Thanks fot all your replies so far!
I am aware of the difference between galvanic corrosion/currents and stray current corrosion
Unfortunately too late to send any pics of the prop; I put the boat in the water Saturday.
I heard about "over-zincing"; I have never measured the cathodic protection voltage of my system for which I would need a silver electrode.
The reason why I added 2 zinc discs to both sides of the propeller
shaft strut, is that some years ago I observed purple-ish coloring of the strut, after I had connected the SSB
auto-tuners RF ground to this bronze strut. There was already a copper mesh connected to it from the engine
ground, thus serving also as the boat's (12V system) ground. Fearing problems due to the SSB
(questionable) I did add the 2 bronze discs and put in high voltage capacitors (1 microF in total) between the ATU and the bronze strut to block any DC and ground loop voltage differences.
The zincs are branded and expensive.
As for the prop: there was no purplish coloring around the small pits. There was however, as every year, a difficult to remove calcified crust on the blades, indeed it made me think about barnacles
- but could they cause the pitting?
As for the dock
power system ground, I checked any leaks
between this ground and the battery
chargers output and 12V negative: infinite resistance. Tested for any faults in the 220V circuit of the water boiler - it seems OK.
There is definitely no connection between the dock
power ground and ships ground or the engine
(...I know...European rules not as stringent...) so I suspect adding in a galvanic isolator
or isolation transformer would be unnecessary.