Corrosion in Marinas - The Hot Dock
By David Pascoe
From Yacht Survey Online
Corrosion in Marinas
The Hot Dock
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
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 ½ 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
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
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 ½ volt, you shouldn't stay connected to if for very long. ½ 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
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
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
American Boat and Yacht Council Safety Standards for Small Craft provides
extensive specifications for all types of boat electrical systems. Contact ABYC at