b393capt wrote:
Nope, I am not parked next to metal hull vessel or bridge.
Boat is one year old, don't know yet whats normal for the prop zinc.
Would it help if, without the zinc fish, I measure current from just
placing my vom meter probe in the water ?


Dan, since you have a zinc on the prop,
the fish should have almost no effect at
all, because it is much farther away
from the prop than the prop zinc. So
there is clearly a problem.

When you disconnected the shore power,
did you actually unplug the cable or did
you just turn off the AC breaker? If you
unplugged it, that pretty much rules out
problems with shore power. With the
cable unplugged, it doesn't matter
whether your galvanic isolator is
working or not.

Here are two quick tests you can make:

1) Measure the voltage between the fish
and the engine stud, noting the
polarity. This may confirm that
something is adding to the galvanic
couple set up by the zinc and the prop.

2. Disconnect the battery positive
terminal wire and see if your symptoms
change. If you have a breaker at the
battery, or a switch, or a fuse, then
there is no need to remove the wire, as
long as absolutely nothing is connected
to the battery terminal. That includes
bilge pumps, etc. Be careful working
around the battery.

I'm still leaning toward the idea that
one of your 12 volt appliances is
causing +12 volts (more or less) to be
set up between the engine/prop and
something like a through-hull. It could
be a short or incorrect wiring to the
appliance. The effect would be to
accelerate the corrosion of the zinc.

If all of the underwater metals are
bonded together then this problem
couldn't occur, of course, but usually
they are not.

Chuck





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