Not every vessel has or needs a shaft brush, so why do some have them while others don’t? It’s a good question that isn’t asked often enough. The use of a shaft brush becomes more critical in applications where shafts and props are not equipped with their own anodes (there may not be enough room on the shaft), or in the event these are consumed or lost. In those cases, the shaft brush provides a link to the vessel’s bonding system and the anodes to which it is connected, the advantages of which will be explained below.
Shaft brushes are specifically designed to maintain electrical contact with a rotating surface. They resemble the brush used in an electric motor’s commutator and should be made from the same material, carbon (some simply use a hunk of copper or bronze, which can be less effective). Conductors used to complete connections between shaft brushes and bonding systems should be a minimum of #8 AWG and if doing double duty as a lightning protection system must be a minimum of #6 AWG; this wiring protocol is true of the entire bonding system.
The vast majority of shaft brushes I encounter are in poor condition, many are corroded, improperly adjusted, or they’ve lost contact with the shaft. Their function is already imperfect at best. Maintaining low resistance contact between moving and stationary objects in a damp, vibration-prone environment is very challenging indeed, and as such, it doesn’t take much to impair their function. If your vessel has one, it is anything but a set and forget piece of gear, it requires regular inspection to ensure it’s working properly.