Stray marina electric current?

[colind3782]

My neighbour in the marina is convinced that damage to his prop is being caused by stray electric current in the water due to other berth holders leaving their shore power connected all the time.

Is there a way of checking for any current in the water, say with a meter or similar?

 

[Lon nan Gruagach]

For current to flow there MUST be a circuit.
If there is a circuit through other peoples earth then it must also pass through your neighbour's earth. HE can stop this with a galvanic isolator.
You can check the current by plugging your shore power cable into the distribution (be very sure the supply is switched OFF) but instead of plugging it into the boat connect just the earth of the shore cable to the earth of the boat socket with a multimeter set to a low current range.

 

[colind3782]

Thanks very much

 

[burgundyben]

Read up about a silver chloride reference electrode.

 

[BlueChip]

Since I fitted a galvanic isolator some years ago, anode wastage is negligible
I always unplug from the mains when I leave the boat as a double safeguard, if there is no circuit there can be no electrolysis

 

[VicS]

My neighbour in the marina is convinced that damage to his prop is being caused by stray electric current in the water due to other berth holders leaving their shore power connected all the time.

Is there a way of checking for any current in the water, say with a meter or similar?

If your neighbour leaves his shore-power plugged in for prolonged periods ( plugged in, not necessarily in use ) then a galvanic isolator is next to essential. However the most vulnerable parts of his boat will be its anodes.
Galvanic isolators are often called "zinc savers" because they reduce the rate of loss of zinc. as BlueChip has found.

If OTOH he does not leave his shore-power connected then there will be little or no point in fitting a GI and unplugging the shore-power lead is the alternative to fitting a GI

Damage to the prop however is unlikely to be caused by the usual galvanic processes that cause high rate of loss of zinc from anodes . Unless its an aluminium prop it is at the wrong end, the noble end, of the galvanic series for that to occur.

I would start by looking closer to home, namely at his boat's own electrical system for defects that might be causing small currents to flow leading to electrolysis.

Checking for a small unexplained current flow, from the batteries would be a good start. I am talking of milliamps here.

 

[Cuan]

This is probably an elementary question, but how do you test for this? What do you but the multi meter probes on?

 

[VicS]

This is probably an elementary question, but how do you test for this? What do you but the multi meter probes on?

Disconnect one of the battery leads from the terminal post then connect your multimeter set its highest DC amps range to measure current in the usual way. Then progress to lower current ranges. ( If you start with a 10 or 20 amp range this will probably mean moving one of the meter leads to the appropriate socket)

If you are unsure about using a multimeter for measuring current study the relevant sections of Tony Brooks' TB training website first. http://www.tb-training.co.uk/cover.html and your meter instruction book.


See also thses two diagrams

 

[Cuan]

Disconnect one of the battery leads from the terminal post then connect your multimeter set its highest DC amps range to measure current in the usual way. Then progress to lower current ranges. ( If you start with a 10 or 20 amp range this will probably mean moving one of the meter leads to the appropriate socket)

If you are unsure about using a multimeter for measuring current study the relevant sections of Tony Brooks' TB training

website first. http://www.tb-training.co.uk/cover.html and your meter instruction book.

Thanks for the diagrams, Very useful. My prop pinked out and had to be replaced. I thought the cause might have been stray currents in the marina even though I only connected to mains when I was there. I have since moved to a mid stream pontoon with no power connected, but would like to check that there are no further issues with the prop, so thought I
Should check that there is nothing going to my prop shaft. Do I put one probe on battery + and the other on the prop shaft and see if there is a reading? And if there is, what is the best way of eliminating the possible causes?


See also thses two diagrams

 

[Cuan]

Sorry my reply disappeared for some reason.
I should probably start a new thread for this. Thanks for the very useful diagram.
My prop pinked out and had to be replaced. I put this down to stray current in the marina, although I was only occasionally connected to mains - why I was there but not when the boat was left.
I have since moved to a mid stream mooring with no power, but I want to check that thee is no possibility of any further problems.
If I am checking that there is no possibility of electrolysis, do I put one probe on the prop shaft, and the other on the + terminal of the battery (after disconnecting the + battery terminal. What do I do to track down the problem if there is a current?

 

[VicS]

Sorry my reply disappeared for some reason.
I should probably start a new thread for this. Thanks for the very useful diagram.
My prop pinked out and had to be replaced. I put this down to stray current in the marina, although I was only occasionally connected to mains - why I was there but not when the boat was left.
I have since moved to a mid stream mooring with no power, but I want to check that thee is no possibility of any further problems.
If I am checking that there is no possibility of electrolysis, do I put one probe on the prop shaft, and the other on the + terminal of the battery (after disconnecting the + battery terminal. What do I do to track down the problem if there is a current?

I'd not like to speculate on the cause of the damage to your prop.

It could simply have been dezincification if the prop was "ordinary" brass.
I cannot see any mechanism by which it would be galvanic corrosion.
It could have been electrolysis due to an electrical fault or due to these mysterious "stray currents".

However before you start trying to make measurements be sure you fully understand how to go about measuring current ( amps). The Sections in TB training are about the best basic on-line tutor that I have come across.
If you are unsure get someone who does understand what's involved to help you............. someone who stayed awake in his school science lessons

To help here in these three photos is how I measured the small current that flows from my car battery when the car is not in use ( I know what its due to , its the clock!)

In the first photo you can see that I have disconnected one battery lead ( for convenience its the negative lead) from the battery post and connected the leads of a multimeter to measure current

The second photo shows the meter set to 10amps with the red lead in the 10amp socket. The meter is reading 0,01 amps ( 10 mA)

In the third photo the meter is set to 200mA (0,2A) and the red lead has been moved to the socket for measuring volts, ohms or mA The reading is 9.2 mA .

 

[Cuan]

Thanks for this, and the links to TB training. I was not always fully awake in science lessons.