new to steel boats #2 neg ground - SailNet Community

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Old 09-03-2011
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new to steel boats #2 neg ground

So I noticed on my 'new' steel boat that the PO has wired up the windlass with a single positive cable, and the Windlass grounded to the hull.

is it kosher to use the hull itself as part of the circuit?
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Old 09-03-2011
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Old 09-03-2011
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There should be no connection between the negative and the hull - it is a surefire way to create a huge problem. Total isolation is the best. On many steel (or aluminum) boats the entire DC system is switched with double pole breakers so that when off there is no chance of a leak through a chafed negative.
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A boat is not a car and should not be wired like one. If the windless is wired to the hull I would be concerned of what else is.
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Thanks everyone. I kind of thought it was hinky but was unsure.

BUT... how the heck do you isolate the Negative completely when the engine block is grounded?

I can certainly see not using the hull as a short-cut for a circuit, but it is going to be grounded and bonded to both AC and DC isn't it?
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Should not on a steel boat. If the engine has a flexible coupling the shaft and prop are isolated. Alternators and starters are available without the case as part of the ground system. If this is done the engine block isn't part of the ground.

The other solution is to isolate the engine circuits except when the engine is running and this is easier. When not using the engine both positive and negative are isolated with 2 separate switches.

Any terminal blocks or switches can be attached to a material like plastic and it attached to the hull for isolation.
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Google "galvanic corrosion" "noble metals" and "zinc protection"
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What's wrong with having the hull being a common point of a circuit (the ground)?

The only time this would ever cause trouble is when connected to shore power. You can use an isolation transformer to isolate the shore power completely. I guess the other option is to isolate the whole hull. But all it takes is one slip up. One metal device that is plugged in and has its grounded chassis touching part of the boat will ground the whole boat to the shore power ground.

You could isolate all the 120v devices on the boat. Then make sure that the device which connects the hull grounded 12v system with the 120v system -- the battery charger -- is isolated. It should have its own internal transformer so isolating it should be easy.

When shore power is plugged in and the grounded hull is hooked to shore ground and shore neutral there may still be a difference in ground voltages by a few volts. That can cause problems with your hull since some current is returning through the water instead of back through the cord.

You can always measure the voltage between your hull and the shore neutral and ground lines. I'm not sure which voltage level starts to become harmful.
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Quote:
Originally Posted by steel View Post
What's wrong with having the hull being a common point of a circuit (the ground)?
The problem with the sort of wiring mentioned in the OP is that the various ground connections will have a different potential. This is ideal for stray current corrosion which can be a very rapid.
Its a very poor idea and should be changed.
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Steel

By grounding to the hull you are providing a way for stray DC current to flow through the hull. Corrosion will result between the anode of the hull and any cathodic metal nearby, with the hull being eaten away. Shorepower is not the only cause of galvanic corrosion.
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