Andina Marie wrote:
Chuck, you're confusing current and voltage.


I try to keep them separate, Ann-Marie,
but there's so much resistance. ;-)

Here is where I was going. The zinc and
the bronze prop are two dissimilar
metals and when they are immersed in an
electrolyte, a voltage can be measured
between them. No current flows through
the water between them until they are
electrically connected, usually by the
prop shaft. Then the current path is
through the water, returning through the
shaft.

There is no measurable voltage between
the prop and the zinc because the shaft
acts as a short circuit. There is an
electric field between the two metals in
the water, however, and if we knew how
to do it, we could measure a voltage at
the surfaces of the electrodes.

This galvanic couple sits in the water
producing a current that involves the
loss of Zn ions until the zinc is
depleted. It should produce no voltages
or currents anywhere else, including
across the galvanic isolator.

There should be no potential difference
between the shaft (which presumably is
bonded to the boat's DC and AC ground)
and the shore power ground due to the
zinc. Think about the boat's 12 VDC
system powering onboard lighting
circuits. There is no reason to believe
any of that 12 volts will show up as a
potential difference between the boat's
DC ground and the shore power ground,
barring some wiring anomaly.

The zinc/bronze galvanic couple no more
makes the boat "alive" than the boat's
onboard 12 VDC system, which also forms
a closed circuit.

Regarding the operation of a
semiconductor diode, it is good to
remember that the voltage across a
forward-biased diode is related to the
current through it. If a voltage is
measured, then there is a current
through it. Alternatively, if there is a
current through it, a voltage can be
measured. The VI characteristic is
highly non-linear of course.

A voltage of 1.2 volts measured across a
pair of series-connected silicon diodes
(like the 1N1190A) suggests a current on
the order of 100 mA or more! That is far
more current than you should ever
measure through a zinc/bronze galvanic
couple on a yacht even if your
measurement were directly between the
zinc and the prop.

Here's a suggestion: next time you
measure a DC voltage across a GI, make a
note of the polarity. From the direction
of electron flow, you can determine
whether the current you observe is
protecting the zinc or depleting it. See
if the polarity is always the same, or
if it is random.

I would try to track down what is
causing those readings.

Chuck

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