According to a recent document published by research specialist MarketsandMarkets, the marine propeller market is expected to be worth US$3.67 billion by the end of 2015, and is projected to grow to US$5.94 billion by 2020. Perhaps more significant, however, is the report’s finding that the key challenge faced by the marine propeller market is the life span of a propeller. This is fairly long, which means that replacement occurs only infrequently.
That said, technological advances are expected to increase demand, hence the growth that has been forecast. Demand has fallen for traditional, heavy types of propeller, made of aluminium or bronze, and there is a premium on the development of new propeller technologies.
This premium has become more obvious as research and development has revealed the extent of the energy savings that can be obtained from improved propeller design and usage. One recent example of this can be seen in MAN Diesel & Turbo’s concept for slow steaming optimisation with de-rating and propeller upgrade, which won the annual Green Ship Technology award earlier this year as a fuel saving and environmentally friendly ship propulsion solution.
This concept optimises a vessel’s operations for slow steaming. It is designed, in particular, for vessels that were originally specified and optimised for higher ship speeds.
The slow steaming, de-rating and upgrading package solution typically starts with MAN B&W’s two-stroke engine, which is de-rated and optimised for a new operating point at a reduced engine speed. This process can include the installation of new MAN turbochargers and fuel injection nozzles. The existing propeller can be retrofitted and upgraded with a new, more efficient MAN Alpha Kappel design with an optimised blade number and design, and this can be combined with the fitting of a Mewis Duct for improving the in-flow to the propeller.
A solution of this kind was developed by MAN Diesel & Turbo’s operations in Copenhagen and Frederikshavn in Denmark. Tank test results gave savings of 17.5 per cent at a ship speed of 12.5 knots for a series of 150,000 dwt Suezmax tankers. Two ships in this series have completed the upgrade and the saving has been verified in operation. Another ship series has shown savings of 15 per cent after the upgrade. The economic benefits for the operators are significant, as are the reduced exhaust gas emissions of CO2, SOx and NOx.
Another example of propeller innovation that has led to environmental and economic benefits can be seen in one of the first commercial applications of the new MAN Alpha Kappel fixed pitch (FP) propeller. The results followed the successful completion of sea trials by Elsabeth C, a 58,500 dwt Supramax bulk carrier owned by Frontmarine Co, a subsidiary of Cypriot company Lemissoler.
This newbuilding is the first in a series of eight sisterships, all featuring single MAN B&W 5S60ME-C8.2 engines with a specified mcr of 8,050kW at 89 rpm. Elsabeth C’s sistership Mirela, the second in the series, has also successfully completed its sea trials.
Lemissoler reports that the Kappel propeller offers significant fuel savings compared to conventional designs. This, together with the new vessels’ lines and the silyl acrylate antifouling paint, contributed to the positive results from the sea trials of the first two vessels. It reports that both ships consumed an average of 23 mtns per day at a speed of about 14.2 knots in ballast conditions during the trials. Elsabeth C also performed a successful crash-stop test that was notable for its lack of noise and vibration.
Thomas Leander, head of propulsion at MAN PrimeServ in Frederikshavn, said: “A key point during the design stage was to address the problem experienced by similar vessels that have an overly-small light running margin. MAN Diesel & Turbo has recently introduced new light running margins of between 4 and 10 per cent for all FP propellers and two-stroke engines. Furthermore, the propeller layout for these 58,500 dwt vessels is 3-4 per cent higher than is normally applied to these vessel types. By introducing these modifications, MAN Diesel & Turbo has managed to ensure safe and reliable operation, while improving the vessel’s efficiency and manoeuvrability during its entire lifetime.”
The MAN Alpha Kappel propeller optimisation has been carried out based on the stern and wake field for the newly designed 58,500 dwt vessels.
The use of computational fluid dynamics (CFD) is also making a big difference in the development of new propeller designs. Wärtsilä is carrying out a multi-year project to establish the development of a method for propeller performance predictions, thruster load determination and ship hull resistance calculations. The project has been ongoing for some time and it was this process that led to the development last year of the fixed pitch propeller FPP Opti Design concept. The design offers fuel savings of up to 4 per cent and highly reliable full scale performance predictions.
Wärtsilä’s FPP Opti Design was the result of highly experienced design engineers having access to sophisticated software and analysis tools. CFD calculations analyse not only the propeller performance but also the interaction between the propeller and the hull. This provides very accurate information, which enables design and parametric optimisation.
“Wärtsilä has more than 100 years of experience in developing high quality propeller solutions. To this huge knowledge resource we have now been able to add a highly sophisticated state-of-the-art design approach that is unmatched in the industry. The hull and the propeller, and the interaction between the two, can both now be taken into account during the design process. The result is a higher level of efficiency and lower operating costs,” said Arto Lehtinen, vice president for propulsion at Wärtsilä Ship Power.
The company offers other solutions that save energy and improve the efficiency of ship propellers. These include the tip rake concept which features an extended tip, smoothly curved to the pressure side of the blade with optimised geometrical parameters developed to achieve efficiency improvements, lower pressure pulse levels, and a quieter propeller.
In addition, the Wärtsilä EnergoProFin is an innovative propeller cap with fins that rotates together with any make of propeller. By weakening the hub vortex, resistance is decreased and propulsion thrust is increased, resulting in fuel savings of up to 5 per cent. Energopac is Wärtsilä’s optimised propulsion and manoeuvring solution that reduces fuel consumption by integrating the propeller and rudder designs. It is fully optimised for energy efficiency without compromising either manoeuvrability or comfort levels.
While design is one means of saving weight, saving fuel and increasing efficiency, materials are another. In recent years research and testing have been carried out for composite carbon fibre reinforced plastic (CFRP) propellers. CFRP has a better strength to weight ratio than the standard bronze alloys typically used for propellers.
This interest has recently seen Japanese classification society ClassNK release the first class guidelines on the use of composite propellers. The publication follows the design and manufacture of a CFRP propeller and its installation on a merchant vessel – a world first – last year, as part of ClassNK’s Joint Research and Development for Industry programme.
The strength and corrosion resistance of composite materials means that they are widely used in fields such as aerospace, automobiles, and wind power generation, but their use has been expanded to other applications, because of their characteristics.
The CFRP propeller weighs around a fifth of aluminium-bronze. But despite its ultra-lightweight composition, CFRP has the same strength, or more, as the aluminium-bronze composite materials used in conventional propellers. Because it is lightweight, propeller shafts can be manufactured with smaller diameters, reducing costs. Furthermore, by taking advantage of the material’s strength, it is possible to produce thinner propellers with smaller blade areas, potentially increasing the propeller’s efficiency.
ClassNK states that in order to apply a composite propeller to a ship, the material must offer performance that is at least equal to existing aluminium-bronze composite materials. Confirmation is also required that the composite propeller as an industrial product can be manufactured with uniform quality.
As part of the testing process, Nakashima Propeller Co successfully installed the first CFRP main propulsion propeller on 499gt chemical tanker Taiko Maru. Installation of the propeller and composite propeller boss cap fins reduced the amount of power that Taiko Maru required to operate during sea trials by 9 per cent compared to conventional nickel-aluminium-bronze propellers.