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Old 01-09-2010, 15:50   #1
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Prop Zinc Is Gone and Now I Have White Spots on the Prop

I'd like advice on my situation. I had a new prop installed in March. I recently cleaned the hull and checked the prop and the prop zinc is gone. There are two prop shaft zincs, one of which is half gone. Is this a normal amount of time to have to replace the zincs ?

There are also tiny white spots on the prop which look like tiny little donuts and are rough to the touch. Is this corrosion or some sort of marine growth ?

Thanks

-tavis
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Old 01-09-2010, 16:01   #2
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Sounds like your zincs are ok. Am I reading it right that you have a prop zinc plus two shaft zincs? The rough spots on the prop might be what's left after barnacles have been scraped off.
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Old 01-09-2010, 16:09   #3
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I started out with a prop zinc and two shaft zincs. The prop zinc has since either fallen off of corroded down to nothing.
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Old 01-09-2010, 16:11   #4
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Our new zincs disappeared in 2 months this year, and never would we have thought this would be possible. We were in the Chesapeake and Potomac and only found out they were gone when we had our girl put on the hard. We're having our electician install a galvanic isolator (sp?) since we've been told shore power can hasten the demise of zincs. The thing is; we were only on shore power about once a week during that time!!! Unfortunately we are completely ignorant of all things electric and hope we're headed in the right direction. Nigel Caulder is an expert and has published many books. I suppose could start there,
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Old 01-09-2010, 16:14   #5
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If you still have one and a half zincs on your shaft I would think this would protect your prop too. By the way there is no "normal" time for zincs to last. A lot depends on your boat and the marina and the boats around you. Mine last about four or five months but I am not in a marina, just anchored or sailing.
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Old 01-09-2010, 16:59   #6
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There is an electrical problem somewhere close to you. The boat next to you, your boat, or the marina power. I would bet a wad of cash that you are in a marina...correct? You need to check your galvanic isolator (hopefully you have one).

The spots on your prop are corrosion.

Is this a new marina slip for you?

Did someone new just move in next to you?

Why did you have a new prop installed? Eaten up with corrosion?

Your shaft may not be bonded.

Be sure to follow up and let us know what you find
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Old 04-09-2010, 21:25   #7
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The reason for anodes on shafts and props in a fully bonded system is the shafts do not make a good electrical contact with the system. For example, the bonding process between the common ground and the shaft is made through the transmission via gears covered with lubrication oils.

I long ago gave up on shaft anodes! Not foolishly but by building shaft brushes (they can be purchased, you need not build your own) and installing them. The brushes which are bonded to the common grounding system make electrical contact with the shafts thereby helping to maintain continuity in the bonding.

Most power boats use too many anodes anyway. I used no anodes on my old Hunter sailboat and after 25 years, there was no degradation of any kind on the external metals. In my Silverton 30, I use one (1) aluminum anode plate mounted to the stern. I also own and use a silver-silver chloride half cell to measure the voltages beween various bonded points and the half cell. THAT IS THE ONLY CORRECT WAY I KNOW TO ENSURE PROPER PROTECTION IS HAD AGAINST GALVANIC CORROSION.

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Old 05-09-2010, 05:25   #8
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Very rapid zinc loss that results in bright, shiny metal being exposed is a clear indication of electrical activity, be it galvanic or stray current, usually the later, since galvanism rarely creates enough current to destroy zincs quickly .

See David Pascoe’s Corrosion Primer:

1 ➥ Corrosion: Buying, Owning and Maintaining Boats and Yachts
2 ➥ Corrosion: Buying, Owning and Maintaining Boats and Yachts
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Old 05-09-2010, 06:23   #9
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GordMay, thanks for the links. That was good reading!
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Old 05-09-2010, 07:33   #10
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Replace them....

You reminded me. I have to replace mine every year. I'm on a mooring and the one on the shaft seems to go quickly while the two on the plate under the prop are 1/3 gone. I presume that means they are doing the job. BOAT...Bring Out Another Thou$and!
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Old 05-09-2010, 08:43   #11
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Originally Posted by GordMay View Post
Very rapid zinc loss that results in bright, shiny metal being exposed is a clear indication of electrical activity, be it galvanic or stray current, usually the later, since galvanism rarely creates enough current to destroy zincs quickly .

See David Pascoe’s Corrosion Primer:

1 ➥ Corrosion: Buying, Owning and Maintaining Boats and Yachts
2 ➥ Corrosion: Buying, Owning and Maintaining Boats and Yachts
I read this, and it is fascinating, but there were a number of statements in it which put me on my guard:

1. "All forms of galvanism involve metal, but all metals don't look like metals. Carbon is a metal . . . "

Carbon is a conductor of electricity, but it is definitely not a metal.

2. "We attach pieces of zinc to the underwater metals of boats to protect those metals. What actually happens is that the zinc reverses the normal flow of current between dissimilar metals. The zinc will emit current that raises and equalizes the electrical potential of all the metals in the system. It does this by releasing electrons, which are positively charged ions of the metal itself. This causes the zinc to erode and disappear. These ions will attach themselves to the other metals, which explains why your props and other metals may end up with a rough, scaly surface; they've become covered with zinc oxide"

Whoa. Reverses the normal flow of current? How is that?

And it does so by "releasing electrons, which are positively charged ions of the metal itself". This is either some kind of massive typographical problem, or utter nonsense. Electrons of course are not ions, nor can they be positively charged, except perhaps in an anti-matter universe I suppose.

3. "Graphite and carbon bottom out the list, being the most highly charged metals."

Graphite and carbon, eh? Who would have guessed both of them?

4. "The conditions created by AC and DC current is not the same, with DC current being the most damaging. The reason for this is that AC current is pulse current that moves in two directions, greatly reducing the corrosion potential for reasons I won't get into here."

"Pulse current" -- is that a technical term? "Reasons I won't get into here . . ." Methinks the author does not really understand cathodes and anodes.


I'm sure that the practical advice is sound, but the theory really had me scratching my head . . .
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Old 05-09-2010, 08:58   #12
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I know it does not seem right, but carbon is a metal.

'A metal is a chemical element that is a good conductor of both electricity and heat and forms cations and ionic bonds with non-metals. In chemistry, a metal (Ancient Greek métallon, μέταλλον) is an element, compound, or alloy characterized by high electrical conductivity'. ...
en.wikipedia.org/wiki/Metal

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Old 05-09-2010, 09:05   #13
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Originally Posted by Dockhead View Post
I read this, and it is fascinating, but there were a number of statements in it which put me on my guard:

1. "All forms of galvanism involve metal, but all metals don't look like metals. Carbon is a metal . . . "

Carbon is a conductor of electricity, but it is definitely not a metal.

2. "We attach pieces of zinc to the underwater metals of boats to protect those metals. What actually happens is that the zinc reverses the normal flow of current between dissimilar metals. The zinc will emit current that raises and equalizes the electrical potential of all the metals in the system. It does this by releasing electrons, which are positively charged ions of the metal itself. This causes the zinc to erode and disappear. These ions will attach themselves to the other metals, which explains why your props and other metals may end up with a rough, scaly surface; they've become covered with zinc oxide"

Whoa. Reverses the normal flow of current? How is that?

And it does so by "releasing electrons, which are positively charged ions of the metal itself". This is either some kind of massive typographical problem, or utter nonsense. Electrons of course are not ions, nor can they be positively charged, except perhaps in an anti-matter universe I suppose.

3. "Graphite and carbon bottom out the list, being the most highly charged metals."

Graphite and carbon, eh? Who would have guessed both of them?

4. "The conditions created by AC and DC current is not the same, with DC current being the most damaging. The reason for this is that AC current is pulse current that moves in two directions, greatly reducing the corrosion potential for reasons I won't get into here."

"Pulse current" -- is that a technical term? "Reasons I won't get into here . . ." Methinks the author does not really understand cathodes and anodes.


I'm sure that the practical advice is sound, but the theory really had me scratching my head . . .
Yours is a good post! I always have trouble reading Pascoe's writings. I get further troubled knowing that many who do read his books believe they understand the mechanics of his theories.

Last--- there are many marine growths that leave "little white spots" on props.

Foggy
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Old 05-09-2010, 09:08   #14
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This should clear it up...Periodic table (metals and non-metals) - Wikipedia, the free encyclopedia

Carbon is a non-metal but right at the metaloid/non-metal borderline. Non-metals can also be conductors of electrons, such as we see with Carbon.
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Old 05-09-2010, 09:12   #15
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Yours is a good post! I always have trouble reading Pascoe's writings. I get further troubled knowing that many who do read his books believe they understand the mechanics of his theories.

Last--- there are many marine growths that leave "little white spots" on props.

Foggy
x 2.

Lil' different from what I understood in the Physics lab, Jus sayin
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