For many years the phenomenon of ‘singing’ propellers has been known and documented in relation to specific vessels. The problem is more of an annoyance to crew-members than a technical risk but it has been noted on both surface vessels and submarines. Singing, however, represented a particular problem on ‘U’ class submarines during conflicts in the 1940s, leaving the vessels open to easy detection.
Resulting in a sound similar to running a finger over the edge of a wine glass, various explanations have been put forward to explain the occurrence of such harmonics. These include combinations of factors such as the propeller size, shape and speed and the asymmetry of flow regimes. A new project carried out jointly by the Wärtsilä Corporation and the City University of London has now identified specific design parameters that increase the risk of propellers being prone to singing with work concluding in December 2015.
Wärtsilä acknowledges that the problem has been recognised in the marine industry for many years and general views have been that hydrodynamic forces at the trailing edge of the blades have excited natural modes in the propeller blades. The latest research, however, has indicated that the problem is more complex. “Our research has shown that the singing phenomenon can be controlled by selecting the proper main parameters of the propeller blades, by careful attention to the flexural modes of the propeller blades, and by careful attention to the specific geometry at the trailing edge of the blades. It has shown that all these aspects are interacting and can prevent the ‘singing’ of propellers,” said Arto Lehtinen, vice president, propulsion, of Wärtsilä Marine Solutions.
The study used Finite Element Analysis tools to assist in identifying factors in propeller designs which were likely to result in the increased risk of propellers being prone to singing. By adjusting design parameters, the study found that the risks of harmonic response could be reduced. Further computational fluid dynamic work (CFD) indicated that improved blade trailing edge design could reduce vortex shedding characteristics and therefore the excitation forces that could encourage the propeller to sing.
As a result of the study, Wärtsilä is now able to more easily identify factors in the design process that increase the risk of singing and the avoidance of this has now been added as a standard design activity for the company’s propeller products. Findings have also now been incorporated in Wärtsilä’s OPTI-Design concept propeller, introduced in 2014. This new fixed pitch propeller (FPP) design offers the potential for up to 4 per cent savings and CFD calculations analysis considers both the propeller performance itself and, more critically, the interaction of the propeller and hull. This approach provides much more accurate guidance to enable optimum propeller and hull designs to be achieved.
Wärtsilä OPTI-Design propellers are proving a popular option in the marketplace and the company announced recently that one of the largest Greek tanker operating companies, TMS Tankers Ltd, had selected the product for new vessels currently under construction in China. The propellers will be made by the Wärtsilä CME Zhenjiang Propeller Co Ltd (WCME) and it is understood that other leading Greek shipping companies have also opted to use Wärtsilä fixed pitch propeller products.
The contract with TMS Tankers Ltd will cover the supply of propellers for four Aframax tankers being built at China’s Hantong Ship Heavy Industry yard. The FPPs will also be fitted with the EnergoProFin system; a finned propeller cap that rotates together with the propeller and is claimed to offer fuel savings of up to 5 per cent in both new and retrofit applications.
“Efficiency and quality are the basic reasons behind the success of Wärtsilä FPPs, especially when supplemented by the EnergoProFin.” said Arto Lehtinen. “We place great value on being selected by this large and successful shipping company, which is an endorsement of Wärtsilä’s propeller technology and evidence that fuel efficiency is still of great importance, even with the current low fuel prices.”