Do I need to electrically bond all skin fittings?
Cathodic protection of skin fittings. It was common practice up to the 1980’s to link all metal underwater fittings with an electrical bonding connected to a hull anode. This was in the belief that any cathodic protection was better than none. In practice that has proved to be counterproductive. In electrochemical erosion processes a current flows between two metals which have a high potential difference (measured as a voltage) from the higher metal on the galvanic scale to the lower. Electrons flow in the opposite direction which results in the lower metal (anode) eroding.
For a sacrificial anode to be effective it needs to be close to the component being protected as potential difference drops over a distance. It is termed ‘line of sight’ in that the anode needs to be able to ‘see’ the propeller or skin fitting which is why most anodes are located near the stern gear. If all the skin fittings are bonded to an anode, they are also bonded together. This means that there is a possibility of an electrochemical reaction occurring between skin fittings which could be of slightly differing alloys. Although their potential difference will much lower, they are likely to be very much closer to each other than to the anode and therefore the potential difference can be actually greater. Best practice now is to ensure that skin fittings are electrically isolated within the vessel.