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Old 10-18-2018, 03:15 AM   #1
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Stray current and engine grounding.

Had sparkies onboard last week checking us for stray current as on the last slip we had a bit of blown paint due to a poor grounding plate and connection.

They suggested we fit one of these when out which i did , redid the grounding wire and they said its better to give the boat some time to settle down before rechecking.



On the check it was noted a slight discrepancy between the rudder and the engine and grounding them was recommended.
After a look around they said running the ground for engine off of alternator negative was as good a spot as any but now I am second guessing them.

I have the gear now and i am running a hefty tinned cable from rudder post to ground plate and from engine to ground plate.

Instead I was going to use a cleaned up bolt sticking out of the block, as one usualy does when connecting earth to engine, theory being that the alternator , due to what it does, may have more chance of actually introducing current to the water than a bolt on the engine.
Plus I feel a bit uneasy about having an integral part of the engine wired up in this way.

Thoughts?
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Old 10-18-2018, 06:15 AM   #2
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Do you have anodes (zincs) on the rudder and shaft? How long do they last?

Ted
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Old 10-18-2018, 06:42 AM   #3
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Do you have anodes (zincs) on the rudder and shaft? How long do they last?

Ted
Guessing we are over zinced.
I reckon the previous owners were adding them and adding them and adding them over the past 40 years.

Replaced them all when we got her.
Have something silly like 6 a side on the keel shoe, 6 on the nozzle and one on the rudder.
Last lift one on the nozzle and one on the nozzle were gone, no nuts remaining.
Most of the remainders were 75% or more good after 2 years.

Sparkies advised against replacing the missing ones.
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Old 10-18-2018, 04:15 PM   #4
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I used to have a steel boat (20 years ago), so the memory is fuzzy. From memory the negative battery cable bolt on the block was where I had mine. Most of the points where the engine or gear touched the hull are electrically insulated or weren't designed for electrical continuity. Unless there is a dedicated negative cable attached to the alternator, my choice would be the above, or another bolt on the same part.

Ted
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Old 10-18-2018, 05:28 PM   #5
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A couple of comments. Firstly, people such as Nigel Calder warn against over-protection (with anodes) of wooden boats. Apparently it can cause severe alkali attack and destroy the wood.

Secondly, (Calder again) an isolated ground alternator is preferable to grounding it via the engine. This avoids high currents running through the engine when the alternator is charging, potentially increasing stray current corrosion.
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Old 10-18-2018, 09:00 PM   #6
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Whoo! Thanks for making that point, Brian. I need to re-read that book!.
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Old 10-18-2018, 09:33 PM   #7
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I have that book somewhere....

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Old 10-19-2018, 04:52 PM   #8
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A couple of comments. Firstly, people such as Nigel Calder warn against over-protection (with anodes) of wooden boats. Apparently it can cause severe alkali attack and destroy the wood.
.
Thanks Brian, we know about the to many anodes issue and there is "some" evidence of wood damage but apparently minor given the age of vessel and thickness of timbers used.

To put it another way, timber guys who worked on it seemed to think it wasn't bad and they had seen plenty of others boats, some in survey that are a hell of a lot worse.

Its taken 40 years to get where it is and the sparkies said the numbers are in acceptable levels or at least will be when the engine and rudder are connected to the earth plate.
Nothing to worry about so they say.

Quote:
Secondly, (Calder again) an isolated ground alternator is preferable to grounding it via the engine. This avoids high currents running through the engine when the alternator is charging, potentially increasing stray current corrosion
Having a read up on this now

Thanks
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