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Old 23-05-2007, 11:09   #1
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Zincs and the 'Hot' Marina

After reading the Only Solar thread I have a question abot plugging in. I have had my boat for several years now and generally don't plug it into shore power unless I need to run a power tool. Normally the batteries are kept in good shape by a 6 watt Siemans solar panal and charge controller or the rare engine run. The marina I am in is considered "hot" by all of the locals, whatever that means, but I'll assume that there is enough of a difference between the electrical potential of the water and the ground on the shore power to cause a current.

Anyway I would like to hear the opinions of the group concerning the life of a zinc and your experiences with marinas hot or otherwise.
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Old 23-05-2007, 11:19   #2
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A "hot" marina can burn your zink in a few weeks.

I never leave shorepower plugged except for short periods.

Have heard horror stories on bad AC systems and stray currents and such...

Presently my prop shaft zink last about 6 months and the rudder bearing zinks about 12 months.

Not in a marina, but rather in a canal with boats docked every 70 feet or so.
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Old 23-05-2007, 12:41   #3
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In a municipal marina here in West Palm Beach, I had to replace my shaft zincs monthly. Cruising, they were in good condition after 5-6 months. I had solar panels, but a lot of shore power on neighboring boats were left on at all times, many times with the power cords dangling in the water. The locals called this place a "hot" marina, too. And it was
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Old 23-05-2007, 15:18   #4
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Well, If you don't connect to shore power, I wouldn't worry about it too much.

As you've pointed out, the current is produced by the difference between the electrical potential of the water and the ground on the shore power. If your boat has no electrical connection other than to water, there can't be a completed circuit and no current flows..,.ie... your zincs last a very long time.

I depend on my solar panels as you do, and do not connect to shore power. I'm going on 3 years in a very active livaboard marina and my zincs are fine.

rick in Florida
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Old 23-05-2007, 17:52   #5
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You can add a galvanic isolator to the green wire coming in to your shore power line in the boat to eliminate the hot factor on your boat if you want to connect to shore power.
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Old 03-07-2007, 15:06   #6
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Galvanic Isolator, zinc, and stray current

A galvanic isolator is a sound investment, and should be installed on every boat that is sharing a shore power circuit with other boats. It does, however, accomplish only one thing. It prevents sharing of your zinc with your neighbors. It is ineffective where stray current (electrical current in the water originating from power sources) is involved. A properly wired isolation transformer does accomplish this end, and in this case a galvanic isolator is not needed or useful.

"Hot Dock" is a pretty broadly applied term. A truly 'hot' dock has contact between its current-carrying electrical circuits and the water. When the same occurs on a boat (without an isolation transformer) and that boat is plugged into the dock's electrical system, the problem is passed to the dock through the safety ground wire. Another condition that causes a hot dock is high resistance in the AC neutral circuit from poor connections as the circuit runs from outlet to outlet. When this occurs, there is voltage on the neutral wire (voltage drop or loss), and less voltage available to the connected devices. It then requires just a single boat with a common fault - a connection between AC ground and Neutral - to be plugged in, and that passes this voltage to the whole dock and the bonding systems of all boats connected.

But, a dock doesn't have to actually be hot to eat zincs. Boats with insufficient zinc will draw protection from their neighbors through the safety ground wire which, in effect, connects everybody together in a common bonding system. Docks with a high proportion of neglected boats will significantly shorten zinc life. A galvanic isolator breaks this connection by blocking current flow at the very low voltage levels involved, but passes current at higher voltage levels to maintain the integrity of the safety ground connection. An isolation transformer creates a new, isolated ground connection that is not shared, thus eliminating the effects of all but the most severe faults. The isolation transformer is of no effect, however, against faults on the boat on which it is installed.

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Old 03-07-2007, 15:40   #7
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Any point in hanging a grounded zinc over the side?
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Old 03-07-2007, 16:33   #8
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I have not connected to the shore power for 6 months and the zincs were very noticibly less used up when we pulled the boat out the other day.
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Old 03-07-2007, 17:03   #9
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Any zinc in the water near your boat, and electrically connected to your underwater metals, is doing the same as the zinc plate on the boat. More zinc does mean more protective current, but not more protection. Overprotection is a big issue on wooden boats, as it causes a buildup of sodium hydroxide (lye) around protected parts, which is very destructive to wood. It's not an issue with glass boats, but overprotection is still pointless, as it is galvanic and not electrolytic protection. Galvanic corrosion is caused by electrical currents that are generated by the interacting metals, and your zinc is part of that interaction. Electrolytic corrosion is caused by current from an external source, and the zinc's part in that is on the wrong end of the circuit along with your other underwater metals.

Now, that said, in the case that you have pulled into a marina on your way and there are a lot of poor looking boats there and you are plugging in and don't have a galvanic isolator installed, adding zinc to your system will definitely compensate for the extra load of the boats drawing galvanic protection through the shore power ground. And, you don't need to have it hanging over your rail, It will do the same hanging off the dock if it is connected to the grounded outlet box, where it only needs 3 feet of wire.

If you have a multimeter on board, you can test your boat and your dock for about 7 more bucks. Get a pencil zinc, drill a hole in it for a wire and solder a length of wire to it. Connect that to the black lead of your meter, and suspend the zinc in the water, after cleaning it with a scotch-brite pad. Put the red lead to your bonding system. The meter readings indicate thus with bronze and stainless protected metals:

Reading DC volts on 2 volt scale:
Minimum protection: 500-550 millivolts (0.5V)
Full protection: 430-480 mv
Overprotected: less than 380 mv

An unprotected bronze fitting of reasonable quality by itself will read about 700 mv; Poorer grade materials have lower readings, with protection levels adjusted accordingly. A metal will be fully protected when its bonded voltage reading is 200 mv less than its unbonded reading. Additional zinc pulls the voltage down, additional protected metals loads the system by pulling the voltage up. Stray current that causes electrolytic corrosion pulls the voltage up dramatically. A voltage reading over 750 mv DC indicates stray current, and there should be no AC voltage at all. ( A small AC reading may be picked up by the meter leads inside the boat if the shore power is on, especially if there are any flourescent lights, motors, or transformers on.)

Test the dock by connecting the red lead to the grounded metal outlet box before you plug in. If it reads higher than your boat, it will draw protection, and if it reads lower it will actually provide protection. If it reads over 3/4 volt DC, or any AC, it is indeed 'hot', though the dock itself may not be the root cause.

A direct-bonded boat (no controller) with a new zinc should have a reading of 80-100 mv, and will be overprotected. As the zinc corrodes, providing current to protect the other metals, the voltage rises, frequently past the minumum protection level by the time the zinc is replaced. Checking your system this way eliminates the mystery, at least as far as knowing what's happening.

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Old 04-07-2007, 00:38   #10
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Any zinc in the water near your boat, and electrically connected to your underwater metals, is doing the same as the zinc plate on the boat.
Hi EngNat, errr that's not entirely true. The ground wire is of too low resistance due to small conductive size and length, to effect a continuation of the bonding system. In fact what actually happens is a voltage is created across that conductor. The Zinc on one boat reacts with the Zinc on the other and the two zincs become the positive and negative of a battery. Even though the two poles are both Zinc.

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Any point in hanging a grounded zinc over the side?
Yes it is a very effective way of monitoring your zincs or even being the total protection. I have used this method for some years now. I simply have a light battery cable bonded to my stearing post and the other end bonded to a Zinc. I dangle the Zinc over the stern. I lift it whenever I care to take a look at what level of protection I have left. Once the Zinc is getting low, I simply bolt on a new one.
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Old 04-07-2007, 03:29   #11
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A couple of points that deserve repetition:

Thanks EngNate:
- Galvanic corrosion is caused by electrical currents that are generated by the interacting metals, and your zinc is part of that interaction. Electrolytic corrosion is caused by current from an external source, and the zinc's part in that is on the wrong end of the circuit along with your other underwater metals.- A metal will be fully protected when its bonded voltage reading is 200 mv less than its unbonded reading.
- A galvanic isolator is a sound investment, and should be installed on every boat that is sharing a shore power circuit with other boats. It does, however, accomplish only one thing. It prevents sharing of your zinc with your neighbors. It is ineffective where stray current (electrical current in the water originating from power sources) is involved. A properly wired isolation transformer does accomplish this end, and in this case a galvanic isolator is not needed or useful.

Thanks Alan Wheeler:
- Wheels’ steering pedestal is bonded, allowing the clipped-on suspended zinc to be electrically connected to underwater metals, which are also bonded.
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Old 04-07-2007, 03:36   #12
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Originally Posted by Alan Wheeler
Hi EngNat, errr that's not entirely true. The ground wire is of too low resistance due to small conductive size and length, to effect a continuation of the bonding system. In fact what actually happens is a voltage is created across that conductor. The Zinc on one boat reacts with the Zinc on the other and the two zincs become the positive and negative of a battery. Even though the two poles are both Zinc...
I believe Alan means to say too high a resistance. A high resistance path* can develop a voltage drop across it’s length (conductor & joints), which should not be a significant factor in any properly designed and installed (low-resistance) wiring.

* Smaller gauge & longer wires, and corroded connections present high resistances
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Old 04-07-2007, 10:05   #13
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Happy 4th everybody - (I'm working in the office). I have to counter Alan. I have been fixing these problems for over 12 years, and that's all I did up to starting my new business in distribution and support, and I fixed everything in my field that came to me. Lots of this stuff followed me to sea, or I followed it, I don't know which... For 10 years, I saw the same boats & docks again and again and again, year after year at haulout, I saw and talked to the divers that cleaned them each month (in So Cal summer). An observant diver can see electrolytic corrosion happening (tiny gas bubbles form on the surface of the metal), and can see the difference between metals that are protected and those that are suffering severe galvanic corrosion. They can detect AC stray current because their eyelids twitch rapidly as they approach an affected part.

Two zincs in the water do not make a battery. (period, ever.) The resistance of wire between them is irrelevant in this point. It's true that even the same brand of zinc will have dissimilaraties between casting runs, but we're talking about minute values of no regard. It doesn't matter what you do with it (except change the composition of one), one zinc does not react with another zinc, it just doesn't happen. 8awg copper wire, common for wiring docks, has 0.63 ohms resistance per 1000 feet. A large sportfisher full of gadgets and all kinds of underwater fittings may draw as much as 100 ma (0.1 amp) of current from its zinc plate (this value is pretty extreme). Volatge loss: E=I*R, if the plate was located 1000 feet away with #8 wire, the voltage loss is 63 mv. If the zinc was "pulling down" 200mv at the boat, it would lose a little less than 1/3 of that, still enough to be barely in the 'protected' range. The ground conductor of a 50 ft 30A shore cord adds another 0.05 ohms, for an additional 5 mv loss. The limiting factor here is not the conductor side, but the ionic path through the water. In both the corrosion and protection processes, every electron that flows through the conductor must be returned via an ion in the water. This path loses effectivenss at distances over 100 feet. In an apparent contradiction, I make bonding systems with a full loop of #6 solid wire, with cross ties and silver-soldered connections, but that's another topic...

Sidenote: Because of the dependency on the ionic path, bonding protection through hoses and pipes is very limited. Inside a restricted passage, full protection will only reach a distance approximately 3 times the diameter of the passage. Owners of Broward (aluminium) yachts have found this out the hard way, with sea water intake standpipes breaking off.

Indeed, if the ground wire had such resistance as to make it ineffective as as bonding, the whole issue of hot docks would rarely be observed. Voltages are not 'created' in wires (unless they are windings in a transformer or generator). Voltage is impressed across the distance of a conductor by the current flowing through it, E=I*R. Georg Ohm died in 1854, and no one has ever broken his law yet. (He was mightier than Moses.)

A zinc simply hanging over the side and connected to the boat's bonding system is (please believe me) of no use whatsoever in determining the protection level of your boat. NO, NO, NO, NO, NO, and I'm not answering to this part of the debate again. Fer cryin out loud, if you've already got this zinc over the side and a wire on it, cut that wire and connect the ends to a multimeter, which you probably also already have, and go by the numbers. Not doing it is like putting duct tape over your engine temp gauge and feeling it with your hand instead.

Non-believers, just humor me, we're talking about $20 - that's if you don't already have any of the stuff. I don't sell pencil zincs or multimeters - but I don't sell thru-hulls either... Make the test setup and use it. Test all your buddies' boats, test yours regularly and keep notes. It's about as close to actually performing magic as you'll ever get.

Too much excess zinc will reduce your level of protection, possibly to nothing. Zincs must not have anything coating them to interfere with contact and reaction with the water, and this includes any sea growth. The growth is kept off because the by-product of the zinc corroding is zinc oxide, an old time antibiotic. If you see moss, barnacles, or slime on your zinc, it's not working, and the growth is a symptom, not the cause. It means that there is not enough protective current being drawn for the surface area of the zinc, thus it does not produce enough zinc oxide to interfere with marine growth. Once the growth takes hold, that zinc might as well be in a bucket of water on the deck. Just as in other electrical systems, current is a matter of demand, it's drawn by the load and limited by the capacity of the source. Increasing capacity far beyond demand is of no beneficial effect. The lack of sufficient demand for protective current means one of two things: The ratio of zinc to protected metal is too high (this goes by wetted surface area), or there is poor integrity in the system distributing that current to the protected parts.

Sailors, pardon references to power boats, electrons have no regard for one over the other.
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Old 04-07-2007, 10:32   #14
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Oughta be a way to suck the charge right out of the water at a "hot" marina, and recharge the ships batteries without having to pay for solar or dockside power.

Maybe feed an MPPT power converter from the water voltage?<G> Step it up and charge away!
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Old 04-07-2007, 14:01   #15
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Two zincs in the water do not make a battery. (period, ever.) The resistance of wire between them is irrelevant in this point.
I am sorry to disagree, but it is a well documented fact. Even the Nigel Calder dude has suggested the fact and shows shuch in one of his diagrams.
The fact of that both hulls have a Zinc each, does not create the battery. The fact that one Hull for whate3ver means, will have a slightly different potential difference in the water in relation to the other hull. This can be due to Ziincs at different corrosion points, the No. of Zincs, the materials the boats are made from and so on. A Battery is indeed created.
Even for protection alone, a good size cable is required to ensure none of the very minute current created by the Zinc, is lost in the cable resistance. Saying that a thin wire connecting across a Marina Circuit to another boat can offer your boat protection from the other boats Zincs is very wrong, even though I did understand you were saying it in jest. Someone that doesn't understand may not understand you had tongue in cheek.

Thanks for the correction Gord. Your corection is exactly what I meant.
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