Stray Electricity in Marinas

nightjar

New member
Joined
26 Jun 2004
Messages
595
Visit site
Having just replaced a ball valve on the engine sea water skin fitting due to what has been described as anodic corrosion, I have just stuck a multimeter between the skin fitting/ ball valve and the engine(-). To my suprise it shows a PD of 0.75V. Is this normal or do I have a problem ?
This reading was with no shore power connected and battery masters off. Connection shore power and batteries did not change the reading.
Help!
 

starboard

Active member
Joined
22 Dec 2003
Messages
3,016
Location
N5533 W00441
Visit site
You just need to take a walk around the pontoons and note how many peoples power cables are running from the pontoon box to the boat but dangling in the water as they go!! you must get some form of leakage from these.....someone once told me a marina is like one large battery, just put a couple of cables in the water and light up the boat...certainally shows the need for cathodic protection whilst moored in one.

Paul.
 

nedmin

Active member
Joined
12 Oct 2002
Messages
1,504
Location
north lincs.
Visit site
If you are in fresh water why not get an aluminium scaffold pole earth it well and stick it in the water and see what happens to it.make sure its nice and clean. Large ships used to trail zinc behind them dont know if they still do.Zinc no good in fresh water, magnesium to reactive. If the aluminium corrodes badly you know you,ve got a problem.Still it might take the problem away from your boat!!!
 

VicS

Well-known member
Joined
13 Jul 2002
Messages
48,544
Visit site
More or less what you would expect. Two dissimilar metals, cast iron and a copper alloy, in an electrolyte, sea water.

You'd find the same out on a mooring.
 

William_H

Well-known member
Joined
28 Jul 2003
Messages
14,054
Location
West Australia
Visit site
The thing is that modern digital multimeters have a very high input resistance. That means that they will sniff a voltage where there is almost no current available. The voltage produced by dissimilar metals is a result of the type of metals ie zinc and carbon will always give 1.5 volts no matter how small the traces of the two materials. A large amount of these materials will light a light bulb but a small amount will see the voltage dissapear with any current conducted. The current flow is what caus3es damage.
So your .75 volts does not represent a serious electrolytic action but rather reflects the materials involved. I will bet that if you use an analogue meter which conducts perhaps 10 microamps to get the reading then you will measure very little voltage. I am not telling you you don't have a problem just telling you the voltmeter reading is not a usefull indication unless it is showing a reading when you thought the bonding wire was supposed to bring components to the same potential. regards olewill
 

tcm

...
Joined
11 Jan 2002
Messages
23,958
Location
Caribbean at the moment
Visit site
galvanic isolators

um, you can rest assured that electricity doesn't run off or leak out of sheathed copper wires.

The problem as williamH alludes is that in a marina, all boats are connected to a common earth, so potentially all of these small batteries are joined together to form a much bigger battery. A solution is to fit a galvanic isolator.
 
A

angelsson

Guest
Re: galvanic isolators

Very interesting thread, not sure how a galvanic isolator works or how they are fitted. would appreciate any answers.
Mike
 

BlueChip

Active member
Joined
24 Aug 2004
Messages
4,849
Location
Bucks/Plymouth
Visit site
Re: galvanic isolators

It just fits in series in the earth line close to the shore power socket
Its designed to put a marginal barrier in the earth line so that any stray currents in the water surrounding your boat are not passed to shore earth through your hull or underwater fittings which maycause erosion.
In the event of any electrical fault on board the earth line is free to do its job.
They are often for sale on eBay
 

boatmike

Well-known member
Joined
30 Jun 2002
Messages
7,045
Location
Solent
Visit site
I tend to agree with you William. I would be more concerned if there was any significant detectable amperage of current flowing rather than an indicated PD between the points in question. Do you agree?
 

nightjar

New member
Joined
26 Jun 2004
Messages
595
Visit site
Many thanks to all who have replied; I will look into amperage and see if its significant. If so then I guess a galvanic isolator will help
Cheers.
 
Top