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  Stray current corrosion (davidww1)
Posted: 12:59:21 pm on 4/5/2014 Modified: Never
 
This is a followup to a comment on another thread, referring to severe corrosion at Burlington. I don't want to hijack a re-powering thread so I'm starting this. Bear in mind that I'm not an expert, but I've heard this story fairly consistently, so I put some credence in it.

Background: http://www.boatus.com/boattech/articles/marine-corrosion.asp

Stray current corrosion was much less of a problem until the CE certification changed the way boats are wired. In the past, boats with AC power ran all three wires, black, white and green, back to the receptacle on the dock. If the receptacle was correctly wired (see the little yellow gizmo at the bottom of <http://www.cc27association.com/fixes/sparestools.html > for a cheap way to find out), power was completely isolated from the boat, even if there was a fault and power flowed to the green wire. CE now demands that the boat's DC and AC circuits share their ground, so any leakage to ground (green) on the AC side can flow to the boat side, which also can flow to the surrounding water. All of a sudden, the water is a little bit, or a lot, hot.

Current flowing to ground, if sufficient, will paralyze a swimmer, who will then drown. Small amounts cause corrosion to less noble metals that act as a ground (which could be sail drives, through-hulls, props, magnesium or zinc anodes, etc.) Somewhat counter-intuitively, the problem is worse in fresh water than salt, as current is more likely to dissipate in the good conductive medium of salt water.

Some boats are made 'hot' by the owner's or a dumb tech's fiddling, but the addition of a badly wired appliance can do the trick - a good boat can suddenly become hot, either kill-a-swimmer hot or corrode-the-neighbouring-boat hot depends on how defective.

My club has had a bad stray current problem - people were spending thousands getting parts fixed or replaced that clearly had been damaged by stray current corrosion. Also some of the sources were well known - boats that constantly tripped their GFI's. Nevertheless, the club elected to rewire the docks entirely. We'll see if that fixes it, but I suspect that the club will have to be vigilant about people making unwise or unauthorized changes.

All this is leading up to the suggestion that if you find that your zincs or magnesium anode(s) are eroding faster than you remember, you probably have a stray current problem. I won't tell you how to fix it, because 1) I'm not an expert; and 2) a lot depends on the politics and general responsiveness of your club or marina. I will suggest, though, that your neighbours may be having the same problem and not really noticed it. A bit of education followed by group grousing might help.



David Weatherston
Towser, Toronto
C&C 27 Mk IV

  Re: Stray current corrosion (Jessy)
Posted: 12:09:17 pm on 4/8/2014 Modified: Never
 
I agree with what you are saying David. When I purchased my boat it was moored in a prominent Victoria BC marina. I went through my zincs in less than 1.5 months - no nasty wiring on my boat! I have since moved my boat to Anderson cove, on its own mooring buoy - My 8 month old zincs are still in good shape! This is a testament to what you are stating. I don't want to stir up a hornets nest, but I will not be mooring my boat in a marina like that again if I can help it.

Peter Lerch
1972 MK I
Sooke, BC