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(Chuck Baier) wrote in message om...
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

A simple check of resistance between the zinc and any components
will show if the zinc is electrically isolated. This can work even in
salt water since the resistance of the salt water is still much
greater than the very low resistance of the metal path. For a
"standard" of no connection, connect to the zinc adjacent to where it
is supposed to be but not touching where it is supposed to be mounted
and conncet the other end to the other metal part. When the zinc is
correctly mounted, the resistance should drop a lot.
I have noticed (and experimentally verified) that having a zinc
correctly connected allows a bronze prop to foul with barnacles
because the difference in electronegativity suppresses the Cu ions
from the bronze from going into solution. When your zinc is NOT
connected, you will have very few fouling problems on the bronze prop.
Some day, I will use this effect to apply zinc to bronze props to
prevent prop fouling by electrodeposition as a sacrificial layer as I
have also noticed that zinc doesn't foul. The use of a shaft zinc
would protect the prop when eventually the zinc coating corroded
through.
A few yrs ago, some guy was applying Cu to bronze props to prevent
fouling but had very limited success. My theory is that the
electronegativity of Cu relative to Zn was the problem.