Paul, Maybe I missed something. Perhaps you can help me to figure how
to connect an ampmeter under water.




(Paul Mathews) wrote in message . com...
"David Flew" wrote in message ...
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

You can use this 'simple' procedure to monitor electrolytic corrosion
before and after the installation of zincs: Connect an ammeter
between the 2 metal structures that are suspected of having galvanic
potential differences (this is done in the water, of course). If
there is a potential, a current will flow in the ammeter. Note the
current, which can range from microamps to amps, depending on the size
of the structures. Then, install the zinc(s) and re-measure. The
current should diminish. You can also sometimes measure the current
in the bonding attachment for the zinc itself. If there is no
current, the zinc is doing no good. With some thought, you can make
sense of the current polarities as well.

Paul Mathews