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Old 15-02-2015, 12:11   #1
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The extreme end of electrolysis

I was recently shown this sail drive by a mechanic. It and it's mate were installed brand new on a catamaran. Three weeks later, after sitting at a mooring with a stray wire on the ground strip, this is what they looked like. Rather remarkable what can happen in three weeks. Don't underestimate the power of electricity to ruin your day.

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Old 15-02-2015, 12:21   #2
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Re: The extreme end of electrolysis

Holy crap!

That's the aluminum portion that dissolved, I assume!?
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Old 15-02-2015, 12:24   #3
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Re: The extreme end of electrolysis

I didn't take a close look at it, just snapped the pic. Evidently the owner was a DIY electrician.
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Old 15-02-2015, 13:28   #4
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Re: The extreme end of electrolysis

Thus can saildrives ruin your day. Friends had one eaten by one day in a marina with a faulty ground.

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Old 15-02-2015, 13:42   #5
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Re: The extreme end of electrolysis

The aluminum went first because it was the less noble metal. But if you wired the boat that wrong you would be replacing the shaft and prop on a shaft drive boat. I've seen it happen.


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Old 15-02-2015, 13:47   #6
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Re: The extreme end of electrolysis

Wow!

Thanks for posting this photo showing the fast damage. Amazing! Scary!
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Old 15-02-2015, 15:04   #7
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Re: The extreme end of electrolysis

Nasty!
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Old 15-02-2015, 15:58   #8
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Re: The extreme end of electrolysis

By "sitting at a mooring" did you mean in the absence of shore power? Or was it in a marina?
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Old 15-02-2015, 16:08   #9
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Re: The extreme end of electrolysis

Quote:
Originally Posted by Dockhead View Post
By "sitting at a mooring" did you mean in the absence of shore power? Or was it in a marina?
I too would like a better description of the failure mode!

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Old 15-02-2015, 17:47   #10
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Re: The extreme end of electrolysis

This wasnt electrolysis. Electrolysis is a slow erosion of metal as a result of differences in galvanic potential, and is normally protected against with sacrificial anodes. This photo depicts stray current corrosion, where DC current either from within or from outside the vessel flows through the component.

Obviously fatal to aluminum saildrives, but can also turn bronze props into swiss cheese.

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Old 15-02-2015, 19:27   #11
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Re: The extreme end of electrolysis

DC corrosion can happen very quickly with a bad leak....
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Old 16-02-2015, 11:30   #12
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Re: The extreme end of electrolysis

Quote:
Originally Posted by DougR View Post
This wasnt electrolysis. Electrolysis is a slow erosion of metal as a result of differences in galvanic potential, and is normally protected against with sacrificial anodes. This photo depicts stray current corrosion, where DC current either from within or from outside the vessel flows through the component.

Obviously fatal to aluminum saildrives, but can also turn bronze props into swiss cheese.

DougR
Indeed, I misspoke.

As for clarification regarding the location of the boat, all I know it was "away from the dock, at a mooring" and that the culprit was the boat's own electrical, but I couldn't tell you more than that.
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Old 16-02-2015, 12:00   #13
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Re: The extreme end of electrolysis

coomon problem with sail drives and IO's needs a galvanic isolator
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Old 16-02-2015, 12:09   #14
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Re: The extreme end of electrolysis

Same thing happened to my sail drives in 2007. Turned out that the shore-power battery charger had gone bad. That's scary unto itself as there were no signs other than the "missing" saildrives.

I installed a galvanic isolator and have had no electrolosis/galvanic issues since..

One of mine is hanging on the wall in Bay Shore Marine at Burt Jabins.
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Old 16-02-2015, 12:13   #15
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Re: The extreme end of electrolysis

Quote:
Originally Posted by Maine Sail View Post
DC corrosion can happen very quickly with a bad leak....
But it has to be DC, right? Can't be AC?
So why do I hear all the horror stories about bad shorepower pedestals in Marina's etc.?
Is that a bunch of nonsense?
How can a neighbors boat be the problem, I've heard that for years, is it true?

On edit, if it can't be AC, then why do I have an isolation transformer isolating my boats ground on the AC side from shorepower?

I'm not doubting, but I am confused.
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