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Old 25-08-2015, 11:30   #1
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Zinc murderer.

My zincs at dock are disappearing at an alarming rate. About one every two months. Is this due to being plugged in for the battery charger? Bad ground? What's going on here?
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Old 25-08-2015, 11:37   #2
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Re: Zinc murderer.

Do you have a galvanic isolator? If not, you probably should.

Your problem is most likely the opposite of a bad ground on your boat. Yours is too good. A galvanic isolator pretty much deliberately disconnects your ground from the marina's, until you put serious current into it. Maybe a bad one on someone else's boat?
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Old 25-08-2015, 13:38   #3
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Re: Zinc murderer.

Quote:
Originally Posted by MarkSF View Post
Do you have a galvanic isolator? If not, you probably should.

Your problem is most likely the opposite of a bad ground on your boat. Yours is too good. A galvanic isolator pretty much deliberately disconnects your ground from the marina's, until you put serious current into it. Maybe a bad one on someone else's boat?
I used to think this, too, until I rewired my boat. The zincs that used to last a mere 6 weeks, are now lasting months, even though I had had an electrical guy drop a meter over the side of my boat once and say there wasn't any stray current so it had to be "someone else's boat." Well, Else's Boat kept following me from marina to marina....until the wiring got upgraded, then it disappeared for good. I still think a galvanic isolator is a good idea and plan to get one eventually, but I wonder if it will actually fix the problem if something from inside is giving off the charge? Anyone know?
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Old 25-08-2015, 21:23   #4
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Re: Zinc murderer.

corrosion is voo doo science. even with proper tools (silver cell etc) and knowledge you may never find the issue.. or sometimes it's simple. but it can be a lot of things on your boat. or it could be any boat in your marina causing it.
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Old 25-08-2015, 21:46   #5
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Re: Zinc murderer.

gamayun is right. It's usually your boat. Things to check, 12 volt items properly wired - all 12v positives to + side, etc. Especially battery chargers. Also, make sure your 120v doesn't swap Hot and Neutral or Neutral and Ground between appliances or sockets.
Sometimes a plug in appliance with a 2 prong plug will be wired wrong and reversing the plug causes a feed back.
When a commercial fisherman, I had a friend eat the blades off his 24" propeller in three week at the dock after he wired in a new radio and reversed the + and -.
My zincs are 4 years old.
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Old 25-08-2015, 21:48   #6
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Re: Zinc murderer.

Quote:
Originally Posted by MarkSF View Post
Do you have a galvanic isolator? If not, you probably should.

Your problem is most likely the opposite of a bad ground on your boat. Yours is too good. A galvanic isolator pretty much deliberately disconnects your ground from the marina's, until you put serious current into it. Maybe a bad one on someone else's boat?
The galvanic isolator is a capacitive coupling designed to allow AC currents of about 60Hz to pass while blocking DC currents. The reason for this is that the water is still your ground for fault currents.

120V AC can kill you and that's why there's a third wire. The third wire is supposed to carry any current that might kill you down the wire to ground. Presumably the ground path through this wire is going have a lower impedance than you and most of the current will flow that way.

But remember this is AC current not DC current. It's an error to refer to 'serious current'. AC current of any magnitude should flow through the isolator but it's not supposed to be doing that. If it is there's a fault in your AC equipment.

Electrolysis is a DC phenomena and that's why there's an isolator. You are trying to break any DC link between the AC system ground and the water/boat ground. It's possible that for various reasons there could be a potential difference between your AC ground and the water/boat. Even a small difference will result in current flowing which will plate or deplete below the water line metals depending on which way the current is flowing.

This isn't my area of expertise but I have put some thought and reading into it and it's my opinion that it's not a simple phenomena and that what most people have to say about the subject is anecdotal. If I had to guess I would guess that less actual science has been done on this subject than people would expect.

Presumably if you have the isolator you should be Ok to remain plugged into shore power. It should be difficult to mess up your wiring. You would have to have below the waterline metal at some potential other than that of the surround water.

In the case I experienced last year I believe my issue was caused by a nearby boat who had some 'professional' wire up their bowthruster backwards (couldn't possibly do that on a metal boat!). I believe that it was proximity through the water that ruined my propeller last year not anything to do with the shore power. But I still wonder and it's all too complicated to be able to go to that guy and force him to cover my propeller even though I never had an electrolysis problem before. How do you prove it? Hire another 'professional'?
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Old 25-08-2015, 21:56   #7
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Re: Zinc murderer.

Quote:
Originally Posted by gamayun View Post
I used to think this, too, until I rewired my boat. The zincs that used to last a mere 6 weeks, are now lasting months, even though I had had an electrical guy drop a meter over the side of my boat once and say there wasn't any stray current so it had to be "someone else's boat." Well, Else's Boat kept following me from marina to marina....until the wiring got upgraded, then it disappeared for good. I still think a galvanic isolator is a good idea and plan to get one eventually, but I wonder if it will actually fix the problem if something from inside is giving off the charge? Anyone know?
I'm not an expert, but my experience bears this out. I installed an isolator in the hopes of fixing corrosion issues but the problem didn't go away until I cleaned the main buss bars months later. There were also SS nuts in between the bars and the connections, which I removed. I think the current was not being given a clean enough path back to ground, thus some current was following a path down my engine block and out the prop shaft.
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Old 25-08-2015, 22:21   #8
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Re: Zinc murderer.

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I'm not an expert, but my experience bears this out. I installed an isolator in the hopes of fixing corrosion issues but the problem didn't go away until I cleaned the main buss bars months later. There were also SS nuts in between the bars and the connections, which I removed. I think the current was not being given a clean enough path back to ground, thus some current was following a path down my engine block and out the prop shaft.
I may have mispoke about some of the details of the galvanic isolator. I see an article on using diodes and having only 0.6V isolation so perhaps they aren't capacitive. But that doesn't change the basic issue of it being the ground fault path.

I believe what you are saying you experienced but I'm having trouble wrapping my mind around your hypothesis. If we imagine your bus bars as resistors it makes sense that current would look for an easier way. What doesn't make sense to me though is that the potential difference to drive the currents in question is between the plates of the battery not between the positive plate and the water. My sense of it is that the current must be trying to get back to the battery and I don't see how it does that by going through the propeller?

I wonder if it's because the isolator isn't a capacitor and perhaps you had a potential difference larger than the diode drop of the isolator? If the drop across your bus bars was greater than the drop through the propeller, water, and then back through the isolator to ground perhaps that would explain it?

Interesting subject, kind of like lightening, lots of experiences but not a lot of hard science. It's funny how many years you can work with V=IR and still get confused. It seems like the current has to go out the propeller through the water find it's way onto the AC ground wire back through isolator and then to the battery. But doesn't it have to go back through your bus bars?

If your charging then the current flows the other way but it seems like the issues are the same.

Now I'm wondering about my isolator and if it's really enough to protect you from some guy who's bowthruster is wired backwards? Well anyway, I've been unplugging my shore power most of the time since then.
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Old 25-08-2015, 23:17   #9
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Re: Zinc murderer.

https://en.m.wikipedia.org/wiki/Electrolysis

Not magic. The worst contributor to electrolysis in a marine application is improperly connected grounds between AC and DC systems.



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Old 25-08-2015, 23:59   #10
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Re: Zinc murderer.

What am I thinking? It could be a transformer.

That would be a good way to do it.

I don't actually know if it's caps, diodes, a transformer or if there's all three types.

But I've been trying to understand what happened to my propeller and the diode hypothesis I put forward above sounds plausible. I like that better than some kind of mystical diffusion of charged particles through the water.

But I believe in mystical diffusion don't get me wrong. What is life if not that. Organic molecules can't do much by themselves they gotta have the mystical diffusion.
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Old 26-08-2015, 00:14   #11
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Re: Zinc murderer.

It is all about current flow. No circuit , no flow.


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Old 26-08-2015, 00:30   #12
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Re: Zinc murderer.

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It is all about current flow. No circuit , no flow.


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Indeed!

For purposes of understanding I've been trying to picture the circuit in question.

If a guy a few boats over tells you he had a bow thruster wired backward and over the course of a few days the entire unit dissolved you have to wonder what that means to your boat.

Basically they put 12V in the water and turned on the battery charger.

It seems reasonable to assume that it's possible that in my location there was greater than 0.6 or 1.2V difference between the water and the AC ground.

If my isolator isn't a capacitor or a transformer but is rather a diode then when the voltage exceeds the diode drop current flows.

That would explain in circuit theory what happened to my propeller if the two metals are such that the cathode (bowthruster) will lose material while the anode (zinc in bronze propeller) is such that is also loses material.

That part is beyond me at the moment without actually looking it up.
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Old 26-08-2015, 01:33   #13
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Re: Zinc murderer.

It is the AC current flowing through the ground that causes the issue. If you bond the neutral on ur boat to ground then it will conduct a portion on the current through the salt water. Boats in fresh water have fewer issues because fresh water is a poor conductor.

A simple test is to put a digital clamp on AC ammeter around ur shore power cable. Because all of the conducting cables are inside the clamp they should negate each other and the meter read zero. If any current is shown, that will reflect the amount of current that has another path to the source. ie: the water is the alternate path.






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Old 26-08-2015, 04:15   #14
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Re: Zinc murderer.

Also take a voltmeter on Dc voltage and attach a neg lead to the negative side on the 12v electrical system and the positive to something in the water (propshaft saildrive etc) you should see .8 to .9 volts

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Old 26-08-2015, 09:05   #15
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Re: Zinc murderer.

To clarify: my boat is at a private dock with no other boats in the vicinity. The power source is a three prong receptical (not a marine type) with ground fault protection. It's not my dock, so I can't change the receptical. I recently had a new Blue Seas panel and all new LED lighting installed. At dock I keep the battery switch on "all" to make sure the Mastervolt 40/3 charges both the starter and house batteries, but all the DC breaker switches off. The AC panel is left on so a fan can move air around to fight mildew. I thinks that's all the particulars . Thanks everyone for the help.
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