propellers
Larger ships will be built with lower design speeds
products. The MAN Alpha portfolio includes fixed and controllable pitch propellers rated to over 40 MW absorbed power. Fixed pitch propeller systems include features for aft ship integration with the rudder or rudder bulb, to enhance efficiency. Designs are based on the need for robust products with good design margins. Material fatigue levels calculated for a 30 year lifetime, considering all possible external loadings in service. The new controllable pitch propellers in the MAN Alpha VBS Mk5 range feature a new optimized flow hub design which is reduced in size and also produces reduced drag. Blade design has been improved to control cavitation, pressure impulses, vibration and noise generation and propellers are optimised for individual ship application and wake field characteristics. Benefits of the revised design include efficiency increases of up to 2 per cent over previous products. MAN is quick to point of the benefits of retrofitting better matched propellers to vessels now slow steaming and the benefits of its “green retrofit” programme. Quoting potential for fuel savings of up to 14 per cent compared to previous specifications, examples of efficiency improvements of over 12 per cent are claimed for the Sassnitz, operated by Scandlines and now fitted with MAN Alpha high skew design propellers. The ferries Ask and Urd are also benefitting from efficiency increases of over 10 per cent from Kappel designs, adapted to new service profiles.
Also responding to market trends for slow steaming, Stone Marine Propulsion, of Birkenhead, UK, has now released further results for the performance of its New Profile Technology – or NPT – propeller. The design delivers higher efficiency and lower fuel consumption than an equivalent conventional propeller but also has
62 I Marine Propulsion I February/March 2012
the advantages of smaller relative diameter, reduced weight and reduced moment of inertia. The smaller optimum diameter, in particular, provides a major advantage when used with ‘new generation’ slow turning main engines, such as the MAN B&W G type, or where existing installed engines are de-rated. Limited diameters also mean that the full potential of very slow turning engines can be realised
whilst
improvements carried out to Aquila, a 57,000 dwt ship based on a Shanghai Ship Research and Design Institute (SDARI) design Changes included de-rating the main engine
maintaining
adequate hull clearances. Recent co-operation between Stone Marine Propulsion and the Danish Shipowner Norden. has resulted in a significant improvement in the fuel efficiency of three 32,500 dwt bulk carriers, now fitted with NPT propellers. Due to engine delivery schedules these three vessels retained their original engine configuration, providing the opportunity for direct comparison of original propeller performance on previous equivalent vessels, with those fitted with NPT propellers. Originally calculated at 3 to 4 per cent, the shipyard trial report showed a much larger than expected improvement of approximately 6 per cent. While accepted that other factors, such as differences in hull condition and measurement accuracy, will have had some influence on the trial results, Stone Marine believes that the results leave little doubt that the forecast efficiency improvement has been achieved and, most probably, exceeded. Class societies are playing an increasing role in verification of improvements. In January 2012, Lloyd’s Register reported the benefits it had verified in reduced fuel consumption of a new bulk carrier delivered to operator Delphin by the Jiangsu Hantong Group. LR supervised the design appraisal, build and sea
trials,
verifying that a 14 per cent reduction in fuel oil consumption had been achieved following
by approximately 1,000kW, a new propeller design, optimised for the reduced rating, and the fitting of a Mewis duct. Meng Cheng Jun, president of the Jiangsu Hantong Group, said, “Hantong completed the design optimisation and the tank test independently, which also brings us the patent right of our own part. Hantong is taking note of the market environment requests and listening to the owners’ needs and will keep on strengthening the optimisation of our vessels.” Germanischer Lloyd continues its close involvement in new propeller designs through the
verification of materials, geometry and
stress analyses. Some of the most significant design implications in which GL has been recently involved, however, have
come from
the requirements of the new Finnish-Swedish Ice Rules and the IACS requirements for polar class ships. “The new rules will require design modifications for propeller blades
to enable
requirements for heavy ice operation to be safely met,” said Dr Junglewitz. “These rules will provide formulae to calculate the additional ice loads as a function of propeller geometry and ice class. Other elements in the shaft line will also need to be designed to tolerate the ice loads imposed.” A further area currently being studied by GL is the impact of the ice rules on propeller hub designs. The general perception has been that hub diameters will need to be increased but this will have the effect of driving propeller costs upwards. “With limited evidence of problems,” said Dr Junglewitz “we need to collect data to gain a better understanding of this issue and the changes in hub design that may be necessary.” MP
www.mpropulsion.com
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