"Russell" wrote in news:1150917215.062248.50070
@p79g2000cwp.googlegroups.com:

I had a diver under the boat today. This boat was in the yard ten weeks
ago. New shafts, props, all new zincs everywhere (tabs, rudders,
shafts, transom, etc.). The diver reports the ZINCS ARE GONE!!! Even
most of the bolts are gone or just hanging. How can this be happening?
If it is another boat, what can I do to find it or to protect against
it without exposing anyone on my boat to AC shock. In the meantime, we
are unplugging from shore power, which is scary, although our bilges
are pretty dry. I read all the posts, but am still confused on whether
it is AC or DC related, and if the isoloation transformer has any
impact. Oh, one more thing. We had a new computerized charger installed
a while back. I wonder if it were wired incorrectly would that cause
the zincs to go like this?



It is DC related because your underwater metal parts are hooked directly
to the electrical system on the dock's ground wire. This amounts to
running a wire from your engine block and underwater metals to every
other boat's underwater metals and the entire bottom of the bay. This
results in some horrific electrolysis currents, you zincs trying to
provide noble metal to protect the whole marina, not just your skeg and
prop, eating it away at an amazing pace. When the zinc was gone, the
next least noble metal started to be used as the underwater battery plate
in the shorted system.

Get a digital multimeter from someone. Unplug the boat at the dock post.
Put the digital multimeter on DC volts. Put one lead of the meter on the
boat's now-open ground pin in the boat power plug. Put the other meter
lead on the ground lug (or the metal electrical box bare metal which is
the same point). You will now measure the DC potential difference
between your boat's underwater metal parts, hooked to the ground pin of
the boat's power plug...and the rest of the world hooked to the marina
ground, including every other boat that's wired like yours is.

This voltage will be around 1-2 volts DC, not AC. Any AC voltage you can
measure at this point indicates something is bad wrong with the marina's
electrical system. There should be no AC voltage between their ground
and your boat's connection to seawater...the underwater metals that are
"grounded" to AC ground.

If you want to see how much current flows when the boat's plugged in,
switch the digital multimeter to DC amps. The internal low-resistance
shunt in the multimeter is a virtual short, allowing the boat and AC
power system to conduct like it was plugged in. That current you see is
why your zincs are eating away just before the nice prop was.....

Many problems are caused by the boaters themselves. One boat has a
faulty battery charger leaking between the DC output and the charger's
ground back to the dock. This results in some DC voltage applied to that
boat's underwater metal parts, much more voltage than just zincs can
produce and, probably the wrong polarity. His boat is now acting like a
PLATING MACHINE!....using your zincs to plate his prop! Some other boat
hasn't had a zinc in years so his boat causes a big potential difference
to yours and others. To find the offender, you need one person standing
watching your DC voltmeter reading high and another person going down the
dock, NOT SWITCHING BREAKERS ON AND OFF, but unplugging boats, one after
another. FRS walkie talkies make this job much easier than yelling
between you. When you unplug the offender, the voltage back on your dock
drops drastically. THAT guy is eating everyone's zincs and props. Show
the marina dockmaster who has a very good reason to get him OFF YOUR GRID
as it's eating the marina's electrical system, too!

Let us know which boat has the flea market automatic battery charger when
you find him..... Japanese battery chargers some have POSITIVE DC hooked
to AC ground!....the only AC ground point on his boat so it doesn't short
his batteries to operate that way. His prop is +14.2V above
yours!...lotsa ground current flows!