Peer-reviewed research conducted by Thordon Bearings into the effectiveness of hydrodynamic lubrication of a ship’s propeller shaft bearing found that the use of seawater-lubricated elastomeric polymer bearings reduces the fuel consumption.
The environmental and financial benefits of adopting a seawater-lubricated propeller shaft system are well documented, but this is the first indication that the arrangement reduces hydrodynamic drag enough to improve fuel economy. of fuel, compared to a conventional oil-lubricated bearing arrangement.
The groundbreaking research is based on a new methodology for calculating the performance of seawater-lubricated bearings, which until now was based on a decades-old theory developed for lubricated propeller shaft bearings in oil.
“The classic rigid surface bearing theory is valid and commonly used for oil-lubricated metal bearings. However, two major factors of bearings lubricated with seawater, namely the low viscosity of the lubricant and its deformability [of the polymer bearing surface]make the application of rigid surface bearing calculations sensitive,” says Dr. Gary Ren, Principal Research Engineer at Thordon Bearings, in his peer-reviewed article published in the July edition of the journal Elsevier Tribology International.
“We believe Dr. Ren’s method is more accurate, as it takes into account the characteristics of seawater as a lubricant, the polymer materials used, bearing pressures, viscosity, friction, etc.” , said Elena Corin, Senior Manager, Special Marine Projects, Thordon Bearings. “This is the first time anyone has investigated whether there are differences in coefficient of friction between the two types of bearings. And there is!”
Corin added that the new methodology provides “strong evidence” to quantitatively support the benefits of a Thordon COMPAC system.
“By applying the results, we were able to determine the minimum shaft speed required for effective hydrodynamic lubrication, the coefficient of friction as well as the thickness of the water film at a given load and shaft speed. “, she said.
“The test results confirmed that the fuel losses due to the coefficient of friction [hydrodynamic resistance] of an oil-lubricated propeller shaft bearing system can be reduced by up to 85% by using seawater-lubricated polymer bearings.
Thordon Bearings used the methodology to compare the fuel consumption of a Panamax container ship and an Aframax tanker operating a 640 mm (25.2 in) diameter oil-lubricated propeller shaft bearing compared to the same vessels operating a seawater lubricated COMPAC open bearing system.
Each vessel was supposed to run on VLSFO (ultra low sulfur fuel oil) at a global average price of USD 890/mt, operating 70% of the time.
“The results confirmed that fuel losses due to the coefficient of friction (hydrodynamic resistance) of an oil-lubricated propeller shaft bearing system can be reduced by up to 85%.
“Fuel costs will depend on the operational profile of the vessel, but we conservatively estimate annual savings of at least $10,000 per vessel,” Corin said. “This is in addition to the other life cycle cost savings associated with a Thordon seawater lubricated bearing system.”
Commenting on the research findings, Craig Carter, Vice President of Business Development for Thordon Bearings, said, “Reduced coefficient of friction is another reason why an oil-lubricated propeller shaft bearing system Seawater pays dividends for ship owners and operators, both commercially and environmentally. When you consider the operational cost of operating an oil-lubricated propeller shaft over 25 years, a seawater-lubricated shaft line pales in comparison.
Thordon Bearings is currently conducting research to assess the carbon emission reduction attributes of ships using a seawater lubricated system to help shipowners meet CII (Carbon Intensity Indicator) requirements.
From 1 January 2023, shipowners will be required to document their annual CII achieved and check it against the annual CII required to determine the operational carbon intensity rating of their vessels. The savings in fuel consumption and the consequent reduction in CO2 emissions are at the heart of the CII measurement.
Source: Thordon Bearings