Even on ships fitted with ICCP or sacrificial anode systems, propeller shaft bearings are vulnerable to corrosion. This is because turning shafts are electrically insulated from the hull by the lubricating oil film in the bearings and by the use of non-metallic bearings in the tail shaft.
The problem can be eliminated if the shaft is earthed to the hull using a propeller shaft slipring. Cathelco supply complete shaft earthing assemblies consisting of a pair of high silver content/graphite brushes mounted in a balanced brush holder, running on a copper slipring with a solid silver inlay track. This combination has been proved to give the optimum electrical continuity.