Shaft earthing slip ring

A shaft earthing slip ring is a brush-and-ring assembly that grounds the propeller shaft to the hull, draining the small voltage the rotating shaft picks up so current does not discharge across the oil film of the stern-tube and main bearings. Without it, shaft potential drives stray current through the bearings, and the spark erosion pits the whitemetal and frosts the journal. A silver or copper-graphite brush rides a clean slip ring on the shaft, connected to the hull by a low-resistance strap; a separate brush often feeds a millivolt monitor that reads shaft-to-hull potential as an early warning. It is distinct from cathodic protection of the underwater hull.