shaft should be bonded when:
1. The voltage test indicates inadequate protection.
2. The shaft zinc forms a significant part of the grounding electrode (ground plate).
and/or shaft-saver type coupling isolates (electrically) the shaft from the engine
Seldom does a small-craft shaft require more than a single
zinc (you mentioned zincs
The common reason for installing a shaft brush is to improve the protection from zincs attached to the propeller
shaft and other underwater parts of the boat. However, owners of some inboard engines have reported that pencil zincs in the cooling
system last up to twice as long with a shaft brush installed. Simply stated, if all underwater metal hardware
is bonded [*1]
, and electrically attached to a zinc it is better protected from galvanic corrosion
. A shaft brush (wiper) provides a better connection between the shaft and the bonding system, than the packing, coupling, and transmission [*2]
*1 A bonded system is when you connect all your immersed metals in the boat to each other, and then to a sacrificial anode. The sacrificial metal will erode protecting the boat metal that is bonded to it.
If a boat is not bonded, and you took readings on all the underwater metals with a DC milli-Amp meter, the readings will be slightly different from each other.
Bonding, in effect, makes all the immersed metals into one piece of metal. The voltage throughout the bonding system will be the same, and will average out depending on the size and voltage of each metal.
One piece of zinc, of the appropriate size, attached to the bonding system as the sacrificial anode, will protect all the bonded metals.
*2 The lubricant in the gearbox works as an insulator preventing good electrical connection between the shaft and the engine.
Not all boats need a shaft brush, and there are cases where one may do more harm than good. Before spending a lot of money
on new hardware
, you should perform (or have performed) the voltage test.
The stuffing box, stern tube and strut should all be attached to the bonding system with at least #6 AWG (Green) wire or copper bonding strap. Obviously the shaft should be provided with a zinc and so should the strut, if provisions are made for it.
BTW: Shaft zinc's are generally expended more quickly than stationary zincs, because of the velocity of the shaft and the abrasion of the water