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You
keep your boat in a crowded marina and there’s a lot of talk
about corrosion and bad wiring. It’s all the marina’s
fault, right?
Not necessarily, and the reason why is due to a
general lack of understanding of what happens in a marina full of
boats. After all, this is not an easy thing to understand, and it
took a lot of research to find out what was going on here. Yes,
the marina’s wiring is involved with all such corrosion problems,
but it may or may not be the cause.
The key to understanding corrosion problems in
marinas involves several things. First, all the boats in the marina
are tied into the marina’s grounding system. Second, all the
boats are also grounded at a second source -- the water.
Thirdly -- a major point here -- the AC power system
on the boat (green wire) is also grounded to the boat’s common
ground point, usually established at the engines. This means that
your boat’s common grounding and bonding system is electrically
connected with the dock’s grounding system.
Before we get any further into this, let’s
be sure that we understand our terms here. The 125 VAC shore system
has three wires (except for 125/250 VAC which has four), one of
which (black) is obviously the current carrying conductor. The others
are the ground (white), or neutral, and the grounding wire (green),
which is the safety ground intended to deal with short circuits
and protect people from electrical shock. Also called a "bond,"
it is attached to the frames of all appliances and other electrical
devices. The ground, which is the negative (-) side of the system,
is supposed to be at earth potential, but this is a normally current
carrying conductor, whereas the grounding wire isn’t.
Anodic means that a metal is less highly
charged than others. Stray current will seek a path to an anode,
and from there travel to ground, the water. When an anode is energized
by an outside source, as the electricity leaves the anode, it carries
molecules of metal away with it. This electrical erosion is true
electrolysis. Cathodic means that a metal is more positively
charged; current will flow from it, and on to one that is anodic,
with no resulting metal loss or erosion. Thus, with a zinc attached
to a stainless shaft, the zinc erodes while the stainless is protected.
Adding an outside source of current accelerates the process.
Galvanism, or galvanic current, is caused
by dissimilar metals, metals with a greatly different electrical
potential that are electrically joined. Galvanism plays the primary
role in the "boat battery" problem described below.
The Boat Battery Next, consider that all
the boats in the marina are connected together electrically by both
of these wires, the neutral and the ground or bond, one of which
is also joined to the underwater metals of the boat (the bond).
The boat’s neutral is never grounded to the boat itself, but
always earth potential of the dock ground. The underwater metals
of the boats, of course, can vary between such things as brass,
bronze, stainless steel -- and aluminum, as in outboard motors,
aluminum boats and stern drives. All of these metals can develop
different electrical potentials galvanic ally in the same manner
as a battery does.
We could have, for example, ten boats on a pier
all in a row, and all tied together via these ground systems. Each
boat in this chain creates a galvanic "cell," just as
the plates in a battery does. The average inboard boat develops
about volt or less via its submerged metals, so that ten boats
connected together has the potential to develop 5 volts since cells
in series increase overall voltage in a line by the value of each
cell.
Electricity is obsessed with finding a path to
ground and will always do so. It will follow the path of least resistance.
If the 5th boat in the row were an aluminum stern drive boat, as
shown in the illustration below, what do you think would happen?
Yes, all those boats with bronze and stainless parts on the bottom,
which are far more noble than aluminum, are going to set up a nifty
little battery with the stern drive or aluminum boat as the sacrificial
anode! The inboard boats will do just fine, but the stern drive
or aluminum boat will see its drives or hull damaged or destroyed.
Much the same thing can happen to boats that underwater
metals of inferior alloys such as low grade bronze or stainless
steel such as the kind that often comes from the orient. Boats with
cheap "active" alloyed metals are going to be anodic to
those with high quality passive alloys. Alloys are often termed
as active or passive based on the amount of alloying metal that
raises or reduces its electrical potential. Only passive metals
should be used on boats. Aquamet 22 stainless shafts are passive,
whereas other alloys may not be, or less passive.
What we have then is a veritable "boat battery"
where each boat on the circuit acts like a battery plate. So it
is that this has nothing to do with the dock’s wiring, though
the marina often gets blamed for these problems.
The Long and Short of it
Unfortunately, this is only the beginning of the electrical problems
experienced in marinas. From here, it gets quite a bit more complicated.
Let’s use our aluminum stern drive boat again as an example.
Let’s say this little beauty is really hot. Or maybe it's an
aluminum hulled boat. Again, it's connected to all the other boats
via the grounding wire. So the current that it is generating is
also being fed back into the docks grounding system, and affecting
all the other boats in the chain. Would that be the boats upstream
or down stream on the wire circuit? See what I mean? It gets complicated.
The size of the immersed metal, and the distance
upstream or downstream on the grounding circuit has a lot to do
with it. A larger area of metal will dissipate current to the water
with far less corrosion damage than a smaller area of metal. This
is because of resistance in the metal that disperses the energy
over a wider area. Yet the very same thing happens over a distance
of wire. The longer the wire, the more resistance in it. The boat
out on the end of the pier is at the end of a very long wire, so
it has a less effective the ground because there is more resistance
in it.
This can translate to good news or bad news, depending
on the electrical potential of your underwater metals. Keep in mind
that sea water is also a grounding source. It's bad news if the
resistance on the dock is high enough to cause the sea water to
become the ground path. This will happen if the resistance in the
dock ground is greater than that of the water, and your metals are
incompatible. The increased electrical potential in the circuit
has now caused them to become highly anodic. Oddly enough, this
may actually help our stern drive boat as the increased resistance
in the line makes it less anodic, but probably not that much. Because
the amount of surface area of metals plays a role, it will likely
be small boats mixed with large boats that will experience the greater
degree of corrosion.
The grounding wire is supposed to be at earth potential.
If it's being charged, then it's not at ground potential. It has
become a current-carrying conductor, meaning that all the boats
on the dock are experiencing more impedance in the ground circuit.
This is going to hurt the boats upstream, but help the boats downstream.
Here’s why: the resistance in the line reduces the current
flow to earth, thus causing underwater metals to become anodic.
This is because our dock wire is not the only ground source; a boat
in sea water is also grounded via the water. When the resistance
in the wire exceeds that of the water, the water then becomes the
ground path. The longer the wire, the greater the resistance, so
the end boats are most affected. When the water becomes
the ground, the underwater metals become anodic.
This is when the general corrosion problems start.
It will first attack the weaker or less noble metals such as zinc
and aluminum until they disappear. If you have a sea cock, perhaps
a gate valve that came from a plumbing supply store that is a poor
alloy, or the wrong alloy fasteners holding things on the bottom,
then these items will go next. By this time, your problem will have
announced itself loud and clear as serious leaks develop.
Sea Water Versus Fresh Sea water always
presents a more serious problem, right? Wrong. In most cases it
does, but there is one important exception. Fresh water is more
resistive by a factor of around 70:1 With the second ground potential
(fresh water) substantially lessened, the problem comes about when
the dock wiring is defective, when the ground and bonds have high
resistance due to corroded connections, etc.
The problem with electrical shocking is heightened
when things like metal framed floating docks are involved, which
provide yet a third grounding source. It is the difference in potential
between these two or three grounding sources that causes our problems.
If the potential within the boat’s metallic systems is greater
than the grounded neutral, then sea water will become the ground.
If in fresh water, you may end up getting shocked when touching
metal parts on both boat and dock. The current flow is going to
be between your body and the dock. Therefore, stray current can
be more serious in fresh water than salt because of poor grounds
on the dock.
Solutions As you can see, the issue of
corrosion problems in marinas can be complex, and even more difficult
to solve from the marina end. You probably find all this very confusing.
Well, you're not along. So do I. In fact, probably no one understands
it fully, but fortunately there are simple solutions.
Faults in the dock wiring should be corrected,
of course. Yet as we have seen, this may be only part of the problem.
The only reasonable solution for the marina "boat battery"
problem (if you have one) is to use galvanic isolators on the shore
power system of the vessel. Transformers eliminate direct electrical
contact by transferring electricity magnetically. Some of the more
high end marinas these days that cater to large aluminum yachts
have such transformers on the dock. This is very expensive since
there’s also a great deal of power loss with transformers,
so you won’t find many -- if any -- marinas catering to smaller
boats like this. It's up to you, the boat owner, to protect yourself.
For small boats with aluminum drives, galvanic
isolators are available that work only with the grounding circuits,
and not the main power feeds. These are okay for boats that are
only running something like a battery charger on AC current. For
larger boats with higher power demands, it is necessary to have
a full sized isolation transformer.
In case you’ve heard of that crazy solution
of disconnecting your green, grounding wire, consider that quite
a few people have been electrocuted as a result of this half-baked
idea. The green is there to protect people against electric shock
and electrocution, so don’t defeat its purpose.
Your Equipment If the dock system takes
a beating, how about your own equipment, like the connectors on
your shore cord and boat? When was the last time you opened them
up and inspected the condition of these things. It is very common
for shore connections to get wet and corroded, both on the male
and female connectors, and at the wire connections to the prongs
within the connector head. Corrosion and damage at these points
not only interferes with grounding potential, but also is one of
the largest causes of fires on boats. High resistance connections
in the power feed causes overheating that can start fires. This
equipment should be serviced at least annually, depending on the
amount of use it gets.
Corrosion on the grounding connectors cause faulty
grounds on your boat which is very damaging to electrical equipment.
Do you know why people so often replace refrigerators and air conditioning
compressors on boats? Yep. Faulty grounds (the neutral) wreck compressors.
Refrigerators at home last forever, but not on boats for this reason.
Reverse Polarity This common
problem is one that should be thoroughly understood by all boaters.
Reversed polarity can exist on the dock or within your own boat,
which should be equipped with a reverse polarity indicator light
on you main electric panel. Regardless of whether the reversing
point is on the dock or in you own boat, this is going to energize
the neutral ground and create an electrical shock hazard. It will
not find its way into your bonding system because these circuits
should never be joined. Most experienced boaters keep a small polarity
tester handy and use it occasionally just to be sure. It's a wise
thing to do, especially if you ever run power tools off a dock outlet.
Battery Chargers Automotive
battery chargers are a common cause of corrosion in boats, particularly
small boats without shore systems. Auto chargers often provide no
isolation between the ac and dc windings and can energize the negative
terminal, which also energizes the boats grounding system. Portable
auto chargers should not be used on boats, and are a frequent cause
of stern drive damage.
Extension Cords Extension
cords should not be used on board boats. Period. Unless it is equipped
with a ground fault current interrupter, or plugged into an outlet
so equipped, and then only for temporary use of power tools. Why
not? Because of the acute electrical shock hazard that accompanies
the basically unprotected connector. Get some water in the connection
and these things short across the terminals to energize all circuits,
including the protective (green) bond.
When Traveling Fortunately, more
and more marinas have become aware of the dangers of faulty dock
wiring, and there are far fewer with bad systems these days. As
you may have noticed, marina electrical facilities take a beating
and quickly degrade. If the power outlets just look bad, they probably
are. If outlets are damaged and exposed to weather, if you find
circuit breakers broken or missing, damaged or wasted conduits (Look
on the underside of the dock). Are there wires and conduits hanging
down, maybe in the water?) If so, then take a pass and don’t
connect up there.
If you’re skilled in the use of a voltmeter,
one thing you can do is to measure the current flow between your
boats grounding system and the dock grounding system. Which way
is the current flowing, and how much is it? Your boat is safe if
the reading is on the positive side with red lead to the boat, black
to dock. If it's reversed, and you’re getting more that volt,
you shouldn’t stay connected to if for very long. volt is
common, but if it reaches up towards 1 volt, there will be an overnight
affect on your zincs. In a week, they can disappear.
If you have an aluminum boat, or a boat with stern
drives or outboards that is connected to shore power in a marina,
consider it a must to have an isolation transformer installed.
Small Boat Marinas These
tend to be the worst offenders, where inexperienced owners will
do some of the craziest things. It's not uncommon to see boats connected
with extension cords, or unprotected three-prong male connectors
and adapters which, when they get wet short across the terminals.
Lacking installed battery chargers, they use portable automotive
chargers, connected to extension cords. I've even seen people take
extension cords and splice a male connector on the female end and
plug it into a receptacle on the boat! Be extra wary of electrical
faults at these marinas.
Answers to Common Questions
How about a stern drive boat with a shore power
system at a private dock? If the wiring system is good, then you
shouldn’t have a problem with it. When in doubt, have the impedance
of the ground checked to be sure that the boat isn’t the ground
source rather than the dock. This is simple and inexpensive to do.
If the zincs on your boat are disappearing very
rapidly, at a rate which leaves shiny, instead of oxidized metal,
you should suspect both the dock wiring and other boats on your
dock. The question becomes who is the culprit and what can be done
about it. Rates of zinc loss are normal when a layer of oxide develops
on the zinc, not when pits appear that leave bright or hard metal
with no oxide. Rates of zinc loss are not predictable in terms of
time as there is a wide range of factors involved.
Does current travel through the water from boat
to boat? No, it doesn’t. Only via the grounding system.
Despite the complexity of the issue, boats with
galvanically compatible underwater metals usually will not be affected
(unless the problem is extreme), while the ones with lesser compatible
metals, the ones that become anodic, suffer the consequences. Oriental
boats with a lot of submerged and questionable stainless steel and
poor quality bronze propellers are often victims as these metals
are active rather than passive, and often contain impurities in
the metal that further adds to the corrosion problem.
Can a boat with a stray current problem affect
those nearby? Yes, but. Remember that all our boats are wired together.
Assuming that the ground connections to each boat are solid, but
the dock ground isn’t, then the grounding wire is going to
be energized. High resistance in the bond will feed current back
into all boats with good connections, those without isolation
transformers. Again, because distance and surface area of metals
dissipates current, those closest are most likely to be affected.
What about marinas with shorts in the system? Won’t
they cause corrosion on my boat? In the case of marinas with short
circuits in the wiring, usually someone is going to get badly shocked,
in addition to which there will be a power drop that will affect
everyone. This sort of thing usually gets discovered rather quickly
as electric equipment doesn’t function properly and the circuit
breakers start popping.
What happens when there is a fault in the neutral
ground? Your lights are dim and electrical appliances run slower,
and eventually burn up. You will get lower readings when you measure
the voltage.
Will my panel meters reflect the problem in any
way? Usually not, although if there is a ground fault you may get
lower voltage readings.
What is a cathodic protection system, and
what about them? These systems protect a boat’s metal hull
by using an electrified anode. The problem with these things is
that when something goes wrong with the system, they can end up
destroying the hull rather than protecting it. This can also affect
other boats nearby. It's not a reasonable solution and is not recommended.
In summary, the "hot marina" is really
either one that has faults in the grounding system, or one or two
boats in an electrical series that are galvanically deficient. If
it's your boat that is suffering the damage, most likely it's your
boat that is the offender. This is why usually only a few boats
on the pier are affected, and not all equally.
This describes but a few, but most significant
factors involved. There can be all sorts of unusual conditions that
can make discovery of the problem very difficult. On the other hand,
if you install a galvanic isolator, at least the corrosion problems
won’t be yours. Whether you need isolators depends on the conditions
where you keep your boat.
* * * * *
* *
References: Your Boats Electrical System,
by Conrad Miller is probably the best and most comprehensive text
on the subject and is usually found at all marine bookstores and
outlets. It covers virtually all subjects and no boat owner should
be without it. Even if you don't read it cover to cover, you'll
find it a very useful reference.
American Boat and Yacht Council Safety Standards
for Small Craft provides extensive specifications for all types
of boat electrical systems. Contact ABYC at 410-956-1050.
See also our article on Corrosion
(with 9 photos).
Posted January 7, 1999
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