Propellers are now just about as efficient as they can get, believes Jens Ring Nielsen, a senior technical engineer for MAN PrimeServ in Denmark.
And so are diesel engines, which gives any discussion about further efficiency improvements in propulsion technology a broader perspective feel.
There is no doubt that significant savings can still be made, as some of the retrofit case studies on MAN Energy Solutions’ website illustrate: two 8,500TEU container ships cut their engine power by 35% yet maintained speed after fitting MAN ES-supplied Kappel propellers; a fleet of tankers saw a 20% energy saving.
Such gains “sometimes [seem] beyond belief” Mr Ring Nielsen said, when he gives presentations about the company’s propeller options, but “it’s not only the propeller we are me looking at,” he said in an exclusive interview with The Motorship. MAN ES’s philosophy is to “look at the complete propulsion drive train in a holistic fashion where components are viewed in terms of the mutual influence they have on each other,” the container ship case study mentioned above noted as it described other changes, such as a new bulbous bow and a propeller boss cap with fins.
MAN ES itself offers “a catalogue of efficiency improving devices,” he said and he identified the pairing of a Kappel propeller with a rudder bulb as a particular combination that “we know always gives a good result.”
That was certainly the case for the tanker project mentioned above, which involved 19 chemical tankers for Odfjell and, as well as a new propeller, included modifications elsewhere, such as to their engines and PTO gearing to better reflect their operating profile.
One area where there is potential to improve propulsion efficiency is to control the flow into a propeller, and Mr Ring Nielsen spoke of cooperating with a number of design bureaux, naval architects and innovation start-ups that have approached MAN ES for advice on such proposals. These devices “always leads to a better working environment for the propeller,” he said.
As an example, he mentioned the growing number of twin-screw vessels with shafts protruding from the hull that are supported by struts. Even these can be shaped and orientated to improve the inflow to the propellers if it is done early in the design process.
Although propeller efficiency now leaves little room for improvement, there is a new focus for design improvements: underwater radiated noise (URN). This has long been a factor in naval propulsion – in which MAN ES has much experience – but it now becoming a concern in the commercial sector, not least for environmental reasons. Two class societies – DNV GL and Lloyd’s Register – offer notations to recognise designs that have reached certain standards in this area.
This requires balancing vibration and cavitation against efficiency to achieve an optimum propeller but there is a trade-off between those two goals, he said.
This has led to a Version 2.0 of the Kappel propeller. The original design is recognisable for its distinctive blade tips and, to the casual observer, it would be impossible to see the difference between that and a Version 2.0 equivalent. But small changes have reduced cavitation and erosion in the tip region, and thus radiated noise, without reducing its efficiency, he said. This is now the standard version that is supplied for ship installations.
These changes were made possible by an optimisation tool that MAN ES developed itself to complement commercially-available CFD software. “We have a strong belief,” he said, that tools such as this should be developed in this way because they are so important to its business.
One driver for the demand for more efficient propulsion has been IMO’s Energy Efficiency Design Index (EEDI), which Mr Ring Nielsen welcomes. It “takes the power away from the economists and moves it back to the engineers,” he said, and it prompts customers to contact the company at a much earlier stage in their projects than before, he said.
EEDI has also made it necessary for shipowners to verify their ships’ performance, encouraging a wider demand for performance verification. This has extended not only to propeller efficiency but also to its cavitation and vibration characteristics, which marks a significant change from when Mr Ring Nielsen first joined MAN ES 30 years ago. Then, only “on a rare occasion would a shipowner want to have his performance verified,” he recalled.
Alongside a greater demand for verification has come a more focused approach to optimising propeller performance. This is particularly important for controllable pitch (CP) propeller arrangements that, in the past, would have been designed to suit a main engine running at 85-90% of its output and at constant speed to suit a shaft generator. But in an era when ships are often running at reduced speed, for CP propellers this means reducing the propellers’ pitch and running them inefficiently.
But Mr Ring Nielsen described a tool that was introduced in October 2019, called EcoOptimizer, that it has developed to be used at the vessel’s design stage. It takes account of the ship’s hull shape and engine performance to develop specific combinatory curves to be used in service for propeller speed and pitch across the speed range to minimise fuel consumption.
This means that the main engine operates at its minimum specific fuel oil consumption while the propeller at its most efficient point, saving up to 20% compared with constant-speed installations. For variable-speed CP propeller installations, savings of around 6% are claimed in the company’s literature. “The key point here is that you agree with your customer where to optimise it,” he said.