Stern tube lubricants; used to lubricate the bearings that support the vessel propeller shaft in the stern tube, are continuously lost to the marine environment while the ship is moving under power. Although the specific amount of lubricant lost in a stern tube application is dependent on the particular stern tube system and vessel type, many reports support the fact that lubricant loss through vessel stern tubes is by far the greatest source of lubricant ingression into the environment of all vessel lubricant applications incidental to their operation. Historically, stern tube lubricants have been based on mineral oil but recently synthetic, environmentally acceptable stern tube lubricants have been made available. The U.S. EPA Vessel General Permit for Discharges Incidental to the Normal Operation of Vessels (VGP) revised in 2013, mandated the use of environmentally acceptable lubricants (EALs) for all oil-to-sea interface applications in vessels constructed on or after December 19, 2013 and all vessels built before December 19, 2013 unless technically infeasible. The VGP specifically defines stern tube lubrication as an oil-to-sea interface application. One type of EAL recommended by the U.S. EPA to replace mineral oil in all oil-to-sea interface applications is based on polyalkylene glycol (PAG) base stocks. PAG based lubricants offer a unique combination of performance and environmental properties.

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