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Old 20-02-2014, 08:44   #1
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DC ground - steel hull

Hi there,

At the moment the situation on my steel hulled boat is as follows: the battery's positive is connected to the control panel with a heavy gauge battery cable. The same cable is led from the negative of the battery to a bolt that is welded to the steel hull. At the control panel, another bolt is welded to the hull and the negative is drawn from there to the control panel.

Now, in an attempt to move the batteries closer to the CP and the solar panels/wind gen/alternator I would like to make a new batterybox right next to the CP. I would also like to eliminate the steel hull as a component in the electrical circuit. I can't imagine the original setup to be good to the zincs...

Now, can I just run batterycable from + to the control panel and from - to the control panel or will I be having problems with it like that? Does any ground need to be incorporated?

Thanks!
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Old 20-02-2014, 09:47   #2
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Re: DC ground - steel hull

See “DC Bond to the Metal Hull”
MetalBoatbuilding.org • View topic - DC Bond to the Metal Hull

And ➥ SmartGauge Electronics - To bond or not to bond - Hulls and electrical earthing
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Old 20-02-2014, 22:27   #3
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Re: DC ground - steel hull

you definatly don't want to be using the hull to pass current... you should have had a wire run.

weither you bond the steele hull or not is a different matter.
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Old 21-02-2014, 09:14   #4
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Re: DC ground - steel hull

Despite the paranoia, passing current through the steel hull does no harm. Electrolysis needs a voltage across 2 submerged electrodes. Current passing though the hull has only ONE electrode, there is nothing else for it to flow to.

The voltage differential from point to point through the hull will be less than 1/1000 a volt and spread out over say 10 feet is 0.0001 volts per foot which is less than all the other voltage potentials floating around in the water.

For ABYC safety compliance the negative has to be connected to the hull anyhow so you may as well use it as one of the conductors.

Even if your starter current flows through the hull (which is unlikely) it only lasts a few seconds so even 1/10 volt over 10 feet for 20 seconds per day is not going to cause any problems.
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Old 21-02-2014, 12:05   #5
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Re: DC ground - steel hull

I would strongly disagree with Andina, I would recommend a floating DC system with no hull connection , same for AC. ABYC is for GRP boats and has little relevance for steel. The problem is not hull currents, but the potential for the hull to cause connection of dissimilar metals and and hence impressed current corrosion.

oh and an isolation transformer on the AC input.

Michael Kastens site has a good summary of the points.

Dave
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Old 21-02-2014, 12:41   #6
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Re: DC ground - steel hull

Quote:
Originally Posted by goboatingnow View Post
I would strongly disagree with Andina, I would recommend a floating DC system with no hull connection , same for AC. ABYC is for GRP boats and has little relevance for steel. The problem is not hull currents, but the potential for the hull to cause connection of dissimilar metals and and hence impressed current corrosion.

oh and an isolation transformer on the AC input.

Michael Kastens site has a good summary of the points.

Dave
Dave, that was not the question. As far as electrolytic corrosion is concerned that's fine if you can isolate the hull from the DC negative but it is usually extremely difficult and may not pass inspection to ABYC standards.

The original question was about passing DC current through the hull, not about isolation with Galvanic Isolators or transformers. Obviously if you decide you want the hull isolated it would not be possible to pass current through it but that was not the question.

My comment stands, the differential voltages caused by current flowing from point to point through the hull are negligible and can be ignored.
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Old 21-02-2014, 13:06   #7
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Re: DC ground - steel hull

MY AC comments are entirely secondary. But if asked about DC, I would say that best electrical practice is to isolate DC from the hull as far as practical, this should include isolated alternators, starters and senders.

Oh course in practice, while this is commonplace on good aluminium vessels, its less common on steel. However that does not negate the principle.

Connecting the hull to DC negative, facilities subsequent impressed corrosion through wet bilges and other inadvertent connections.

I fully agree that the differential voltage across the hull plating are not generally an issue, though resistance at welds can be an issue in high currents, though not applicable here

As to ABYC, I would argue it is not applicable to steel vessels in the main

Michael Kasten also agrees saying

"Keep in mind though that the ABYC rules represent the consensus of the US Marine Manufacturers Association, and are therefore primarily aimed at satisfying the requirements aboard GRP vessels,"

dave
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Old 21-02-2014, 13:20   #8
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Show me a case were a steel hull has rotted away from using the hull as a dc ground.
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Old 21-02-2014, 15:28   #9
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Re: DC ground - steel hull

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Originally Posted by d design View Post
Show me a case were a steel hull has rotted away from using the hull as a dc ground.
All steel hulls rot away from any number of issues.

Dave
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