Stray electrical currents and a steel hull ?

[Jassira]

I have a steel hulled Bruce Roberts and whilst doing a little rewiring I noticed I get a voltage reading (equal to the battery voltage) between the positive battery terminal and the hull, I get nothing from the negative terminal.

The ohm metre tells me there's no continuity between the hull and the batteries.

I have always been careful not to use the hull as an earth and believe everything has an insulated return.

Do I have a problem, am I looking for a negative cable shorting to the hull somewhere, any advice on how to proceed greatly appreciated.

 

[PCUK]

I had my previous steel boat for 30 years and from new the neg' was connected to the hull. I never used it as an earthing point and always wired everything with two cores, one to the pos' and one to the neg' Never had any electrolysis problems (if that is the correct term)!

 

[Jassira]

Further investigation points toward the Xantrex battery monitor, I'll check I've wired it correctly and report back...

Thanks for your help

 

[BabaYaga]

and believe everything has an insulated return.

Likely this means that your engine block is also not connected to battery negative. But if not, I remember having read that the content of carbon black in rubber hoses can make these somewhat conductive. Possibly enough to get a voltage reading from an otherwise insulated hull?

 

[KellysEye]

To stop stray current our steel yacht was fitted with a galvanic isolator on the wire of both 12 and 240v volts. I did connect an earth for our SSB bolted to a frame in the bilge, the reason is the earth makes up half the aerial metal boats have the strongest signal and the longest range, for GRP boats copper foil along the bottom of the hull we saw and American cat with that.

 

[Jassira]

To stop stray current our steel yacht was fitted with a galvanic isolator on the wire of both 12 and 240v volts. I did connect an earth for our SSB bolted to a frame in the bilge, the reason is the earth makes up half the aerial metal boats have the strongest signal and the longest range, for GRP boats copper foil along the bottom of the hull we saw and American cat with that.

I do have a galvanic isolator but it's only wired to the 240v, I don't understand how or why I might connect the DC circuit as well. I thought it's function was to block very small currents flowing through the 240v earth connection.
Following on from my earlier post re: battery monitor, further investigation seems to rule it out. I now find that whilst I have no continuity between the battery negative and the hull (as measured with the ohm function on my multimetre) even with the negative battery lead disconnected I still get 24v+ between the positive terminal and the hull. I guess a piece of equipment somewhere doesn't have an insulated earth/return ?

 

[Shearmyste]

I may be incorrect but measuring the voltage to the hull from the positive will always give the potential difference and therefore will show the batteries charge?

 

[Jassira]

I may be incorrect but measuring the voltage to the hull from the positive will always give the potential difference and therefore will show the batteries charge?

I had wondered that, perhaps I'm looking for a problem I don't have

 

[Shearmyste]

My head thinks you might be, you may be able to measure if there is any current flow from the battery to the hull, some of the brains on here should give you some guidance as my electrical knowledge is big power stations,

 

[rszemeti]

I'm still trying to work out why you would care.

It is a phsyical impossibility to have a reading of 0V from the negative and the postive terminals to the hull, even if not strongly bonded, there will always be a potential difference from at least one terminal and possibly both!

Hull to negative would be my choice.

 

[halcyon]

I'm still trying to work out why you would care.

It is a phsyical impossibility to have a reading of 0V from the negative and the postive terminals to the hull, even if not strongly bonded, there will always be a potential difference from at least one terminal and possibly both!

Hull to negative would be my choice.

Normally isolated, with two bulbs and push button switches, one pos to hull, one neg to hull. Press pos button if any neg leakage to hull bulb will glow, press neg button any pos leakage to hull, bulb will glow.

Mains isolation transformers are common for mains supply, also allows for shore supply voltage if Global sailing.

Did one mains charger for a steel boat where the service and engine negatives were isolated even during mains charging.

Very much what the owner wants.

Brian

 

[Jassira]

I'm still trying to work out why you would care.

It is a phsyical impossibility to have a reading of 0V from the negative and the postive terminals to the hull, even if not strongly bonded, there will always be a potential difference from at least one terminal and possibly both!

Hull to negative would be my choice.

My concern is electrolysis (if that's the right word).
Rightly or wrongly I have tried to insulate my electrical circuit from the hull and thought there should be no connectivity between the two.

Why is it physically impossible to isolate the two, after all, if I disconnect the battery completely there is 0 voltage, so I don't understand why if the cable insulation is intact and all of the equipment has insulated returns, then why shouldn't I expect 0 volts ?
Are you saying I ought to bond the negative to the hull and that this situation is normal and to be expected ?

 

[Jassira]

Normally isolated, with two bulbs and push button switches, one pos to hull, one neg to hull. Press pos button if any neg leakage to hull bulb will glow, press neg button any pos leakage to hull, bulb will glow.

Mains isolation transformers are common for mains supply, also allows for shore supply voltage if Global sailing.

Did one mains charger for a steel boat where the service and engine negatives were isolated even during mains charging.

Very much what the owner wants.

Brian

Sorry if I'm being a bit thick Brian but would light bulbs tell me something different to the voltage setting on my multimetre ?

 

[RollingStone]

I have a steel hulled Bruce Roberts and whilst doing a little rewiring I noticed I get a voltage reading (equal to the battery voltage) between the positive battery terminal and the hull, I get nothing from the negative terminal.

The ohm metre tells me there's no continuity between the hull and the batteries.

I have always been careful not to use the hull as an earth and believe everything has an insulated return.

Do I have a problem, am I looking for a negative cable shorting to the hull somewhere, any advice on how to proceed greatly appreciated.

I have a steel hulled Bruce Roberts and whilst doing a little rewiring I noticed I get a voltage reading (equal to the battery voltage) between the positive battery terminal and the hull, I get nothing from the negative terminal.

The ohm metre tells me there's no continuity between the hull and the batteries.

I have always been careful not to use the hull as an earth and believe everything has an insulated return.

Do I have a problem, am I looking for a negative cable shorting to the hull somewhere, any advice on how to proceed greatly appreciated.

Jassira, did you ever get this sorted out? I have an identical problem and its puzzling me...

 

[TernVI]

I have a steel hulled Bruce Roberts and whilst doing a little rewiring I noticed I get a voltage reading (equal to the battery voltage) between the positive battery terminal and the hull, I get nothing from the negative terminal.

The ohm metre tells me there's no continuity between the hull and the batteries.
....

This is consistent with a high impedance connecting the battery - to the hull.
Too high for the ohm-meter to register
Not as high as the impedance of the voltmeter, which can be very high, many megohms.

I'm not sure what current best practice is, but AIUI, the idea is that no currents should pass through the hull. Sometimes 0V is bonded to the hull at one point only, sometimes it's connected via a resistance.
Sometimes it's just connected accidentally via damp orsomething like input filters on some equipment.

 

[Mark26]

I had my previous steel boat for 30 years and from new the neg' was connected to the hull. I never used it as an earthing point and always wired everything with two cores, one to the pos' and one to the neg' Never had any electrolysis problems (if that is the correct term)!

Me too, exactly the same as PCUK says in the quote above, mine is 30 years old with no signs, kept in a big marina and constantly on shore supply, while there.

To Jassira, don’t worry yourself with problems that may not exist. Have a look next time it’s out of the water for evidence of electrolysis, there may be none.
In the mean time that’s exactly what your anodes are there for.
If it ain’t broke don’t fix it.

 

[halcyon]

Sorry if I'm being a bit thick Brian but would light bulbs tell me something different to the voltage setting on my multimetre ?

Well it saves getting the multimeter out, allows you to check daily in a second or two both positive and negative leakage, has a lower resistance so not so susceptible to the vagaries of multimeters, the intensity of the bulb's illumination gives a guide to how bad the fault is, simple for crew to operate, can be used in the dark.

Brian

 

[Jassira]

I've not managed to find a solution but a couple of years later it's not caused any obvious problems. I'm out of the water after 2 years afloat the anodes show some erosion but they're still serviceable.
The only place I was able to find connections to the hull were via the engine temperature gauge through the throttle and gear cables and possibly the exhaust mounts. I've not found a way to rectify these but as I say they don't seem to of caused any issues

 

[fisherman]

Some anecdotes. One old fisherman having a new boat fitted with anodes, (long time ago) which he had never met "I'm not paying for any more of them they just disappear"
My GRP crabber was I thought well protected. The hauler was in a steel cabinet on the wood and GRP deck. The hot dip galvanise fell off it in one sheet. I could only imagine that stray current was coming aboard in the wet ropes.
A wood boat owner left loose wires in the bilge, he was very untidy, the roves on the rivets vanished.
My anodes would last over a year in Porthleven, disappear in three months in Newlyn. Welding repairs on steel boats not strapped to earth was blamed.
My bilge alarm was wired with the neg going down to the float switch in the wet, so no pos in the bilge water.