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Old 24-01-2015, 15:09   #1
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DC ground with a flexible coupling on the drive shaft

We have a problem where our boat is not properly DC grounded. Our DC negative bus is wired to the engine, but the drive shaft has a flexible coupling which prevents the ground path to the water.

The wire size to the engine is 1/0, but that size wire will not work to make a bridge over the coupling due to the clearance of the the transmission.

I thought about using a solid piece of copper, but getting it cut correctly would be a pain and I'm not sure what gauge would be required.

Does anybody have any knowledge of a collar with brushes that could be used to ground to the shaft? Something that can go on without taking the shaft out.
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Old 24-01-2015, 15:28   #2
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Re: DC ground with a flexible coupling on the drive shaft

What type of coupling do you have? Most of the common ones provide a strap for continuity if wanted. If not, all you need is a short piece of strip copper to bridge two offset bolts on the insert. No real machining required - just an inch or so strap with holes drilled in it the size of the bolts.

You are way over-thinking this with a collar and brushes. I bet your hardware store carries copper hanging straps for a buck that will work and already have the holes in them - just cut off a piece the right size so that the existing holes line up with any two offset bolts on the coupler.

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Old 24-01-2015, 15:29   #3
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Re: DC ground with a flexible coupling on the drive shaft

the engine wire size has nothing to do with it. generally the shaft is bonded to the boat bonding system with a shaft bush. with 8awg green.
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Old 24-01-2015, 15:49   #4
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Re: DC ground with a flexible coupling on the drive shaft

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Originally Posted by ReMetau View Post
We have a problem where our boat is not properly DC grounded. Our DC negative bus is wired to the engine, but the drive shaft has a flexible coupling which prevents the ground path to the water.

There isn't anything "not proper" with this and you really don't need to do anything. DC ground has nothing to do with connecting the shaft/sea/DC circuit and DC grounds to your battery.

BTW - you really wouldn't want the current to go though the engine to the shaft into the sea if you want your zincs and prop to stay around..
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Old 24-01-2015, 17:05   #5
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Re: DC ground with a flexible coupling on the drive shaft

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Originally Posted by colemj View Post
What type of coupling do you have? Most of the common ones provide a strap for continuity if wanted. If not, all you need is a short piece of strip copper to bridge two offset bolts on the insert. No real machining required - just an inch or so strap with holes drilled in it the size of the bolts.

You are way over-thinking this with a collar and brushes. I bet your hardware store carries copper hanging straps for a buck that will work and already have the holes in them - just cut off a piece the right size so that the existing holes line up with any two offset bolts on the coupler.

Mark

You are right. I was way over thinking it. I decided to go with 1" flat braided copper for ease and flexibility.
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Old 25-01-2015, 06:34   #6
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Re: DC ground with a flexible coupling on the drive shaft

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You are right. I was way over thinking it. I decided to go with 1" flat braided copper for ease and flexibility.
Braided copper can corrode very quickly in that environment. Check it often. You might want to consider tinned braid, stainless braid or solid strap instead.

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Old 25-01-2015, 06:45   #7
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Re: DC ground with a flexible coupling on the drive shaft

If you have a piece of marine grade co-ax cable use this with correct size ring terminals. The wire does not carry high current so large size braid is not necessary.
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Old 25-01-2015, 06:50   #8
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Re: DC ground with a flexible coupling on the drive shaft

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... Does anybody have any knowledge of a collar with brushes that could be used to ground to the shaft? Something that can go on without taking the shaft out.
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Old 25-01-2015, 09:55   #9
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Re: DC ground with a flexible coupling on the drive shaft

My flexible coupling came with a metal spring that gave electrical continuity between the transmission output flange and the flange on the prop shaft. It was to be positioned in the center hole of the flexible coupling.

When I installed the flexible coupling during an engine/propshaft alignment session with my local boat mechanic/electrician, he said I didn't need it. So it wasn't installed.

I don't know, but I know I haven't had any electrical problems since the installation of the flexible coupling two seasons ago.
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Old 25-01-2015, 11:25   #10
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Re: DC ground with a flexible coupling on the drive shaft

You do not need any external DC ground. It is irrelevant electrically and could cause corrosion problems. The batt- IS the DC ground. If you have AC you Do need an external ground but the regs vary. In Europe it must not be connected to the DC neg, in USA it should be. prefer separate AC and DC grounds mainly because if the ground poit corrodes and breaks it is possible to get a fault which makes all you DC kit AC live. Can also lead to radio noise. I use tow bronze bars through bolted to the hull, one for AC ground, one for radio ground.
PS the starter neg normally ground the engine block for DC. Paralleling grounds can cause RF noise through ground loop oscillations.
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Old 25-01-2015, 15:54   #11
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Re: DC ground with a flexible coupling on the drive shaft

I have seen braided copper method, have also seen slip rings, think all large vessels do that.

The idea is that hull anodes could then protect the propeller, without electrical current through the bearings/shaft coupling that may do electrolytic damage to the bearings.

If you have a shaft anode, that should provide anode current that will be contained by the propeller shaft pair and give you shaft protection separate from hull.

Nothing wrong with the braided method, ideally it will remain redundant.

Lightening protection for insulated hull also need consideration, I wouldn't want to use shaft to take it all, but better than having nothing at all, (bang).
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