Water Lubrication System

A water lubrication system uses water rather than oil as the lubricant in a ship’s stern tube and shaft bearings, with the rotating shaft drawing in seawater that forms a hydrodynamic film between shaft and bearing. The coefficient of friction falls below 0.001 once the wedge forms, and the bearing needs no oil and no environmentally acceptable lubricant, so a stern tube leak poses zero oil-pollution risk under the US VGP and MARPOL Annex I. Water also carries roughly twice the specific heat of oil for cooling. The limit is low seawater viscosity, which cuts load capacity and demands wear-tolerant bush materials.