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post #1 of 24 Old 07-18-2016 Thread Starter
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12V ground on the boat

I am beginning the electrical system refurb on my boat today, starting at the batteries, main switch battery multiplexer, and motor electrics. (Perhaps I will do a write up of what I find a little later as I get into it a bit more.) My question for today concerns the ground leg.

First off, there are two grounds on the boat. The ground we are all familiar with is the battery negative terminal. This ground is tied together throughout the boat: batteries, instruments, lighting, etc. all on one bus. The positive side of the electrical system is +12.6V or thereabouts under normal conditions with a charged battery and no charging current from either the alternator, solar charge controller, or mains charger. (It may useful to note that the voltage of the system represents charge potential, and that electrons flow from the negative side of a power supply to the positive leg, which some find counterintuitive. Be that as it may, my understanding is that the +/- nomenclature is a historical artifact of early electrical engineering.) At any rate, the second ground is "earth ground", and on my boat is accessible by way of a connection to the keel-bolts --> iron keel --> saltwater.

On this boat there is a 2" wide strip of copper affixed to the keel-bolts and runs to a 4AWG or so cable which runs to the engine block. The aluminum mast is electrically connected to the keel as well to provide protection from lightning strikes. Incidentally, the VHF antenna is at the top of the mast with the cable running down the backstay and forward into the cabin. I'm sure a lightning strike will cause the radio to explode spectacularly. At any rate, I question the wisdom of grounding the keel and battery ground through the engine casing. I speak some Electron, but not enough to fully understand the subtleties, and it is my suspicion that the battery ground may in fact float around but not quite at the earth ground. If this is true, my lack of EE expertise is not sufficient to know whether there is an advantage to tying the battery ground to the keel, or whether it is a bad idea on general principles, lightning strikes notwithstanding.

Any sparkies out there with opinions on the wisdom of the existing arrangement here?

Disclaimer: Not an EE.

1974 PY-26
Volvo-Penta MD-6A

Last edited by Frogc; 07-18-2016 at 07:58 PM. Reason: Disclaimer
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post #2 of 24 Old 07-18-2016
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Re: 12V ground on the boat

I can't make much comment other than I will be watching. I think there is some disagreement as to whether to bond (tie all your metal parts together to the ground) or not. I will say that I have only seen the strip of copper tape on SSB installs, is that the radio you are talking about with the antenna running up the back stay? normally the VHF antenna is run down the mast.

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post #3 of 24 Old 07-18-2016 Thread Starter
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Re: 12V ground on the boat

miatapaul,

Not sure my original description was clear enough; the copper tape runs from the keel bolts to the engine block. There does not seem to be any direct connection to the VHF cable. The cable itself appears to be something like RG-213 (taken from the radio manual as recommended low-loss spec for runs longer than 50'). No idea if that is what it is as the cable does not have the usual markings on the sheath, but it is about 1cm dia. It is possible the installer did not want to drill the rather substantial hole(s) which would be required to run it without chafing or a tight radius bend. The manual does recommend grounding the cable at the PL259 connector at the back of the radio with #10AWG "to the ship's ground system ... at the nearest ship's ground connection point", which I have added to my to-do list.

I will have to acquire a handheld VHF unit as backup against failure of the mast antenna.

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Re: 12V ground on the boat

Grounding, bonding, lightning grounds, circuit grounds, would suggest you read some of the printed books, buy them online used, from some of the long-published authors. There are disagreements about "grounds" and why which system is best for particular purposes. Sometimes it is a matter of personal choice and belief and it helps to see how exoerts disagree.

I can't see why the copper strap from keel to engine, it might just be a half-executed idea a PO had. Personally I would rather run a high capacity cable from the engine/battery common to the keel bolt, it should carry more current than copper strapping, which is usually (better and) used as an electrical counterpoise for a HF radio.

The VHF coax running down the backstay may sound odd but it might make sense. I once had "today only" to run a new antenna coax before a 3-day race, and the mast spat out the fish tape no matter how we tried to run it. So, the new cable got temporarily run outside the mast and covered with packing tape to secure it for the week. Not a perfect solution but given the options...now doesn't a secure run down the backstay seem more sensible?(G) Something to move inside the mast, when you get around to it.
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post #5 of 24 Old 07-19-2016
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Re: 12V ground on the boat

It's possible that the copper strip from engine to keel boats is not an attempt to ground the engine, but rather the reverse - an attempt to bond the keel bolts to the engine ground.

The engine usually gets grounded via the prop and prop shaft. No copper strips required.

Experts differ on whether bonding is a good idea or not - but they are all agreed on one thing - it's all or nothing. The absolute worst case is an incomplete bonding system not connected to ground, for example two through-hulls connected to each other, but not to ground. Unfortunately, as bonding systems get old and don't get maintained, this sort of thing happens through ageing - bad connections, a PO who wondered what that wire is for, and so on.

I recently spent a merry weekend restoring my bonding system to an actual complete system with everything connected. You could tell what wasn't connected well by the colour of the bronze.

Bristol 31.1, San Francisco Bay

Last edited by MarkSF; 07-19-2016 at 02:19 PM.
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post #6 of 24 Old 07-19-2016
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Re: 12V ground on the boat

I had a much longer reply before the system lost it. The gist of it is this: "Ground" is really just a reference point that you choose and call it 0 volts, and as such, there is only one ground in any electrical system. If the DC system is totally isolated, then it's called a "floating ground," and for practical reasons it's usually designated as the negative pole of the battery.

But since your DC negative connects to the starter on the engine, there's an electrical connection all the way through the engine to the propeller shaft, and the ground in your DC system is thus an Earth ground. The strap between the keel and the engine block is part of the lightning-protection system, not the DC electrical system. There's no positive connection to complete the circuit, and no current should normally flow through it.* Lightning has such high voltage that its current flowing through the mast and keel induces voltage in nearby chunks of metal, like engines. The inverse-square law means that the induced voltage is much lower than in the bolt itself, and where there's voltage potential, current can flow. Best it flow through the copper strap, rather than leap through any sailors who might get caught in between.

* There is a completed circuit through seawater, though, so keep your zincs fresh or you might get galvanic corrosion.
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post #7 of 24 Old 07-19-2016 Thread Starter
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Re: 12V ground on the boat

Quote:
Originally Posted by hellosailor View Post
Grounding, bonding, lightning grounds, circuit grounds, would suggest you read some of the printed books, buy them online used, from some of the long-published authors. There are disagreements about "grounds" and why which system is best for particular purposes. Sometimes it is a matter of personal choice and belief and it helps to see how exoerts disagree.

I can't see why the copper strap from keel to engine, it might just be a half-executed idea a PO had. Personally I would rather run a high capacity cable from the engine/battery common to the keel bolt, it should carry more current than copper strapping, which is usually (better and) used as an electrical counterpoise for a HF radio.
The reason for the copper strapping is actually quite mundane; the copper runs up out of the bilge hatch, under a bit of carpet and then up to a much beefier AWG10 cable routed aft through the quarter-berth.

Quote:
The VHF coax running down the backstay may sound odd but it might make sense. I once had "today only" to run a new antenna coax before a 3-day race, and the mast spat out the fish tape no matter how we tried to run it. So, the new cable got temporarily run outside the mast and covered with packing tape to secure it for the week. Not a perfect solution but given the options...now doesn't a secure run down the backstay seem more sensible?(G) Something to move inside the mast, when you get around to it.
That's a pretty good reason. But, if I had access to the top of the mast to mount an antenna, I'd fish down or use a plumb bob if possible and snag the string as it passed the target hole. I don't think the run up the backstay is necessarily bad, but I would have used cable-ties rather than snaking it around the cable a couple of turns. Pretty sure cable-ties were invented before 1994.
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post #8 of 24 Old 07-19-2016 Thread Starter
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Re: 12V ground on the boat

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Originally Posted by JonG View Post
I had a much longer reply before the system lost it. The gist of it is this: "Ground" is really just a reference point that you choose and call it 0 volts, and as such, there is only one ground in any electrical system. If the DC system is totally isolated, then it's called a "floating ground," and for practical reasons it's usually designated as the negative pole of the battery.

But since your DC negative connects to the starter on the engine, there's an electrical connection all the way through the engine to the propeller shaft, and the ground in your DC system is thus an Earth ground. The strap between the keel and the engine block is part of the lightning-protection system, not the DC electrical system. There's no positive connection to complete the circuit, and no current should normally flow through it.* Lightning has such high voltage that its current flowing through the mast and keel induces voltage in nearby chunks of metal, like engines. The inverse-square law means that the induced voltage is much lower than in the bolt itself, and where there's voltage potential, current can flow. Best it flow through the copper strap, rather than leap through any sailors who might get caught in between.

* There is a completed circuit through seawater, though, so keep your zincs fresh or you might get galvanic corrosion.
I measured the potential between the floating ground of the -ive battery terminal and the engine-keel connection and measured a fluctuating 300mv difference between earth ground and the battery system. Looks like there is a lot of noise there, but I can't really see it with a cheapo DVM and my portable oscilloscope is on the other side of the Georgia Straight.

I scraped and did the underside anti-fouling paint on Saturday (recovered Sunday) and got a fresh zinc on the keel. The only trace of the previous zinc was its steel bar mounting strip. I will have to do some reading (also as per hellosailer) to try and find out about the practical implications of this ground potential difference etc. In the audio world a floating "ground" condition causes well-known hum artifacts, and in typical mains wiring is an obvious hazard.. No idea what occurs in the marine environment such as whether it will affect galvanic corrosion one way or another.

Your point about induced currents during a lightning strike is noteworthy. 30MV is nothing to sneeze at even if the current is relatively low; the power is necessarily considerable. I am glad I selected a keel-stepped mast but confess I was thinking structural rather than conductivity when considering its advantages.

Many thanks to all who replied.

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Re: 12V ground on the boat

A short postscript to my last post:

Reading a bit about sacrificial anodes and such made me realize that the voltage differential I was seeing between earth ground (via the keel) and the battery - terminal was in fact the process of corrosion underway on the zinc anode installed on Saturday when I did the anti-fouling paint on the hull and keel. The fluctuation in voltage I observed on the DVM was apparently a result of electrolysis occurring in the vicinity of the sacrificial anode and mediated by variations in the chemical composition of the sea water occurring in real-time as the process was underway and localized in that area. (This is not to say that there is no voltage potential difference between the battery -ive terminal and earth ground.) This explanation is perhaps not as clear as it could be, but I am necessarily temporizing here.

In any event, I realized that as the MD6B is raw water cooled, it would be foolish to insulate the engine from the keel and I therefore reconnected the keel-engine bond in order to include the motor in the sacrificial zinc anode system. In other words, the zinc anode on the keel will also protect the engine from internal corrosion by way of sea water in the water jacket, and hopefully also the sail drive.

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post #10 of 24 Old 07-21-2016
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Re: 12V ground on the boat

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Originally Posted by Frogc View Post
In any event, I realized that as the MD6B is raw water cooled, it would be foolish to insulate the engine from the keel and I therefore reconnected the keel-engine bond in order to include the motor in the sacrificial zinc anode system. In other words, the zinc anode on the keel will also protect the engine from internal corrosion by way of sea water in the water jacket, and hopefully also the sail drive.
I (for one) am not sure that's true. Most RWC engines have their own anodes (usually pencil anodes) installed in the cooling system someplace for this purpose.

Anyways, Volvo produce very good clear and concise diagrams and instructions on how to properly connect, maintain and operate their engines.. you better check the manual.


EDIT: FWIW, to help protect the engine from internal corrosion my FWC Volvo MD2040 is fitted with an "earthing relay" specifically intended to ensure the engine is not earthed under normal conditions.. but I realise your engine may be quite different, hence my suggestion to RTM.

-
A bad day on a boat beats a good day in the office

Last edited by Classic30; 07-21-2016 at 01:47 AM.
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