Your prop is actually creating electrical current as it turns. The current sets
up galvanic corrosion of your prop / shaft and strut sacrificing metal to the
water. The zincs are there to sacrifice zinc instead of your prop brass and
shaft / strut steel. The shaft zinc takes the brunt of the corrosion.
My shaft and strut zincs seem to ware out at the same rate and are replaced
at the same time. The rate of decay is also effected by the stray current at
your dock and the amount of time your boat is connected to shore power.
I work at keeping my shore power on only to charge batteries and turn it
off when I am not on board. This will all change when I go to 12V fridge
and will need to leave my shore power on all the time. I am changing zincs
out 2X per year now and expect that to increase to 4X per year without
some kind of galvanic isolator.
The Bottom Cleaner cleans off the zinc area with an abrasive cleaning pad
prior to install.
Happy Zincs