Shaft Anodes

pgtips

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Sorry if this is a daft question, but boat is out of the water at the moment for anti-fouling & stuff and I'm replacing the anodes, which includes one on the prop shaft.

The question I have is, does the shaft below the anode need to be free of paint for it to work properly?

Thanks in advance

Paul
 

rob2

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Yes, ideally. At least, the purpose is to make as good an electrical connection as possible between shaft and anode when you clamp it up - so paint isn't going to help!
 

Ruffles

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Since noone else has responded.

Shaft anode needs electrical contact with the shaft to work.

BTW its generally considered a bad idea to paint a stainless steel prop shaft since it needs exposure to oxygenated water to avoid corrosion. I made the same mistake and was ticked off.
 

pgtips

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Thanks for this. I did think this was the case, but wasn't sure and knew someone here would reply.

Didn't know about not painting the whole shaft though! I had stripped it back to bear metal and was going to re-paint it, as the previous owner had done, but perhaps I shouldn't??
 

bryeomans

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Tip - if you haven't already gone back in - before tightening up the anode around the shaft wack it with a hammer to bed it firmly on to the shaft and then tighten up the clamp screws-that way you should not lose it in five minutes of running. My marine engineer told me that one but I imagine you have to be careful not to bend the shaft!
 

MarkGrubb

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for piece of mind, if you have a multimeter, then check for a good electrical connection between the zinc anode and prop. after you've fitted it.
 

VicS

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[ QUOTE ]
Tip - if you haven't already gone back in

[/ QUOTE ] Tighten the bolts first, hold a heavy weight against one half of the anode to absorb the shock then hit the other half of the anode with a decent sized hammer Then tighten the bolts some more.

Don't whack it without the "dolly" to absorb the shock or you might damage the shaft/ bearing /gland/ P-bracket and it wont have the same effect anyway. Use the heaviest thing that you can hold, a sledge hammer for example.
 

pgtips

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Thanks for the tips on this. Will give this a go, but will make sure there's something in place to act as a "dolly", as could do without a bent shaft!?

Is it right about not painting the shaft??
 

VicS

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[ QUOTE ]
Is it right about not painting the shaft??

[/ QUOTE ] No, dont paint the shaft. you definitely do not want paint between the anode and the shaft. There has to be a good electrical connection between the two. Paint does not stick well to stainless steel anyway without the correct prepartion and primer (except to places you don't mean to get it!).

Personally I would not fit an aode to a stainless shaft with a broze prop attached anyway unless I found a problem that needed an anode as a solution.

Normally a bronze prop should be OK. A stainless shaft is potectected by "anodic protection" by the bronze (just the opposite of cathodic protection). Stainless steel is normally protected from corrosion in well oxygenated water by the oxide film on its surface and when in a circuit with a copper alloy such as bronze the electrochemistry promotes the formation of this oxide film. Once you stick an anode on a bit of stainless steel the electrochemistry destroys this film so you loose its protection and you end up with the anode protecting the StSt in the same way as it would ordinary steel ie by cathodic protection.

The proximity of a big bronze prop to the anode simply means that the anode dissolves away rapidly. The MGDuff website does give arecommended minimum distance between anode and prop.

I am not trying to persuade you not to fit your shaft anode but I am pointing out the reasons why I would need a good reason to fit one. The anode manufacturers will advise fitting one. They will advise fitting one to every thing, after all they are in busines to make money by selling anodes. I am quite convinced many thousands of anodes are fitted every year unnecessarily, hull anodes as well as shaft anodes.
 

VicS

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[ QUOTE ]
pencil anodes in engines. Anyone ever seen these in Mg

[/ QUOTE ] Have you looked at MG Duff's or other anode manufacturers websites?

Without checking on any details my first reaction would be that magnesium is so much more reactive than zinc or aluminium that what would amount to a fairly small anode would not last very long. In sea water there is about 1 volt difference between Mg and cast iron or 0.6 to 0.8 volts between it and aluminium. (I dont have figures for fresh water).
 
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