So, what is this all about anyhow? The concept here centers around bonding of the shaft to the hull for the purpose of mitigating corrosion. The idea is that propeller shafts really don't make a very good electrical connection to the rest of a boat's bonding / cathodic protection system since the propeller shaft is connected directly to the transmission (reverse gear) at the back of the engine. Electrical continuity through the transmission is always a bit dicey, especially when underway. So, how do these shaft brushes do in this situation? Usually not too well frankly. Most of us that deal with matters of marine corrosion don't actually recommend them. The failure rate for these brushes, and especially something as jury rigged as the one in the photo above is just too high. I've checked many of these over the years for electrical continuity and found an open circuit between the shaft and the brush.