The technical reason why barncles love growing on a bronze propeller
or shaft, even when painted with strong cupper based antifouling paint
, is like follows:
Stainless and marine quality bronze are cloe in nobility and does not fight each other galvanically. So, as long as the shaft and the propeller is electrically isolated from all other underwater metal parts
, they need no protection zinc mounted. Shurley you have seen many old sailboats having rudder
hinges with bronze bearings and stainless shaft. No zinc, no corrosion
problem, yet 30+ years of use.
When a zinc anode is installed on a propeller / shaft, it starts a fairly strong galvanic process to prevent any and all metal ions to leave the propeller and the shaft.
That may seem OK, but since bothe the cupper in the bronze and the cupper in the antifouling paint
is then completely prevented from getting loose. Since the cupper ions is what stops the barnacles
, the protection from the antifouling is lost
This problem has come up when the tin based antifouling (luckily for the environment) was stopped. That paint was not dependent on ion release, but other mechanisms. Now we try to rely on Cupper for protection, and that gets ruined by the zinc anodes.
If contemplating to take teh zinc away ONLY do this when:
1) The propeller is made of proven marine quality bronze
2) there i absolutely NO electrical
connection (ground) between the shaft/propeller and ANY other metal object under water. (Isolated shaft coupling, engine
not connected to the bonding system, if any, or two-pole main engine
cutoff switch, off during harbour stay) This can be checked with a multimeter for ohms, on land, it shall exceed 10000 ohm between propeller and any ather underwater item.
Basically, the commonly practized bonding system used in USA boats cretes more problems than it solves. Lightning
protection to sea should be installed by sparc-gap isolaters.