roll-Off Passenger) ferries aimed at lowering fuel consumption and improving operating efficiency. These include, Podded Contrating Rotating Propellers (CRP’s), Wing Pods and Wing Thrusters. These all have one important feature in common: they depart from the traditional twin shaft line setup, and instead use a propeller mounted on the center line skeg combined with either one or two azimuthing propulsors. The new concept features a Contra Rotating Propeller pair with the forward propeller mounted on the center line skeg, and the aft propeller on a pullingtype steerable mechanical thruster located directly aft of the forward propeller. When looking from the outside, this setup looks very similar to the existing CRP arrangement with an electric pod. It also acts in the same way hydrodynamically. However, the difference is on the inside of the thruster. The electric motor is a ferry and also economically superior to all other machinery alternatives on the market today. The aft thruster is driven mechanically by a medium-speed diesel engine located in the center of the vessel above the shaft line of the forward propeller. This means that the engine is located higher up than where normally situated in ferries. The engine compartment penetrates the main deck. The forward propeller is driven in the traditional manner by one or two engines. T h e engines are located in the center of the vessel at tank top level. The machinery forms a very compact package that is higher than normal, but much narrower. The machinery can also be pulled further aft than in a twin shaft vessel, since the single skeg hull form allows the reduction gear to be located further aft. The thruster engine, because of its high location, is also Table 1. Dimensions of RoPax Ferry Concepts very far aft. Twin shaft 186m 170m 27.7m 7m 10m 28,500 6,700 5,150t 12,000 18,700t 2,410 CRP 2-stroke CRP Wartsila 16V46 191.9m 186.6m 175.1m 170m 27.7m 27.7m 7m 7m 10m 10m 29000 28500 6,600 6,600 5,150t 5,150t 12,300 11,900 18,900t 18,500t 2,420 2,410 View of the Ropax vessel showing its engine arrangement NEW SHIP DESIGNS New ferry designs were conceived using different machinery solutions. One design is of a ferry with conventional twin-screw machinery, while all the other have a CRP propulsion arrangement but with different main engines. The five machinery alternatives studied were: • Twin shaft, 4 x Wärtsilä 9L38 • CRP, 7-cylinder Wärtsilä RT-flex60 + Wärtsilä 8L38 • CRP, Wärtsilä 16V46 + Wärtsilä 8L38 • CRP, 2 x Wärtsilä 8L46 + Wärtsilä 8L38 • CRP, Wärtsilä 8L64 + Wärtsilä 8L38 The vessels are designed to offer the same capacity and performance: • Lane meter 2400 m • Payload 5150 ton • Speed 24 knots (15% SM, 85% MCR) The ship design was kept similar, but the main dimensions were varied to offer an optimized solution. This provides the ideal way by which to compare different machinery alternatives. ROPAX PARTICULARS The ferry represents a contemporary RoPax vessel with 2,400 lane meters for cargo and day facilities, for a limited number of passengers. The main dimensions are presented in Table 1. The CRP ferry with a main engine below the main deck can have the same dimensions as JANUARY 2008 MARINE LOG 15 Length, OA Length, BP Breadth Draft, design Depth GT DWT Payload LWT Displacement Lane meters replaced by a mechanical drive system with two 90° bevel gears. The most obvious benefit of this is that it allows for fully diesel-mechanical machinery, while at the same time significantly lowering investment costs. SAVINGS ACHIEVED VIA NEW MACHINERY ARRANGEMENT The novelty of the new propulsion concept is in the machinery arrangement that makes it both practically feasible for www.marinelog.com MACHINERY ALSO SUITABLE FOR LARGE MAIN ENGINES The new machinery arrangement also provides some new options when it comes to engine selection. The narrow and high machinery is well suited for a large main engine that is higher than normal. The main engines could be, for example, a 2-stroke engine or the very largest medium-speed engine on the market, the Wärtsilä 64. The new arrangement efficiently overcomes some of the problems associated with high engines in ferries. With a conventional propulsion solution, two large engines would close up the entire beam of the main deck. Since the engines in the new proposal are behind each other, two engines will not take up more than two lanes on the main deck. http://www.marinelog.com
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