We have been eating up zincs, and just had to replace both props. Had
the marina check and they said the power cord gound terminals on our
twin inboard were grounded to the thru hulls. This is in sal****er.
Any ideas on what could be going on, why this is an issue and what to
do to correct it?

Russell wrote:
We have been eating up zincs, and just had to replace both props. Had
the marina check and they said the power cord gound terminals on our
twin inboard were grounded to the thru hulls. This is in sal****er.
Any ideas on what could be going on, why this is an issue and what to
do to correct it?


Hello Russell,

I assume this is a relatively recent problem that
isn't correlated with any change(s) in the boat or
its surroundings that you are aware of.

A very common problem is that a nearby boat with a
depleted zinc is obtaining protection from your
zinc via the marina shore power cable (the green
grounding wire, specifically) as the return path.

If so, you can protect your zincs from this
"theft" by installing an isolation transformer or
a galvanic isolator. A third solution is to
isolate your boat's prop and shaft from the shore
power grounding wire. Of course, "pulling the
shore power plug" will solve the problem albeit
while adding a new one. Might be a good precaution
though until the source of the problem is identified.

There are other, less common, problems that could
cause sudden and rapid depletion of zinc (and props).

If you want to pursue it yourself, get a copy of
"The 12 Volt Doctor's Practical Handbook" from
West Marine or BoatUS. It's all in there. Not
really very difficult and there's no danger of
"breaking something".

But if you have more money than time, hire a
marine electrician or corrosion specialist. Get
the book in either case, since it costs a lot less
than a single zinc.

Good luck.

Chuck

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Russell wrote:
We have been eating up zincs, and just had to replace both props. Had
the marina check and they said the power cord gound terminals on our
twin inboard were grounded to the thru hulls. This is in sal****er.
Any ideas on what could be going on, why this is an issue and what to
do to correct it?

For safety reasons, the ABYC specifications call for the shore power
cord to be grounded to the DC ground which includes the underwater
metal attached to the engine(s).

Your cheapest protection is a galvanic isolator. They are on sale for
under $100 at Defender
http://www.defender.com/product.jsp?path=-1|328|303336&id=605562
One isolator can handle a 50 amp shore cord or two 30 amp.

Andina Marie wrote:
Russell wrote:
We have been eating up zincs, and just had to replace both props. Had
the marina check and they said the power cord gound terminals on our
twin inboard were grounded to the thru hulls. This is in sal****er.
Any ideas on what could be going on, why this is an issue and what to
do to correct it?

For safety reasons, the ABYC specifications call for the shore power
cord to be grounded to the DC ground

OK

which
includes the underwater
metal attached to the engine(s).


I don't think ABYC actually requires this part. It
is often done, but not always.

There is an increasing number of new boats that
are designed with electrically isolated props and
shafts. Without a connection to the shore power
ground, there is little chance for galvanic
currents to be set up with a neighboring boat. In
fact, even through-hulls can be electrically isolated.

It is actually a safe and trouble-free approach,
but it requires an insulating shaft coupler. And,
of course, some special attention to an RF ground,
if there is one. Usually a capacitor with high
reactance at 60 Hz can effectively isolate the
grounding plate.

Chuck

Your cheapest protection is a galvanic isolator. They are on sale for
under $100 at Defender
http://www.defender.com/product.jsp?path=-1|328|303336&id=605562
One isolator can handle a 50 amp shore cord or two 30 amp.


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"Andina Marie" wrote in news:1150291413.115563.258890
@g10g2000cwb.googlegroups.com:

For safety reasons, the ABYC specifications call for the shore power
cord to be grounded to the DC ground which includes the underwater
metal attached to the engine(s).


What safety reason is that? The engine doesn't have any AC-powered
equipment, neither do any of the DC-operated equipments. There's no reason
to AC ground anything not hooked to the AC power line.

Have you got a reference to ABYC about this? I'd like to read what it
says, other than "for safety reasons"....

"Russell" wrote in news:1150267403.137578.163050
@p79g2000cwp.googlegroups.com:

Any ideas on what could be going on, why this is an issue and what to
do to correct it?



That's easy. The marina's power system is hooked to ground in a hundred
places, as well as dangling in the seawater under the docks.

There is a complete circuit for your electrolysis hooked to all these
wonderful grounds and underwater conduits because some idiot grounded
your AC power ground to your underwater fittings and outdrive. So, YOUR
zincs, and when they're gone, YOUR OUTDRIVE is protecting the marina's
electrical system from corrosion, like it does to EVERY metal thing it is
hooked to...

Now, where is the issue. First place to look is your battery charger,
the common point to both battery DC systems, the negative terminal of
which is hooked to the engine/outdrive, and the AC power system's ground.
It's supposed to be isolated....Let's test it.

Drag the power cord from the dock post down into the boat so we can get
to the AC ground pin on the dock plug of your boat any ol' place we
want.....

Do you have an ohmmeter and know how to use it?

If yes, measure the resistance from the ground pin on the plug to the
engine block. It SHOULD be open...no connected...infinity resistance.
If it's connected together and shows a low resistance, unplug the battery
charger from the AC power supply, breaking its ground from the AC system,
and measure from the AC ground pin to the engine block, again. If it
opened up, the charger is where they are connected. If it's still
connected, we'll have to start tearing into the power panel to find out
where some idiot interconnected them....probably after reading some of
the posts on this newsgroup where it all gets bonded together to satisfy
some old wives tale that has screwed up many a zinc and prop.

Other places the AC power line can ground the engine block a

**hot water heaters with both AC and engine cooling system heating...
**a jumper wire added to the inside of the power panel from the AC ground
connection to the DC negative bus by some idiot.
**any place else where the systems come together....including seawater-
cooled air conditioners, another electrolysis problem altogether.

The case of the air condition is grounded to the AC line ground....
The case of the air conditioner is also hooked to the seawater by the
seawater passing through its seawater-cooled heat exchanger. There might
as well be a wire hanging over the side and no way of changing it. The
only way I know of to isolate them is feed the AC line through a galvanic
isolator or big isolation transformer like the big yachts use.

Good luck. I can hear your new zincs fizzing away clear over here!

Until you fix it....UNPLUG THE BOAT!