It's the Pits

[dmr]

I have no expertise in this subject but do know a bit about electricity and am struggling to understand this. The boat has very low electrical resistance as its made from thick steel, so even 7 amps flowing through the steel is only going to cause a minute voltage (Ohms law) so I don't see how this caused your pitting. And... the current path will be between the exhaust and the connection to the battery and so quite localised to the rear of the boat, but you had pitting all along the sides of the boat, well away from this current path. I still think there must have been a potential difference between your boat and something else in the water (like another boat) to have caused your pitting.

 

I am concerned here as we have a rigidly mounted engine so it is inevitable that some alternator current is flowing through the hull. Just done a dry docking this summer, wish I had had a better look at the baseplate now.

 

.................Dave

[Guest]

I have no expertise in this subject but do know a bit about electricity and am struggling to understand this. The boat has very low electrical resistance as its made from thick steel, so even 7 amps flowing through the steel is only going to cause a minute voltage (Ohms law) so I don't see how this caused your pitting. And... the current path will be between the exhaust and the connection to the battery and so quite localised to the rear of the boat, but you had pitting all along the sides of the boat, well away from this current path. I still think there must have been a potential difference between your boat and something else in the water (like another boat) to have caused your pitting.

 

I am concerned here as we have a rigidly mounted engine so it is inevitable that some alternator current is flowing through the hull. Just done a dry docking this summer, wish I had had a better look at the baseplate now.

 

.................Dave

You likely would anyway even if not rigidly mounted. Exhaust, stern tube etc.

I assume this is why some marine engines have an alternator that is insulated from the engine block?

I too am a bit puzzled how this could cause pitting along the entire boat, but stranger things have happened.

Edited December 23, 2016 by Guest

[jddevel]

Reading and now following this topic I`ll pick up on the comments about lead shielding or possibly aluminium. I wonder what the effect will be on my sailaway lined out with both aluminium foiled Celotex with joints foil taped and Tyvex Airguard also containing aluminium. Comments please.

[Tom and Bex]

Surely this must be a very common setup on boats? If this is what caused the pitting why are other boats not similarly affected?

 

I would still be looking for other causes - particularly around the mains hook up. Might just be a transient fault that's not reoccurred whilst you've been checking. Have you been able to find out if other boats on the mooring have suffered any problems?

 

Tom

[dmr]

I think lead shielding is all about blocking radio waves so is not really significant here. Your aluminium lining is an interesting point. Its very thin so even if there were any stray hull currents they would predominately flow through the steel hull rather than your aluminium. However where the aluminium touches the steel then there will be the potential (!!!!) for dissimilar metal corrosion, especially if there is any condensation. I think its unlikely to be a problem and if it is it will be the aluminium that goes.

 

.............Dave

[dmr]

You likely would anyway even if not rigidly mounted. Exhaust, stern tube etc.

I assume this is why some marine engines have an alternator that is insulated from the engine block?

I too am a bit puzzled how this could cause pitting along the entire boat, but stranger things have happened.

 

Yes, when strange things happen they can be very strange and its best not to discard them just because they don't make logical sense, but it would also be dangerous for the op to conclude that he has found the source of the problem and so not to consider other options.

 

We almost never connect to shore mains but if we did I would likely replace our GI with a transformer. Our only regular mains connection is on our annual 2 week visit to Liverpool and that's full of boats and salty water. The anodes do fizz a bit but I really hope we don't get too much pitting in just two weeks.

 

..............Dave

[nicknorman]

I think lead shielding is all about blocking radio waves so is not really significant here. Your aluminium lining is an interesting point. Its very thin so even if there were any stray hull currents they would predominately flow through the steel hull rather than your aluminium. However where the aluminium touches the steel then there will be the potential (!!!!) for dissimilar metal corrosion, especially if there is any condensation. I think its unlikely to be a problem and if it is it will be the aluminium that goes.

 

.............Dave

and hopefully the shell is painted on the inside prior to installing the cellotext - paint is an isulator!

I have no expertise in this subject but do know a bit about electricity and am struggling to understand this. The boat has very low electrical resistance as its made from thick steel, so even 7 amps flowing through the steel is only going to cause a minute voltage (Ohms law) so I don't see how this caused your pitting.

.................Dave

Maybe it's something to do with the magnetic field?

[dmr]

and hopefully the shell is painted on the inside prior to installing the cellotext - paint is an isulator!

 

Maybe it's something to do with the magnetic field?

 

Never heard of a magnetic corrosion mechanism, I really should be doing some boaty jobs today but now I have something else to Google.

 

..............Dave

[dmr]

I wonder if the stray potentials that cause pitting could be easily measured by dropping a probe into the water a little way from the boat and measuring any small voltages (AC and DC) between the water and the boat?. A suitable device, including some data logging, could be quite easy to produce and would be a good tool for boaters who keep a boat in a marina with a long term shore connection.

 

...............Dave

[Bee]

I do not really understand anything much more than basic electrolysis but all the boats I have seen with intact 'paint' - epoxy, bitumen - even Dulux, have had few new pits or rust 'flowers'. I really wonder if electrical thingys are often blamed for pits when really the problem is simply overdue painting or some other paint problem.

[jddevel]

The lead shielding was a reference to the comments made way back in the beginning of this topic in Sept. in concerning washing machines. Subject has moved on a bit now.

[Keeping Up]

OK, a few more measurements and some commentary.

 

As far as I can tell from my clamp meter measurements (some of which are somewhat unreliable due to the low dc current and strong magnetic fields) the resistance of my "normal" negative return cable to the engine block, including the terminations, is 0.25 mOhm (which is about right). This suggests that the resistance of the secondary, parallel path via the hull and exhaust system, has a resistance of just 6 mOhm.

 

Re the comments about the neg-hull bond being needed to ensure that the fuse blows in the event of a fault caused by, for example, chafing of the insulation. Consider the case where there is no bond from negative to hull::

 

Each of the cables in the system is appropriately fused, so no overheating of any cable can occur in the event of a fault (whatever resistances are involved). Therefore the only source of a hazard is at the place of failure, when that failure is unable to carry the fusing current safely. In certain circumstances, if the resistance of the exhaust increases with age, it may limit the fault current to a value that is below the fusing current but is above that which can be borne by the fault - thus increasing the hazard. However, also possible (and I think more likely) it may limit the current to one which the fault can safely carry - thus decreasing the level of hazard.

Let me try this with a numerical example. Suppose a cable can carry 120 Amps, and is protected by a 100 Amp fuse. A fault could occur which - for the sake of argument - may cause a hazard if it were to carry more than 50 Amps. Assuming that the fault itself has no significant resistance, then if the resistance of the exhaust system were to increase with the passage of time from its present 6 mOhm to any value up to 100 mOhm, then the fuse will blow and no hazard exists. On the other hand if it were to increase to any value above 250 mOhm, then the fault itself can safely carry the current and no hazard exists; in fact the resistance of the exhaust system has removed the hazard! Only if the resistance has increased from 6 mOhm to somewhere between 100 and 250 mOhm has the hazard level been increased by the omission of an earth bond.

 

On this basis the requirement for a neg-hull bond seems to be somewhat dubious, especially if a side-effect is that the hull could be eaten away. All the same I can appreciate that there are some circumstances where it could be required, and so I am grateful for the suggestion above that the system could be bonded by a cable from the block down to a point adjacent to the point where the exhaust meets the hull. In fact I have identified a bracket which is welded to the hull to support the silencer to which it is also welded, so that would be an ideal point for connecting a hull bond which cannot possibly cause any stray currents. The one downside that I can see, is that it would no longer be adjacent to the ac earth-hull bond, whereas it would not seem logical to insert an extra 3 metres of cable into the ac earth path and re-route it to the other end of the engine bay.

 

I still have the problem that people who know about DC systems say this must have been an AC fault; people who know about AC systems say it must have been a DC fault. Anybody who knows little about electrical systems, simply say that it is obviously an electrical fault! So the question still remains, if it were an AC fault - maybe something to do with the shore hookup - what sort of fault could cause this type of problem and by what mechanism?

 

 

 

Edited to correct mathematical error

Edited December 31, 2016 by Keeping Up

[WotEver]

"Re the comments about the neg-hull bond being needed to ensure that the fuse blows"

 

That's not why. It's so that, in the event of a fault, the hull won't sit at +ve potential which would cause corrosion.

 

Take a look at Gibbo's musings: http://www.smartgauge.co.uk/earthing.html

[Keeping Up]

"Re the comments about the neg-hull bond being needed to ensure that the fuse blows"

 

That's not why. It's so that, in the event of a fault, the hull won't sit at +ve potential which would cause corrosion.

 

Take a look at Gibbo's musings: http://www.smartgauge.co.uk/earthing.html

 

I was responding specifically to those comments above which had said that that was the only reason for the bonding. All the same, an accidental fault from positive to hull would not cause a problem if the exhaust system bonds the engine block to the hull; even with a relatively high resistance (compared to 2.5 mOhm) it would blow the fuse.

Edited December 30, 2016 by Keeping Up

[WotEver]

I was responding specifically to those comments above which had said that that was the only reason for the bonding.

I appreciate that. I only posted so that no-one would think that those who said that were correct.

[Keeping Up]

I appreciate that. I only posted so that no-one would think that those who said that were correct.

OK, understood.

[Keeping Up]

Well there are a couple of new bits of information - although they still don't give me a conclusive answer.

 

The first is that the boat which was moored next to mine, and which shared the same mains supply for the 2 years in question, was lifted out for survey a few days ago - and despite its not having been blacked for 8 years the hull was still in excellent condition. This would seem to rule out the possibility that there was a fault on the supply.

 

The second is that last week we arrived at Northampton Marina and plugged in, only to have the on-shore RCD trip about 5 minutes later. After resetting, it would then trip instantly whenever the battery charger was switched on. As I didn't have a low-current ac ammeter handy, I left it switched off and returned a few days later; everything then worked perfectly and I couldn't find any fault with the charger which showed just 1.5mA current to earth. All the same, as the charger was getting rather old (15 years at least) I decided not to trust it, and fitted a new one. Further testing at home has shown no further problems with the old charger, but EITHER the RCD at the marina had become so sensitive that it was tripping on just 1.5mA which seems unlikely, OR ELSE the charger has an intermittent fault which hasn't been enough to trip either my on-board RCD or that at the mooring BUT was enough to trip the marina RCD (which may be more slightly sensitive, who can tell). If it was the latter, it would suggest an intermittent fault which is marginally close to 30mA of AC being fed out through the hull.

 

I don't know, it's still so inconclusive - but I now have a new battery charger as well as a re-wired negative connection.

[Guest]

Well there are a couple of new bits of information - although they still don't give me a conclusive answer.

 

The first is that the boat which was moored next to mine, and which shared the same mains supply for the 2 years in question, was lifted out for survey a few days ago - and despite its not having been blacked for 8 years the hull was still in excellent condition. This would seem to rule out the possibility that there was a fault on the supply.

 

The second is that last week we arrived at Northampton Marina and plugged in, only to have the on-shore RCD trip about 5 minutes later. After resetting, it would then trip instantly whenever the battery charger was switched on. As I didn't have a low-current ac ammeter handy, I left it switched off and returned a few days later; everything then worked perfectly and I couldn't find any fault with the charger which showed just 1.5mA current to earth. All the same, as the charger was getting rather old (15 years at least) I decided not to trust it, and fitted a new one. Further testing at home has shown no further problems with the old charger, but EITHER the RCD at the marina had become so sensitive that it was tripping on just 1.5mA which seems unlikely, OR ELSE the charger has an intermittent fault which hasn't been enough to trip either my on-board RCD or that at the mooring BUT was enough to trip the marina RCD (which may be more slightly sensitive, who can tell). If it was the latter, it would suggest an intermittent fault which is marginally close to 30mA of AC being fed out through the hull.

 

I don't know, it's still so inconclusive - but I now have a new battery charger as well as a re-wired negan.tive connectio

Interesting.

May I ask did you connect the new negative close to the exhaust? I am considering similar. Cheers.

[Keeping Up]

Interesting.

May I ask did you connect the new negative close to the exhaust? I am considering similar. Cheers.

Yes indeed. It connects to a bracket that had been welded to the hull to support the exhaust. As this is on the other side of the engine, it now connects to the stud on the engine that is provided for the starter battery rather than that which is provided for the domestic circuit.

 

The AC earth connection was also moved so that the two are still close to each other.

[Guest]

Yes indeed. It connects to a bracket that had been welded to the hull to support the exhaust. As this is on the other side of the engine, it now connects to the stud on the engine that is provided for the starter battery rather than that which is provided for the domestic circuit.

 

The AC earth connection was also moved so that the two are still close to each other.

Cheers, seems sensible to me. A bit of rewiring ahead for me I think.