Stray currents?

[mogmog2]

This might be a daft question, but I'm trying to catch up on all this electrickery:
Many of the boats around ours are festooned with shore power leads. Given the nature of the boats (Centaur, Sadler 26 and other older boats), I suspect they are just plugged into battery chargers, but even that presumably can give rise to stray currents (if the isolator switch was on, it would be connected to anything/everything in the boat)?

I can't work out: If we are NOT on shore power and therefore not connected to the mains, can electrically leaky neighbours affect us? We are presumably earthed/connected to the same earth through the water, as the mains earth is?

Should I be concerned?
Many thanks

 

[pvb]

In most circumstances, with a fibreglass boat, if the shore power is disconnected (ie unplugged) there's little risk of damage due to electrolytic corrosion.

 

[PaulRainbow]

If you were connected to the shore power and your boat was wired to current regs your shore power earth would be connected to your DC negative, which is invariably connected to your anodes. So, there is a direct electrical connection between your anode and shore power ground. If another boat had the same connection, your anode would be connected to his boat. If his anode was depleted, he'd be using yours. To combat that you would fit a galvanic isolator. As you are not connected to shore power, you cannot be affected by such galvanic corrosion.

 

[Lon nan Gruagach]

The mains earth isn't the same (exactly) as the water, that's where all these problems come from. For there to be any issue there must be a circuit, since you have only one possible connection (the water), there cannot be a circuit.

Now then, why isn't the mains earth exactly the same as the water? Well, at the far end of the cable, at the earthing spike, it is exactly the same. However there are many metres of less than infinitely thick wire between you and that point. Since anyone with any metal, be it skin fittings, props, anodes or even a good old lump of iron hull, and aren't using a galvanic isolator, will be injecting current into the earth connection, there will be a voltage difference between the mains earth and the water.

 

[superheat6k]

Whereas galvanic currents are unlikely to affect you, stray currents are completely different, and much will then depend upon how your boat's fittings are bonded and the position of your boat in or near a stray current flow through the water.

AC current even at low power is of far higher energy than galvanic (chemical) DC current. If your external metalwork is bonded then this can provide a lower resistance path for a stray current than the water, which even when salty will be of higher resistance than the bonded fittings, so the stray current can hop on and off along its route. That said if the boat were laying at 90 degrees to the stray current it might cary virtually nothing.

So is the reason for your question because you have a specific problem with anode wear ?

I do have some excellent publications available on this somewhat complex subject - PM me if you would like some links to them.

 

[mogmog2]

Whereas galvanic currents are unlikely to affect you, stray currents are completely different, and much will then depend upon how your boat's fittings are bonded and the position of your boat in or near a stray current flow through the water.

AC current even at low power is of far higher energy than galvanic (chemical) DC current. If your external metalwork is bonded then this can provide a lower resistance path for a stray current than the water, which even when salty will be of higher resistance than the bonded fittings, so the stray current can hop on and off along its route. That said if the boat were laying at 90 degrees to the stray current it might cary virtually nothing.

So is the reason for your question because you have a specific problem with anode wear ?

I do have some excellent publications available on this somewhat complex subject - PM me if you would like some links to them.

Hi, thanks for this and the offer. I haven't got a problem. I was a bit concerned that all the boat's around me are hooked up and are of a sort that the electrics may not be that great. And I'm trying to learn the basics, not least as I want to make sure our bonding is correct.

I'm also new to this type of boating - in the olden days when I started sailing, my dad's yacht had a battery which judt lasted for two weeks and he took it home to charge and only gin palaces had mains. Our Centaur, which is primitive by today's standards has a battery charger installed in a semi-fitted manner and I'm trying to assess the implications of using it as getting the batteries out to take them home is very awkward.

I've got Nigel Calder's weighty tome but it's heavy going and mostly not relevant to us.

 

[Tranona]

There is no need to bond anything on a Centaur apart from perhaps the shaft and prop. The seacocks are bronze or DZR so do not need anodes. Our club marina is full of mostly older boats, including several Centaurs and nobody seems to have any problems, although anybody who connects to shorepower is required to fit a galvanic isolator to their boat.

For charging your battery, unless you have large capacity banks you may well find a modest solar panel enough. If you are going to use shorepower for the charger then might be worth fitting a GI, but if it is just short term to top up the battery then probably not necessary.

 

[PaulRainbow]

There is no need to bond anything on a Centaur apart from perhaps the shaft and prop. The seacocks are bronze or DZR so do not need anodes. Our club marina is full of mostly older boats, including several Centaurs and nobody seems to have any problems, although anybody who connects to shorepower is required to fit a galvanic isolator to their boat.

For charging your battery, unless you have large capacity banks you may well find a modest solar panel enough. If you are going to use shorepower for the charger then might be worth fitting a GI, but if it is just short term to top up the battery then probably not necessary.

He really only needs a galvanic isolator if his shore power earth is connected to his DC negative. Not likely on a Centaur, but worth checking just in case.

 

[mogmog2]

There is no need to bond anything on a Centaur apart from perhaps the shaft and prop. The seacocks are bronze or DZR so do not need anodes. Our club marina is full of mostly older boats, including several Centaurs and nobody seems to have any problems, although anybody who connects to shorepower is required to fit a galvanic isolator to their boat.

For charging your battery, unless you have large capacity banks you may well find a modest solar panel enough. If you are going to use shorepower for the charger then might be worth fitting a GI, but if it is just short term to top up the battery then probably not necessary.

Thanks. The only seacock that had a bonding wire is the engine raw water inlet. That has snapped off and was on my list of things to do. Being original, should I not reconnect this? The anode is less than a metre away.

As a stopgap I have connected a 1W panel I had lying around. I haven't done the calcs as I've had bigger fish to fry, but last time I measured the 85Ah battery, which I bought recently, it showed 12.9v which I think is ok?
Our plan is to use the boat this season pretty much as is and not rush into improvements until we know what we need. This will include upgrading the wiring & switchboard/distribution and to include a solar panel to cover our relatively modest daily usage.

PaulRainbow
A previous owner had put a Duraplug socket in the stern locker which leads to a double domestic 13A socket in a quarterberth and from the back of that, another rubber socket in the engine compartment where there is a battery charger fixed to the side. This is unplugged and not connected to the battery. I haven't plugged into this as I haven't yet checked if it is safe conceptually or physically.

So as I now understand that as the AC and DC circuits aren't connected in any way, there shouldn't be stray currents? So am I correct in thinking that if I turn the 12B switch to off and put a battery charger on the battery without taking at least the negative boat cable off, there is a path from AC to DC?
Many thanks.

 

[fisherman]

If you were connected to the shore power and your boat was wired to current regs your shore power earth would be connected to your DC negative, which is invariably connected to your anodes. .

Mine wasn't. My engine electrics were insulated return, the engine and stern gear were not connected to DC neg. The anodes were wired to stern gear, engine/gearbox and, rightly or not, seacocks. (40 years old and counting). The keel band had its own anodes which degraded much more quickly than the others on rudder, (wired inside) and hull (ditto). I readied another boat for a trip to IoM from Newlyn, and fitted all new anodes. It had been fitted with a lorry starter, no insulated return, thus connecting DC neg to engine etc. On arrival there were no anodes left. Newlyn is very bad for degradation, rumoured to be due to welding work on steel boats without earthing them to the quay. Certainly I would lose anodes in less than twelve months, when in Porthleven they would last two years +. I tried to get a definitive answer on anode wiring and even M G Duff would not commit.
I had deck fittings, a steel hauler cabinet, and a steel tripod sitting on the rails. The cabinet lost its hot dip galvanise coating in one sheet, the bracket on top of the stem degraded where the water dripped from it and down the GRP hull. I can only guess that stray current was coming onboard via the potropes, and connecting to the machinery via the wire braids in the hydraulic hoses...or not. The whole issue seems to depend on witchcraft. (Good name for a boat purchase mag there)

 

[PaulRainbow]

PaulRainbow
A previous owner had put a Duraplug socket in the stern locker which leads to a double domestic 13A socket in a quarterberth and from the back of that, another rubber socket in the engine compartment where there is a battery charger fixed to the side. This is unplugged and not connected to the battery. I haven't plugged into this as I haven't yet checked if it is safe conceptually or physically.

So as I now understand that as the AC and DC circuits aren't connected in any way, there shouldn't be stray currents? So am I correct in thinking that if I turn the 12B switch to off and put a battery charger on the battery without taking at least the negative boat cable off, there is a path from AC to DC?
Many thanks.

The only thing you need to consider is continuity between your anodes and the shore power earth. If the AC earth is not connected to the boats anodes then plugging in the battery charger will not cause a problem, it matters not that the charger is connected to the AC input and DC output, that does not provide a path for galvanic corrosion. It would be perfectly safe to leave the battery charger plugged in, it would be perfectly safe to have it switched on, irrespective of the position of the isolator switch.

Slightly different issue, not all battery chargers are equal. Some are safe to leave connected to your batteries for long periods, others are not. What charger do you have ?

 

[Graham376]

Thanks. The only seacock that had a bonding wire is the engine raw water inlet. That has snapped off and was on my list of things to do. Being original, should I not reconnect this? The anode is less than a metre away.

This side of the pond, advice is not to bond through hulls/seacocks. Your easiest way of charging batteries without worrying about likely effect of shore power connection is to fit a solar panel. Even a small 25w one will have batteries fully charged ready for the weekend.

A bit off topic, have a look at what happens to steel with some salt water and a battery, useful for security marking - https://www.youtube.com/watch?v=Bj0M81Hn5-0

 

[mogmog2]

The only thing you need to consider is continuity between your anodes and the shore power earth. If the AC earth is not connected to the boats anodes then plugging in the battery charger will not cause a problem, it matters not that the charger is connected to the AC input and DC output, that does not provide a path for galvanic corrosion. It would be perfectly safe to leave the battery charger plugged in, it would be perfectly safe to have it switched on, irrespective of the position of the isolator switch.

Slightly different issue, not all battery chargers are equal. Some are safe to leave connected to your batteries for long periods, others are not. What charger do you have ?

Thank you.
The battery charger is a basic Gunson or Halfords type car charger. Haven't yet investigated it but is the type along the lines of my own car charger and assume it has fast & float charge manual switch at best but nothing more sophisticated than that. I'll have to live without a modern charger for the time being,not least because solar is the direction in which I want to head.

 

[VicS]

Thank you.
The battery charger is a basic Gunson or Halfords type car charger. Haven't yet investigated it but is the type along the lines of my own car charger and assume it has fast & float charge manual switch at best but nothing more sophisticated than that. I'll have to live without a modern charger for the time being,not least because solar is the direction in which I want to head.

A non automatic charger will probably be capable of fully charging the batteries. Quite quickly too if it has a sufficiently high max amps output for the battery capacity.

If not supervised it will also be capable of over charging the batteries !

The beauty of even a relatively inexpensive automatic charger is that not only will it rapidly charge the batteries if it has adequate amps output but it can safely be left unsupervised without fear of overcharging.

I have a Ring 8 amp Smartcharrger. An old model and an impulse buy from Halfords a good few years ago. I am not suggesting it is the bee's knees but I can happily leave it connected 24/7 if I choose too. It is the right size for car batteries but a larger one ( 12 or 16 amps) would have been a better choice for larger batteries.

 

[PaulRainbow]

Thank you.
The battery charger is a basic Gunson or Halfords type car charger. Haven't yet investigated it but is the type along the lines of my own car charger and assume it has fast & float charge manual switch at best but nothing more sophisticated than that. I'll have to live without a modern charger for the time being,not least because solar is the direction in which I want to head.

By all means give the batteries a quick top up when you return to the marina, but don't leave it on when you're not there.

Solar is the way to go, so fit as much as you can. It can also be useful to have a mains charger, depending on your batter bank size and usage, so you can keep the batteries charged up and use your equipment if you're in a marina and there isn't much Sunshine.

 

[mogmog2]

Thanks everyone. That's one piece of the jigsaw in place