STAYING AHEAD OF THE GAME

Coming up with propulsion solutions that meet green targets is a key investment areas in the current climate. Investment in conversion kits for diesel engines is just one area of opportunity for propulsion specialists, who are having to deal with the ongoing problem of propeller noise

One recent example is a collaboration by marine design and engine developer ScandiNAOS, Chalmers University of Technology and the Swedish Maritime Administration (SMA), which have launched a project to develop dual-fuel kits for conversion of new and existing diesel engines to methanol operation.

The methanol dual-fuel kit will be generic and capable for retrofit to a large range of existing diesel engines of different brands. The kit will target engines up to 1000 kW and will accelerate the transition to low emission fuel and sustainable operations for marine and industrial applications.

The project will last for 24 months and has a budget of SEK8,600,000, with 50% funding by the Swedish programme for

Strategic Vehicle Research and Innovation, (FFI). Leading methanol producer Proman and the Methanol Institute are providing industry funding.

During the work, ScandiNAOS will develop and implement a dual fuel kit in a pilot boat owned and operated by the SMA. The SMA already operates a methanol-powered pilot boat equipped with a single fuel compression ignited methanol engine, which completed successful trials in December 2021, a conversion supported by the FASTWATER consortium programme.

The adoption of dual-fuel kits will enable conversion of more ships more quickly, since a conversion kit can be cost-efficiently applied to existing engines while maintaining the fuel flexibility to run on either methanol, marine gas oil or hydrotreated vegetable oil.

The SMA has a target to remove fossil fuels from its fleet by 2045. Methanol as a fuel for combustion engines provides a number of opportunities for engine optimisation. Chalmers University of Technology has assigned a Postdoctoral research position for the duration of the project to develop and test advanced combustion strategies to be applied in the next generation of methanol single and dual fuel engines.

The pilot boat is expected to be ready for field trials in the third quarter of 2023, a process that will go on for nine-12 months during which the dualfuel kit will be tuned and optimised based on operational experience and from the results of the research and laboratory tests carried out by Chalmers University.

Bengt Ramne, managing director of ScandiNAOS comments: “We are excited to get a chance to apply a dual fuel kit on an SMA pilot boat and continue the great co-operation with the Swedish Maritime Administration to reduce the carbon footprint of its fleet.”

Lucien Koopmans, head of the Energy Conversion and Propulsion Systems division at Chalmers University of Technology, says: “A quick and powerful transition towards a decarbonised transport future starts with conversion of the existing fleet.”

Greg Dolan, chief executive of The Methanol Institute, adds: “We are delighted to be co-sponsoring this project, which builds on the successful 2021 trials and will establish a practical process for the conversion to dualfuel methanol operations safely and at reasonable cost.”

Blade Performance

The slightest deviation in the machining, polishing and finishing of ships’ propeller blades could result in underwater radiated noise and cavitation, even if defects are within the maximum tolerance allowed by classification societies and the ISO 484-1 standard.

A Canada Transport-funded study on the impact of manufacturing tolerances on propeller performance –carried out by Memorial University of Newfoundland, DRDC Atlantic Research Centre and propeller manufacturer Dominis Engineering – found the slightest change in propeller geometry resulted in “significant” cavitation, and much earlier than previously thought.

The behaviour of a section of propeller blade with leading edge defects of 94µm, 250µm and 500µm were studied using computational fluid dynamics (CFD) at the DRDCAtlantic Research Centre, and Memorial University of Newfoundland, in a threeyear project that concluded last year.

Project lead, Dominis Engineering president Bodo Gospodnetic, says: “Experimental results show that current widely accepted propeller manufacturing tolerances as stated in the ISO standard need to be thoroughly evaluated and investigated further.”

The current tolerance for a defect to the leading edge of a propeller blade is 500µm (0.5mm).

Ship propellers are manufactured according to ISO 484-1, with the majority of propellers made from castings rough machined on CNC mills and then finished using robotic and manual grinding. However, robotic and manual grinding of propeller surfaces introduces inaccuracies and deviations from the approved design, which can lead to cavitation, erosion, noise, vibration and loss of propeller efficiency.

“The leading edge is a very challenging area to manufacture accurately, yet it has a strong influence on sheet, streak and vortex cavitation,” says Gospodnetic.

Researchers found that a ship with a “defective” propeller must travel at a given percentage slower than a vessel with a “correct” propeller to operate below the cavitation inception speed and remain quiet.

For example, a ship with a propeller defect of 0.5mm would have to sail at 45% of the speed of a defect-free propeller to avoid cavitation noise. The smaller the defect, the less speed reduction is required to remain quiet.

“The 0.5mm defect tested is one of the tightest ISO 484-1 propeller manufacturing tolerances yet it has been demonstrated that it affects cavitation inception significantly and detrimentally. The rules need tightening up,” says Gospodnetic.

ISO 484-1:2015 has been a standard for propellers since 1982 and although the standard was reviewed in 2015 and 2022, the allowable tolerance and geometry remains unchanged.

“We know that 80% of underwater radiated noise comes from the propeller, but if ships are legislated to be quiet in sensitive habitats such as the Juan de Fuca Strait then they will have to limit their speed to below the cavitation inception speed,” says Gospodnetic.

While initial CFD studies show how very small defects can influence cavitation inception, the research partners are looking for funding to continue their investigation in a second phase model tests in a cavitation tunnel.

Cutting The Noise

A propeller technology capable of substantially reducing the underwater radiated noise (URN) generated by ships’ propellers has been developed by Oscar Propulsion together with the University of Strathclyde.

The patented PressurePores™ system reduces propeller tip vortex cavitation by applying a small number of strategically placed holes in the propeller blades. The addition of these pressure-relieving holes allows ships to operate with a more silent propeller.

Lars Eikeland, marine director at Oscar Propulsion, says: “Underwater radiated noise is one of the most adverse environmental by-products from commercial shipping, yet unlike other forms of marine pollution, there is currently no international legislation in place to prevent or reduce this source of environmental damage.

“Increasing noise levels, especially in the low-frequency range, is disorientating marine fauna and disrupting their communication signals, leading to behavioural changes or extinction. We now have a costeffective, easy-to-apply solution that prevents this from happening.”

Following four years of comprehensive computational fluid dynamics (CFD), modelling and cavitation tunnel tests during the solution’s development phase at Strathclyde, it was demonstrated that PressurePores can reduce cavitation volume by almost 14% and URN by up to 10dB.

Results were further verified in tests on the sub-cavitating propellers on Princess Royal, a 19m research catamaran operated by Newcastle University. And last year, CFD Finite Element (FE) propeller stress tests were successfully completed in accordance with classification society DNV rules.

“We have found the optimum number of holes required to reduce the noise. So long as the right number of holes are placed in the most effective positions, a cavitation sweet spot can be achieved,” says Eikeland.

“It’s not a case of simply drilling holes into the blades, as this will affect the propeller’s thrust capability. We know exactly where to place the holes for maximum efficiency and for optimum noise reduction.”

It is interesting to note that propeller cavitation can generate as much as 188dB of underwater radiated noise and can be heard by marine fauna 100 miles away.

According to the US National Oceanic and Atmospheric Administration, anything above 160db can pose a significant risk to marine life.

Commenting on the impact noise has on marine life, Eikeland says: “Noise levels in the ocean due to maritime activity have been increasing for decades and are expected to double by 2030.

“URN can cause irreversible damage to marine wildlife through stress, habitat displacement, reduced reproduction, lost feeding opportunities and even death, greatly changing the marine ecosystem and impacting biodiversity.”

Eikeland continues: “PressurePores has a major mitigating effect on propeller cavitation and URN and can be incorporated into new propellers or retrofitted to existing propellers either in drydock or possibly in-water.”

While Oscar Propulsions technology is suitable for all types of vessels, they are particularly suitable for naval vessels, yachts, fishing fleets, offshore vessels and cruise and research ships operating in sensitive environments. The technology can be applied to all types of propellers, including pods and thrusters.

Recent innovations are improving the performance of vessels – and manufacturers are also trialling new uses of old favourites