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Old 11-13-2012, 11:30 AM   #1
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Bonding system / electrical current

The last time I had my shaft zincs changed I noticed that one of them was not degraded at all. I believe the problem is that the shaft isn't bonded to the rest of the system. On one side I have a wire that connects the shaft to the transmission which jumpers the two together where a plastic shaft saver is separating the two. This wire broke on the other side. Until I can get this fixed I just used a wire with battery clips on both ends to temporarily connect the shaft to the bonding system when not under way.

I was curious about what kind of current would flow between the shaft and the bonding system, so I put a multi tester on it and I'm reading about .25 volts flowing between the transmission and the shaft. That means the red lead on the transmission and black lead on the shaft. I get the same if I place the red lead on a nearby thru hull fitting which is bonded to the rest of the system. Turning off all electrical systems, battery switches, breakers, and main 120V switch has no effect, but physically unplugging the shore power cord reduced or eliminated the voltage flow.

I do not have a galvanic isolator.

In addition, my boat has three steel (I assume) pilings in the general vicinity, and I understand that the galvanic incompatibility leads to an electric current.

Do I have stray power entering the boat via the ground of my shore power or is this normal galvanic corrosion and the zincs are doing their job by degrading?
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Old 11-13-2012, 01:33 PM   #2
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Well you definitely want to jumper your plastic Drive Savor so that the prop shaft zinc will be connected to the other underwater parts, like thruhulls.

The adjacent steel piles are grounded to earth, so I don't think you are going to suffer any harm to your boat's underwater metal as a result.

Your test of measuring voltage with or without the shore power connected tells you something is passing through the ground wire. Install a galvanic isolator to block that voltage whether it is coming from the steel pilings or an adjacent boat.

But typical zinc life is 3 to 12 months. Any shorter than that and I would be concerned about an internal DC fault or DC leakage to your shore power ground from another boat. Chase down any DC wires that drop into the bilge water for the former and install a galvanic isolator, or isolation transformer (better) for the latter problem.

David
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Old 11-13-2012, 02:00 PM   #3
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If one zinc is degrading and the other is not, if sounds like connection/contact of the zinc to the shaft. Did you clean the zinc white coating off. Zincs will produce a white coating to the point it does not protect very well as current will flow to the least resistance.

As to the 0.25 volts, check the voltage at the shore power cord, using the AC green ground wire/round plug hole and the neutral. By memory an isolator is good for 0.125 volts, a very small voltage, so voltage over will pass through and/or blow the diodes. You might want to talk to the marina to checked, but many large marinas are hot, so that might be the way it is.

As mentioned before, are you zincs going fast then the norm for the marina and or boats around you?
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Old 11-13-2012, 06:04 PM   #4
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My zincs degrade at a normal level. I'm assuming now that I've jumpered the drive saver the shaft zinc on that side will degrade normally. The zincs were installed correctly so that's not the problem. I'm really wondering about the .25 volt reading and whether or not that is normal. I guess an isolator can't hurt at this point and can only help.
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Old 11-14-2012, 06:26 AM   #5
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A galvanic isolator wouldn't hurt. It blocks current in both directions up to 1.2 volts. The following is a good read on the topic: Galvanic Isolator Explained

Without the isolator you become part of the pool and your zincs may be protecting other submerged grounded metal objects in the vicinity.
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Old 11-14-2012, 07:24 AM   #6
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Quote:
Originally Posted by Egregious View Post
............. I do not have a galvanic isolator.
.............
I would suggest installing one, regardless of any other issues.
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Old 11-14-2012, 07:38 PM   #7
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It should be noted that most high amp items, charger, heater,water heaters, produce some stray currant. I took a reading that started at 0.28, but I turned off. the high amp iyems, which reduced to 0.16 which is about normal. So make sure high amp items are off.
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Old 11-15-2012, 04:33 PM   #8
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Quote:
Originally Posted by Phil Fill View Post
It should be noted that most high amp items, charger, heater,water heaters, produce some stray currant. I took a reading that started at 0.28, but I turned off. the high amp iyems, which reduced to 0.16 which is about normal. So make sure high amp items are off.
The reading was made with all breakers 12 and 120 all off and the main 120 switch off and the breaker on the shore power pedestal off. I think the telling thing here is that physically removing the shore power plug was the only thing that changed the reading. That should confirm that an isolator is the way to go.

Thanks for the replies all.
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Old 11-15-2012, 06:20 PM   #9
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[QUOTE=Egregious;112909] That should confirm that an isolator is the way to go. QUOTE]

Maybe. The only real way is the Isolation Transformers. A Charles 93-ISOG2/6-A Isolation Transformer is about $575. So a pair is a little over $1,100. Problem solved.
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Old 11-15-2012, 08:35 PM   #10
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[QUOTE=JD;112949]
Quote:
Originally Posted by Egregious View Post
That should confirm that an isolator is the way to go. QUOTE]

Maybe. The only real way is the Isolation Transformers. A Charles 93-ISOG2/6-A Isolation Transformer is about $575. So a pair is a little over $1,100. Problem solved.
Galvanic isolators do the trick for the vast majority of situations at a much lower cost. If you don't mind the cost, weight and space, transformers are theoretically better.

Here's a link to a thread on building your own isolators for $10.

http://www.trawlerforum.com/forums/s...-diy-7738.html
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