I'm in the middle of a repair of problems caused by electrolysis. One
destroyed bronze prop, several destroyed SS bolts which hold the stuffing
box plate in place. I found the connection from the electrode to the steel
parts adrift due to a grounding, but that's recent and not the cause of the
damage. I feel the underlying cause is that the shaft, prop and stuffing box
/ plate / bolts were insulated from the main zinc by the skeg bush. It
won't happen again, and an additional zinc on the prop shaft is clearly
required.
But this all got me to thinking - how can one check the effectiveness of the
zincs? It's easy enough to swim under the boat in summer and look at the
extent of marine growth, make sure the zinc is still there, etc. But is it
possible to check that the zincs are actually doing their job of maintaining
a voltage difference? I could easily connect a voltmeter to the rudder
shaft or the prop shaft - but what could I use as a reference point, and
what are the appropriate voltages? I'm specifically NOT looking for a
commercial piece of electronics, just a simple test I and other owners can
do now and then. We've got pretty basic wooden boats, mostly 18 to 25 ft
long, mostly over 20 years old - some going back a lot longer than this.
All moored permanently in Mordialloc Creek Victoria Australia - we try to
slip them every year and re-do the antifoul etc, but sometimes the
maintenance gets a little delayed. It would be nice if we could at least
check that the zincs are working ....

Any reference sites or suggestions?

Regards
David Flew