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Just had survey on potential boat.


RedsfanUk

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13 minutes ago, robtheplod said:

May I ask how you measured this?  thanks!

Steel rod stuck in river water, one probe on that, one probe on hull of barge.
I was in plastic canoe floating on the river, holding the rod. 

Also tested against metal pontoon, boomed off bank with steel poles. Similar difference between bank and barge. 

Shoreline earth point would have been about 100m away.

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On 17/02/2020 at 15:26, Alastair said:

Volts. 

 

Meter just in water didn't pick up much. I suspect because of area of probe/low conductivity of water. 

The current flow only needs to be in milliAmps to do major damage (and be a danger to swimmers).

I've checked this whilst at the boat. Not sure how to interpret the results though, connected a wire to a mooring peg and put it in the water - connected this to + on voltmeter, and connected the other - end to boat hull. I get 0.8v. I tried this with the boat connected to shoreline, whilst disconnected, with and without a GI connected and always got 0.8v .. so not sure what (if anything) this is telling me, especially as I get 0.8v when the boat isn't connected to shoreline at all?  Can you point me in the right direction?  ta

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6 minutes ago, Alastair said:

I think if you have some metal in none-pure water, then there will probably be a very small voltage when doing something like this. 0.8V isn't much, given the accuracy of most meters. 

Is your 240V system earthed to the hull?

to be honest... I don't know!

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On 17/02/2020 at 12:54, Alastair said:

(snip)

Out of curiosity, I tested the hull of our barge (since sold) vs the river water. 20V difference.
A galvanic isolator was purchased, fitted - insignificant difference.

 

 

If the potential difference is really 20V, a galvanic isolator will be well into conduction, assuming the mains earth is at a similar potential to the water.

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4 hours ago, robtheplod said:

I've checked this whilst at the boat. Not sure how to interpret the results though, connected a wire to a mooring peg and put it in the water - connected this to + on voltmeter, and connected the other - end to boat hull. I get 0.8v. I tried this with the boat connected to shoreline, whilst disconnected, with and without a GI connected and always got 0.8v .. so not sure what (if anything) this is telling me, especially as I get 0.8v when the boat isn't connected to shoreline at all?  Can you point me in the right direction?  ta

Do the same test with the meter leads reversed (-ve in the water etc). Does the reading on the meter change to -0.8V?

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If you put a Zn plated mooring pin into the water and put a voltmeter between the pin and the hull you will see a voltage of about half a volt because of the galvanic effect of Zn to steel, if you do it near the back end of the boat it may be a bit higher because you could be measuring the difference between the brass prop (Also connected to the hull via the prop shaft) and the Zn plated pin.  
So I would expect to see a voltage.  Not sure this helps. 

  • Greenie 1
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On 15/02/2020 at 08:13, matty40s said:

Normal corrosion occurs generally at the waterline, slow to occur, deep red/ brown colour and often results in delamination as it progresses. 

Galvanic corrosion occurs at all levels underwater, is typified by bright orange ''pustules' of rust in rashes, when washed off leaves bright silver pits. It normally doesnt happen near a working anode....here, less than 2 feet either side is protected.

20200214_161926.jpg

 

So this is an example of galvanic corrosion? To be honest it looks the hull of any pressure washed boat in reasonable condition. Most boats that I've seen have some corrosion all over, not just at the waterline  Anyway, if this really is galvanic corrosion why did it happen? Was the boat left on shore power without any form of mains earth isolation for example?

Edited by blackrose
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8 hours ago, Chewbacka said:

If you put a Zn plated mooring pin into the water and put a voltmeter between the pin and the hull you will see a voltage of about half a volt because of the galvanic effect of Zn to steel, if you do it near the back end of the boat it may be a bit higher because you could be measuring the difference between the brass prop (Also connected to the hull via the prop shaft) and the Zn plated pin.  
So I would expect to see a voltage.  Not sure this helps. 

Think wet battery

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12 hours ago, WotEver said:

Do the same test with the meter leads reversed (-ve in the water etc). Does the reading on the meter change to -0.8V?

yes it does reverse to -0.8v  so the 'wet' battery theory seems to work?

Edited by robtheplod
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1 hour ago, robtheplod said:

yes it does reverse to -0.8v  so the 'wet' battery theory seems to work?

Yup. 


Try it again with a lump of untreated steel instead of a mooring pin. Hopefully you’ll see nowt :)

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