Here's an item that may be of interest to folks who keep a boat in the
water at a marina. It has been technically reviewed, and after a few
minor tweaks is now considered accurate by a local specialist in the
field......

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Could Shorepower Sink Your Boat?
The Corrosion Series, Part I

John Q. Boater is fastidious about maintenance. Almost every weekend
that he isn't actually using his boat, John can be found at the marina
changing fluids, checking connections, topping off batteries, and
cleaning or recleaning surfaces that already look immaculate to any
less critical eye. Most of John's friends admire his dedication to
keeping his pride and joy in bristol condition; but when John is ready
to wrap up a maintenance project and return home he habitually takes
an action that exposes his boat to needless wear, tear, and abuse- he
connects to the shorepower grid at his marina.

John Q. is quick to observe that the shorepower connection will allow
his charger to keep the batteries topped off. Should a through hull or
stuffing box begin leaking before he returns to the boat, the fully
charged batteries will better serve the bilge pumps. John Q. may have
a refrigerator or hot water heater running while he's not aboard, and
may rely on AC powered heaters to prevent freezing in the cabin or
engine room during the winter months. The many benefits of maintaining
an AC shorepower connection are not without some serious tradeoffs.

Many of the most critical components of a boat are made of metal and
are either partially or totally submerged. Shafts and props are
obvious examples, as well as struts, rudders, and through hull
fittings. Some of these metal items are critical to the propulsion or
control of the boat, while others are literally preventing a
disastrous flooding of the bilge. The moment that a boat is immersed
in water, an electrochemical process known as "galvanic corrosion"
begins. Each type of metal or alloy has a unique voltage or potential
with respect to the water. When two types of metal, (for example, a
stainless steel shaft and a bronze propeller) are immersed in water or
any other solution that will conduct electricity, electrons begin
flowing from the less "noble" metal in the pair to the more
"noble" (more positively charged) metal.

The physics of this corrosion processs are closely associated with how
and why a flashlight battery works; the flow of electrons between two
dissimilar charges becomes electrical energy. When electrons are
flowing between two types of metals immersed in a conductive solution,
the result can be referred to as an "electrochemical galvanic
corrosion cell". The more negatively charged metal will degrade and
corrode as electrons are drawn away by the more positively charged
material. The comparatively negative material in a galvanic corrosion
cell is referred to as an "anode", while the more positive material is
called a "cathode". Electricity can flow from the anode to the
cathode at voltages as high as 1000 millivolts- (one volt). When
additional external voltage is available to drive the transfer of
electrons, the destruction of the anode can occur in a surprisingly
short period of time.

Protecting an individual boat

Boaters normally rely on "sacrificial" anodes to protect the more
vulnerable, less noble metals used in a boat. Electrical energy flows
along the path of least resistance, and the more noble metals will
establish a galvanic corrosion cell with the least noble metals
available. Sacrificial anodes are intentionally less noble than any of
the vital metal parts of a boat, and as long as sufficient anodic
material remains there will be little or no loss of electrons from
critical components. Zinc is commonly used as an anode, and many
boaters routinely refer to all anodes as "zincs".

Zinc is a fine anode for a vessel normally moored and operated in salt
water, but can develop an oxide-like coating when used in fresh water.
The coating or "crust" essentially disables the zinc by insulating it
from the water. (The coating can be cleaned off with a wire brush). If
the zinc is not cleaned off, the next least noble metal component will
begin acting as a default anode. Magnesium is a superior anode for
fresh water use, but is not suitable for use on a wooden boat under
any circumstance and should be restricted to boats that will see very
little or no sal****er exposure. (Magnesium degrades very quickly in
seawater).

Our hypothetical boater, John Q, undoubtedly changes his anodes at
every haul out and likely hires a diver at regular intervals to
perform a visual inspection. Assuming John Q's boat is fit with an
appropriate amount of properly selected anodes, assuming his boat is
properly wired and grounded, and assuming his dock is properly wired
and grounded, John Q shouldn't have to worry too much about
electrochemical galvanic corrosion. That is, of course, providing that
he has his own private dock and there are no other vessels hooked to
shorepower nearby. (Oops!)


The problem with shorepower:

AC service to a proper shorepower connection consists of three
conductors. Power is supplied to AC devices through a "hot" wire, and
returns to the grid through a "neutral" wire. Some simple appliances
and fixtures, (such as a desk lamp, for example), rely on a two-prong
plug and use only the hot and neutral conductors to create a circuit.
More sophisticated devices use a three-prong plug and include a
"ground" wire.

The purpose of the ground wire is to provide a safe pathway to earth
for any electrical current that might become present outside a
normally functioning circuit. For example, a loose connection in an AC
device with a metal case could result in a "ground fault" condition
and the presence of an electrical charge on the case. When a human
being touches an ungrounded component and an electrical charge is
present, that charge will attempt to use the human being as a means to
complete an electric circuit. Even 110-volts flowing through the body
can be fatal. Electricity follows the path of least resistance, and
the presence of a ground wire provides an easy means for any stray
electrical charge to complete a circuit without flowing through a homo
sapien. When connected to a shorepower grid, any stray current
collected by the ground wire is conducted to a grounding point-
typically a large copper rod driven into the earth on shore.

Perhaps a majority of cruising boats have AC generators aboard, and
manufacturers are mindful that even boats initially sold without an AC
generator may have a generator added in the future. Accordingly,
standard industry boatbuilding practices also ground a vessel's AC
circuits to underwater metals as well as to the shorepower connector.
(Rather obviously, there is no shorepower ground available at sea).
The green ground wire in a shorepower system actually creates a
circuit that includes all vessels connected to it, with the ground
wire functioning as one conductor in the AC circuit and the water that
the vessels are floating in functioning as the other. Additionally,
stray low-voltage currents introduced through a vessel's shorepower
ground wire can flow through the vessel's underwater AC ground system
and accelerate corrosion of metal items connected to the DC bonding
system.

When shorepower is added to the equation, the additional stray voltage
traveling through the ground circuit will accelerate the activity of
galvanic corrosion cells. As surely as some of the vessels on a common
dock will be improperly wired, others will be inadequately or
improperly "zinced". Boats without adequate anodic protection tend to
rob the anodes from boats moored nearby. A conscientious owner may
apply fresh zincs with a reasonable expectation of six months to
perhaps a year of service from the anodes, but if a boat with faulty
wiring or inadequate anodes is connected to the same shorepower grid
the fresh zincs may erode in 60 -90 days. Once the sacrificial anodes
are gone, the least noble portions of shafts, struts, props, rudders,
and other underwater fittings will begin corroding instead.

Boaters often blame the marina itself, and rumors of a "hot dock"
circulate throughout the community. Unless there is an obvious
problem, (such as a wire with a damaged insulator hanging in the water
or a "wet" junction box), experts say that most supposedly hot dock
problems actually originate aboard some of the vessels moored at the
location.


What not to do:

During discussions of this subject with Lori Hogan, (co-owner of Boat
Electric in Seattle), she emphasized that boaters should ignore any
advice to "cut the green wire" as a means of isolating their onboard
electrical system from the shorepower ground system.

"A few years ago," Lori remarked, "one of the regional boating
magazines ran an article about dealing with galvanic corrosion. Much
of what they had to say was useful, but I couldn't believe my eyes
when I read their advice to 'cut the ground wire' just inside the
boat's shorepower connector. That sort of advice could literally get
somebody killed. Without the ground wire in place, there's a greater
risk that a person could become an unwilling ground and complete the
circuit. To make matters worse, even after we contacted the publisher
with our concerns we noticed that one of the boating yellow pages
books reprinted the article later that year, with the same potentially
deadly advice still remaining. No matter what, nobody should cut the
safety ground wire in their AC electric circuits."

Two proper solutions:

Boaters unwilling to supply anodic protection for neighboring boats on
a common shorepower grid, (and/or unwilling to run the risk of
premature anode corrosion and the damage that will inevitably follow)
can take two fairly simple steps to mitigate the shorepower problem.

A galvanic isolator can be installed on a vessel's ground wire,
immediately inboard of the shorepower connector and ahead of any
connection to selector switches, distribution panels, or AC devices.
With a galvanic isolator in place, AC current will continue to pass
through the isolator and back into the shorepower grid. Should a
ground fault occur aboard a boat, the green ground wire will still be
able to do its job and reduce the risk of electric shock. Low voltage
and DC currents of the type that sneak aboard uninvited from adjoining
vessels and create most of the shorepower-related galvanic corrosion
issues on a boat will not pass through the galvanic isolator. Newmar
is a popular brand of galvanic isolator, and is available for purchase
through Boat Electric. Most boaters would consider the modest price of
a galvanic isolator reasonably affordable, as well as a good
investment.

A more sophisticated, reliable, (and more expensive) option involves
the addition of an isolation transformer. When installed immediately
inboard from a vessel's shorepower connection, an isolation
transformer effectively separates an individual boat's electrical grid
from the shorepower system on the dock. In simple terms, the isolation
transformer uses whatever shorepower current is available to generate
a new, isolated AC service circuit. Power spikes, polarity concerns
and other issues associated with marina-supplied shorepower are of no
concern when power is supplied by an onboard isolation transformer.
The power produced is uniform, predictable, and there is no risk of
accelerated galvanic corrosion due to stray currents in a common
ground wire.

"Isolation transformers used to be pretty well out of financial reach
for a lot of boaters," observed Lori Hogan, "but one of the brands we
carry, Iso-Guard, has developed a new model that is a lot less
expensive and we are selling quite a few more isolation transformers
than ever before. If a boater wants to be entirely certain that poor
quality shorepower service and the common ground wire aren't messing
up his own electrical system, an isolation transformer is the only way
to go. On a dock where there are a lot of boats drawing quite a bit of
AC power, boats at the end of the line often get less than 110 volts
delivered. An isolation transformer with a 'boost feature' (such as
the Iso-Boost) will correct that voltage problem."

Boaters seeking friendly and informative advice regarding galvanic
corrosion or other electrically related issues will find helpful
answers available through a variety of experts in the industry. (Our
own "go to" source is Boat Electric, 206-281-7570 or www.boatelectric.com).

We'd like to thank Dick Troberg, a marine surveyor specializing in
issues concerning corrosion for assisting us with the technical review
of this article. He can be reached at
425-483-7076

Thanks Chuck.

Wasn't there someone in one of the rec.boat groups that was
complaining about zincs and other metalic deterioration happening
after a new client at their marina moved in?

Found out he wasn't using shore power as of to say, but rather a
permanently plugged-in , ungrounded battery charger, and several
boats around him were starting to develop an unseemingly fast
corrosion.

it makes sense to me.....