Peer-reviewed research by a bearing manufacturer into the hydrodynamic lubrication efficiency of a ship’s propeller shaft bearing has found that the use of seawater-lubricated elastomeric polymer bearings reduces 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 resistance enough to improve fuel consumption, compared to a conventional oil-lubricated bearing arrangement. The research is based on new methodology for calculating the performance of seawater-lubricated bearings, which, until now, has been based on decades-old theory developed for oil-lubricated propeller shaft bearings.
Dr Gary Ren, Thordon Bearings’ Chief Research Engineer, said: “Classic rigid surface bearing theory is valid and commonly used for oil-lubricated metal bearings. However, two major factors of seawater-lubricated bearings, namely low lubricant viscosity and deformability [of the polymer bearing surface], make the application of rigid surface bearing calculations susceptible.”
Elena Corin, Thordon’s Senior Manager, Special Marine Projects, added: “We believe Dr Ren’s method is more accurate, because it takes into account the characteristics of seawater as a lubricant, the polymer materials used, bearing pressures, viscosity, friction and so on. This is the first time anyone has investigated whether there are differences in friction coefficient between the two types of bearings. And there are. By applying the results, we were able to determine the minimum required shaft speed for hydrodynamic lubrication efficiency, the coefficient of friction as well as the water film thickness at any given load and shaft speed. Test results confirmed that fuel losses due to the friction coefficient [hydrodynamic resistance] of an oil-lubricated propeller shaft bearing system can be reduced by as much as 85% by using seawater-lubricated polymer bearings.”
Corin believes that the new methodology, published in the journal Tribology International, provides “strong evidence” to quantitatively support the benefits of a Thordon Compac system. The company has used the methodology to compare the fuel consumption of a Panamax containership and an Aframax tanker operating a 640mm (25.2in) diameter oil-lubricated propeller shaft bearing versus the same ships operating an open seawater-lubricated bearing system. Each vessel was assumed to run on VLSFO at a global average price of US$ 890/mt, operating for 70% of the time.
Corin said: “The results confirmed that fuel losses due to the friction coefficient (hydrodynamic resistance) of an oil-lubricated propeller shaft bearing system can be reduced by as much as 85%. Fuel costs will depend on the operational profile of the vessel, but we conservatively estimate annual savings of at least US$10,000 per vessel. This is in addition to the other life cycle cost savings associated with a Thordon seawater-lubricated bearing system.”
The company is now carrying out research to assess the carbon emissions reduction attributes of vessels using a seawater-lubricated system to help shipowners meet CII requirements.
