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Old 21-03-2014, 06:52   #1
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Isolating steel hull from the electrical circuit

Hi there,

I'm working on my electrical sytem and have just relocated the batteries. In the old setup the negative of the battery was connected to the steel hull, as was the engine block. I would like to just close the circuit without using the steel hull as a conductor, but the whole situation is sort of confusing me atm. I've made the following sketch from what I can remember:



Does this sketch make sense at all? Is removing the steel hull from the circuit as easy as lengthening the cable coming from the engine block and just attaching it to the relocated batteries? I'll be at the boat all weekend, so I can further comment on the situation once I get there. If at all possible I'd like to tackle the job this weekend though...

Thanks!
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Old 21-03-2014, 08:03   #2
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Re: Isolating steel hull from the electrical circuit

If you are trying to isolate the hull for electrolysis reasons it can be done but it is quite difficult, requiring insulating engine mounts and an insulating coupling to the prop shaft which can be done with a rubber flexible joint. But it also means all the engine controls must be isolated from the hull and finding every possible connection to the hull can be daunting. Things like gauges and idiot lights may all have to be re-wired if they currently use the hull for a ground.

If you are trying to isolate the hull from the electrical circuit to prevent current flowing through it, it is not necessary. Current flowing through the hull even at starter motor currents causes only a few thousandths of a volt differential and spread out over that distance it will have no electrolytic consequences.

Connecting a heavy copper cable from battery to engine block will not produce any significant improvement over the current (pun not intended but good) circuit.
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Old 21-03-2014, 08:21   #3
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Re: Isolating steel hull from the electrical circuit

I redid all wiring and nothing uses the hull as a negative atm. It's just the bloody engine . The enginecontrols won't pose an issue either, but the mounts may (they're newly replaced, but I don't know if they have a bolt right through them connecting engine and hull.

I also relocated the batteries, which is one of the reasons for changing the current setup. If I was to keep the hull as a conductor in the circuit, that would mean having to weld a bolt onto the hull at the new battery location in order to attach the battery negative to the hull again. Since the new location is close to the engine itself, I'd like to avoid that and just go directly with a piece of salvaged battery cable .
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Old 21-03-2014, 08:43   #4
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Re: Isolating steel hull from the electrical circuit

On my previous (steel) boat the negative was connected to the engine. It had flexible mounts and the ground was thru the shaft (without the drive saver) just like a glass boat. If you have a drive-saver (rubber connecting the shaft to engine coupling) the shaft is also isolated.
I would remove the connection to the hull and leave only the one to the engine.
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Old 21-03-2014, 09:38   #5
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Re: Isolating steel hull from the electrical circuit

Lots of people miss the negative ground connection of the VHF antenna on the mast.
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Old 21-03-2014, 21:31   #6
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Re: Isolating steel hull from the electrical circuit

An isolated return system, will have a bond between hull and neg bats.

If you have properly wired the on-board power system with a true isolated return, then no current need flow between the bats and the hull.

The cable between the bats neg bus and the hull are in fact just a bonding cable. That means by design no current should ever flow in this bonding situation. It is only there in case of fault.

And to assure that the ground potential of all metals are the same.

Lloyd

Quote:
Originally Posted by Orchidius View Post
I redid all wiring and nothing uses the hull as a negative atm. It's just the bloody engine . The enginecontrols won't pose an issue either, but the mounts may (they're newly replaced, but I don't know if they have a bolt right through them connecting engine and hull.

I also relocated the batteries, which is one of the reasons for changing the current setup. If I was to keep the hull as a conductor in the circuit, that would mean having to weld a bolt onto the hull at the new battery location in order to attach the battery negative to the hull again. Since the new location is close to the engine itself, I'd like to avoid that and just go directly with a piece of salvaged battery cable .
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Old 21-03-2014, 23:55   #7
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Re: Isolating steel hull from the electrical circuit

Quote:
Originally Posted by Orchidius View Post
I redid all wiring and nothing uses the hull as a negative atm. It's just the bloody engine . The enginecontrols won't pose an issue either, but the mounts may (they're newly replaced, but I don't know if they have a bolt right through them connecting engine and hull.
There are two approaches with metal boats. Ground everything at one and one only point, or use a completely isolated negative system.
I understand you are doing the later and I believe it is superior.

Isolating the engine is quite easy. You need:

An isolated alternator. (many are as standard, or can be converted)
Dual return senders (water temp and oil)

Rather than an isolated starter which is difficult to source) most boats use a large solenoid and briefly connect the negative to the engine during the time the starter motor is running.

When the engine is running it is electrically connected to the hull, but not connected to battery negative. There is not any battery negative connection to hull other than the few seconds starting the engine.

It is important with this system to ensure isolation, dual CB are ideally fitted disconecting both the positive and negative side of unused circuits.
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