Stray current / galvanic current corrosion

PK123

PK123

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Oct 5, 2022
33
Mirage 27 Crescent Beach Marina
April 1st , I fitted some zincs to my prop shaft and put the boat in the water with a new prop. Previous owners had let their prop perish and was pinked and so replaced at that time.

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Early May, I fitted a galvanic isolator to protect against stray current especially as I am next to a distibution panel for a boat house .

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Early June my Zincs have all but fizzed away.

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It seems incredibly aggressive corrosion given the short space of time (about 8 weeks) in the water - Maybe about 5 weeks or so between the boat being launched and the galvanic isolator being fitted. All underwater fittings are bonded.The prop was quite covered in barnacles as was the shaft - from what I understand , another indicator of galvanic current. Just wondering if this is more to do with stray current before I hooked up the isolator?

I’ve just refitted the zincs to see what the next couple of months look like but would welcome any thoughts or checks that I could make . I’m thinking if haul her out again and painting the prop with GlideCoat Prop Optimiser which ive heard good reports - any thoughts?

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bonding system is currently wired to both keel bolt and engine block
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Don S/V ILLusion

Don S/V ILLusion

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Sep 25, 2008
7,251
Alden 50 Sarasota, Florida
hard to give a definitive answer via the internet.

Look for the most common causes, I.e., bilge pump wiring, bad bonding wire connections, etc… which seem to be the most common causes. 2 months indicates a severe problem.
 
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Ralph Johnstone

Ralph Johnstone

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Jan 4, 2006
6,940
Hunter 310 West Vancouver, B.C.
PK123 said:
I’m thinking if haul her out again and painting the prop with GlideCoat Prop Optimiser which ive heard good reports - any thoughts?
Click to expand
That will do absolutely nothing to protect your prop against stray currents.

PK123 said:
The prop was quite covered in barnacles as was the shaft - from what I understand , another indicator of galvanic current.
Click to expand
Just an old wive's tale. Every so often I'll find the prop taken over by barnacles but I've never had problems with attacks on the zincs.

Sorry but you're going to have to remove the source of current. Until then, you might consider hanging a well connected zinc plate over the side. Don't bother with all the expensive crap for connecting the interior shaft to the wire from the overboard zinc. Check the continuity between the engine block and the shaft. It it's "0" connect the block to the wire while at the dock. If it's above "0", correct the discontinuity.

This will give you some breathing room while you search for the problem.
 
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Stu Jackson

Stu Jackson

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Feb 26, 2004
22,894
Catalina 34 224 Maple Bay, BC, Canada
Nigel Calder's Boatowners "bible' has very good information about this subject and the troubleshooting necessary to find the cause(s) and resolve them. Like most things, requires you to do some more research and homework. Good luck, glad you checked.
 
BigEasy

BigEasy

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Jun 21, 2004
2,652
Beneteau 343 Slidell, LA
PK123 said:
I’ve just refitted the zincs to see what the next couple of months look like but would welcome any thoughts or checks that I could make .
Click to expand
Agree with everyone, that prop paint is not going to make any difference with galvanic or stray current corrosion. Suggest you purchase a corrosion reference electrode to test your hull potential (without any protection) and retest after adding zincs & isolator to ensure that your boat is adequately protected. Can always add a supplemental drop zinc if necessary. As the zincs become depleted, you can retest to determine the level of protection that remains. If you suspect that an accessory onboard is causing a problem, you can test that also. Considering the amount of damage that stray current & galvanic corrosion can cause ($$$), the test electrode cost is well worth the expense.
 
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