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Old 25-10-2020, 12:59   #1
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Zinc disappearing fast - not all locations - guidance?

Maybe someone can point me in the right direction as I attempt to solve this puzzle? Need some clues...

Bought the boat last Nov, launched late July. Boat just hauled out. I was looking at zincs. In 3 months in the water this season the frigoboat zincs are gone!!!

The big hull zinc is really badly pitted and deteriorated. (all zincs were new)

But the prop-shaft and prop zincs are perfect.

Might this be related to the survey finding below and the boat being plugged into shore power at the slip for the last 3 months? Or bad marina environment?

Survey finding>>Shore power supply: 180 Milliamps were measured in the shore power cord, suggesting a possible onboard electrical leak, re-evaluation with a different reliable shore power supply is recommended, service as required

Where would you start solving this?

thank you for any guidance you can provide.
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Old 25-10-2020, 13:30   #2
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Re: Zinc disappearing fast - not all locations - guidance?

Could be a shore power leak or just a stray current leak. These can be tough to find. My last boat had a stray current problem. I hired a pro for 8 hours. He determined it was a stray current leak but couldn't find the source. I learned enough watching and talking with him that I bought a reference cell and found the problem with about another 4 hours of searching.

Order one of these. It comes with a booklet that will get you started.
https://www.boatzincs.com/corrosion-...ode-specs.html
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Old 25-10-2020, 13:40   #3
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Re: Zinc disappearing fast - not all locations - guidance?

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Originally Posted by dougweibel View Post
Order one of these. It comes with a booklet that will get you started.
https://www.boatzincs.com/corrosion-...ode-specs.html
Thanks! had no idea this tool existed. Will have to wait until spring but this will help then.
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Old 27-10-2020, 13:10   #4
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Re: Zinc disappearing fast - not all locations - guidance?

I suspect its a DC leakage...I would look at the bonding of all the metal parts and make sure the connections are good, because the zinc with no loss are probably NOT bonded and then I would look very hard at the bilge pump/float switch wiring and/or any wiring that can/could have got wet in the last three months, I would be pretty certain that's where you will find the culprit device that is leaking current.
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Old 27-10-2020, 13:23   #5
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Re: Zinc disappearing fast - not all locations - guidance?

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Originally Posted by Cowpoos View Post
I suspect its a DC leakage...I would look at the bonding of all the metal parts and make sure the connections are good, because the zinc with no loss are probably NOT bonded and then I would look very hard at the bilge pump/float switch wiring and/or any wiring that can/could have got wet in the last three months, I would be pretty certain that's where you will find the culprit device that is leaking current.
Thanks, are you saying that if the engine were correctly bonded the shaft zincs should have also been deteriorated?

The frigoboat DC system was a retrofit. Might that installation have missed bonding the heat exchangers? should I check that first?

I'll be checking the whole bonding system.

My concern was correlating the 180mA shore power cord "leak" with my spent zincs.
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Old 29-10-2020, 12:59   #6
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Re: Zinc disappearing fast - not all locations - guidance?

Yes, the fact that some zinc are deteriorating and some not is a sign that the bonding isn't right or zincs are not installed correctly. The exception to this is some of the Volvos saildrives are electrically isolated from the engine, so the zincs should hardly get used. 180Ma leak might be consumption, if it were a leak the RCD would have tripped at 20-25Ma's. Any other info you can think of will be helpful for giving you suggestions.
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Old 29-10-2020, 13:18   #7
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Re: Zinc disappearing fast - not all locations - guidance?

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if it were a leak the RCD would have tripped at 20-25Ma's. Any other info you can think of will be helpful for giving you suggestions.
The RCD or an ELCI would trip on an AC leak, not a DC leak.

AC can cause corrosion but not enough to notice in a decade or so.

The issue here is DC
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Old 29-10-2020, 13:24   #8
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Re: Zinc disappearing fast - not all locations - guidance?

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180Ma leak might be consumption, if it were a leak the RCD would have tripped at 20-25Ma's. Any other info you can think of will be helpful for giving you suggestions.
Not much more to tell. But when the boat was on the hard this winter and I was getting it ready, when I plugged an extension cord into the building then went inside and turned on shore power the GFI (RCD) would often trip. A few tries would get it to work.

The boat was also repowered right before I bought it so maybe they missed bonding the whole engine.

Looks like I have some work to do.

All the zincs are attached correctly.

thanks for the attention.
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Old 29-10-2020, 13:34   #9
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Re: Zinc disappearing fast - not all locations - guidance?

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Originally Posted by boatpoker View Post

The issue here is DC
thanks boatpoker, I think he was trying to also address my 180mA survey finding but not tying that to my zinc dilemma.

I need to find my DC issue, my focus will be bonding system first then maybe half-cell exploration... I guess
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Old 29-10-2020, 13:40   #10
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Re: Zinc disappearing fast - not all locations - guidance?

Quote:
Originally Posted by boatpoker View Post
The RCD or an ELCI would trip on an AC leak, not a DC leak.

AC can cause corrosion but not enough to notice in a decade or so.

The issue here is DC
OP said there was current leakage from the shore power which is AC.
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Old 29-10-2020, 13:50   #11
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Re: Zinc disappearing fast - not all locations - guidance?

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Originally Posted by Ween View Post
thanks boatpoker, I think he was trying to also address my 180mA survey finding but not tying that to my zinc dilemma.

I need to find my DC issue, my focus will be bonding system first then maybe half-cell exploration... I guess
If the surveyor tested properly that 180mA should refer to a DC leakage.

Each type of metal has a millivolt range of acceptability.
180mV is not within the range of these metals ...
Bronze: -500 mV to -700mV
Steel: -750 mV to -850 mV
Aluminum: -800 mV to -1,050 mV

Check the obvious spots first. i.e. bilge pump, abraded wires in bilge water, poorly sealed conductors in bilge water etc.

PS. You don't have to buy a corrosion meter. If you have a decent multimeter with a mA range you can just buy the silver silver chloride half cell on a 20' lead.
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Old 29-10-2020, 14:00   #12
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Re: Zinc disappearing fast - not all locations - guidance?

Quote:
Originally Posted by Cowpoos View Post
OP said there was current leakage from the shore power which is AC.


Shore cords often can and do carry dc currents from your boat or others. These currents can cause rapid loss of anodes.
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Old 29-10-2020, 14:04   #13
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Re: Zinc disappearing fast - not all locations - guidance?

Quote:
Originally Posted by boatpoker View Post
That 180mA refers to a DC leakage.

Each type of metal has a millivolt range of acceptability.
180mV is not within the range of these metals ...
Bronze: -500 mV to -700mV
Steel: -750 mV to -850 mV
Aluminum: -800 mV to -1,050 mV

Check the obvious spots first. i.e. bilge pump, abraded wires in bilge water, poorly sealed conductors in bilge water etc.
Apologies I assumed since the finding was on shore power the reference was AC. Honestly I have no idea how that is measured and what is "off" in the boat to measure it. I assumed it was shore power off on boat but ac current measured through cable. I should have asked...

clueless how a DC load could be on a "turned off at the boat" shore power cord. I was ignoring AC issues in this dilemma since I have a galvanic isolator in the mix. Now if I sound really ignorant I probably am.
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Old 29-10-2020, 14:09   #14
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Re: Zinc disappearing fast - not all locations - guidance?

Quote:
Originally Posted by Ween View Post
Apologies I assumed since the finding was on shore power the reference was AC. Honestly I have no idea how that is measured and what is "off" in the boat to measure it. I assumed it was shore power off on boat but ac current measured through cable. I should have asked...



clueless how a DC load could be on a "turned off at the boat" shore power cord. I was ignoring AC issues in this dilemma since I have a galvanic isolator in the mix. Now if I sound really ignorant I probably am.


With the shore breakers on the boat off the ground is still connected.

The galvanic isolator should block these stray DC currents, but they are able to fail.
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Old 29-10-2020, 14:14   #15
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Re: Zinc disappearing fast - not all locations - guidance?

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Originally Posted by Sailmonkey View Post
With the shore breakers on the boat off the ground is still connected.

The galvanic isolator should block these stray DC currents, but they are able to fail.
Aha, I might know enough now to be dangerous. Thanks to everyone.
Everyone says check bilge pump and any submerged wiring.
Check bonding for all external metal.
Check isolator (not sure how to test but will find out).
Then use half-cell to hunt it down.
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