Anode Query

PeteCooper

PeteCooper

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Before my boat was lifted out she had been in the water for about 3 years. I normally have her lifted every second year, and the propshaft anode always needs replacing - just about completely gone. This time however most of the anode is still there - so has anybody any ideas why? I confess to playing about with some of the internal electrical stuff with more enthusiasm than knowledge and wondered if I may have disconnected something? I will replace the anode anyway but the lack of erosion seems odd.
Grateful thanks for any explanation.
 
Has anything about your boating changed? Possibly a poor quality anode, change of material not suited to the water you are in, was the anode in good electrical contact with the prop shaft?
 
PeteCooper said:
Before my boat was lifted out she had been in the water for about 3 years. I normally have her lifted every second year, and the propshaft anode always needs replacing - just about completely gone. This time however most of the anode is still there - so has anybody any ideas why? I confess to playing about with some of the internal electrical stuff with more enthusiasm than knowledge and wondered if I may have disconnected something? I will replace the anode anyway but the lack of erosion seems odd.
Grateful thanks for any explanation.
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The shaft anode is there primarily to protect the galvanic couple between the stainless steel shaft and the manganese bronze (=brass) propeller. There are several possibilities as to why it might not have eroded.

1. It has a different composition to those bought previously. This has been quite a problem for owners of saildrives who bought cheap anodes. In some cases the saildrive casing has corroded to protect the anode! A very small amount of iron in zinc anodes, typical of recycled anodes, is sufficient to reduce its ability to protect by a great deal.
2. The anode is not making good electrical contact with the shaft, or possibly the prop is not in good electrical contact with the shaft.
3. The prop has been painted or has an impervious oxide film on its surface.
4. The prop is dezincified (hopefully not)

Most unlikely that any changes to electrics in the boat have had any effect.
 
The first thing to do is find out whether there is continuity from the anode to the prop/shaft. Using a cheap multimeter is good enough and you should get only a few ohms resistance between the two. Just a point of concern, if the anode has not been erroding it is likely to have grown a covering of hard white encrustment and that must come off to get an electrical contact and also for it to work in te water. I found it came off, albeit slowly, by pecking at it with a chipping hammer - just like cleaning up a weld.

If there is not continuity or too high a resistance, then work your way along the connecting parts til you find where the problem lies. Overzealous clearing out of "unused" wiring has been known to disconnect the anode and fitting a new flexible drive coupling can introduce an insulating component.

Rob.
 
rob2 said:
The first thing to do is find out whether there is continuity from the anode to the prop/shaft. Using a cheap multimeter is good enough and you should get only a few ohms resistance between the two. Just a point of concern, if the anode has not been erroding it is likely to have grown a covering of hard white encrustment and that must come off to get an electrical contact and also for it to work in te water. I found it came off, albeit slowly, by pecking at it with a chipping hammer - just like cleaning up a weld.

If there is not continuity or too high a resistance, then work your way along the connecting parts til you find where the problem lies. Overzealous clearing out of "unused" wiring has been known to disconnect the anode and fitting a new flexible drive coupling can introduce an insulating component.

Rob.
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Its a prop shaft anode so no wiring involved and the existence of a flexible coupling is irrelevant.

It is still appropriate to test for good electrical contact with the shaft though and of prop with shaft. I'd think poor contact the most likely explanation.

Vyv suggests a poor quality anode as a possibility. I can understand how that could lead to major problems on a sail drive leg but not so sure that it would explain lack of apparent action when bolted on a stainless steel shaft together with a copper alloy prop.

Not sure I understand why a dezincified prop would be the cause either but perhaps I misunderstand him.
 
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rob2 said:
If there is not continuity or too high a resistance, then work your way along the connecting parts til you find where the problem lies. Overzealous clearing out of "unused" wiring has been known to disconnect the anode and fitting a new flexible drive coupling can introduce an insulating component.

Rob.
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The OP said it's a shaft anode. I assumed it was on the shaft, so no wiring involved.
 
VicS said:
Vyv suggests a poor quality anode as a possibility. I can understand how that could lead to major problems on a sail drive leg but not so sure that it would explain lack of apparent action when bolted on a stainless steel shaft together with a copper alloy prop.

Not sure I understand why a dezincified prop would be the cause either but perhaps I misunderstand him.
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Apparently a very small iron content can render a zinc anode ineffective, which will cause it not to corrode preferentially. http://www.boatzincs.com/use_milspec_a18001k.html is a bit of a promotional site but it refers to a US Mil spec that I assume is genuine.

Dezincification not intended as a serious cause but copper and stainless steel together seem to do pretty well, as opposed to brass and stainless steel.
 
vyv_cox said:
Apparently a very small iron content can render a zinc anode ineffective, which will cause it not to corrode preferentially. http://www.boatzincs.com/use_milspec_a18001k.html is a bit of a promotional site but it refers to a US Mil spec that I assume is genuine.

Dezincification not intended as a serious cause but copper and stainless steel together seem to do pretty well, as opposed to brass and stainless steel.
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Thanks for the link. I'm sure its a genuine explanation ....... Would it also be the reason one sees it advised not to clean an anode with an ordinary steel wire brush do you think?
 
Even attached directly to the shaft, a thin barrier of corrosion between the shaft and the Zinc could reduce its effectiveness. It is critical to clean off the shaft with a light grit before installing. Maybe that? As others said - you can check resistance with a cheap meter.
 
Thanks for all the advice.
Yes it is a shaft anode - I can't remember where I bought the last one - it may be of poor quality. I'll replace it and check that both the new anode and the prop are making electrical contact with the shaft, relaunch and forget about it:)
Thanks again.
 
PeteCooper said:
Thanks for all the advice.
Yes it is a shaft anode - I can't remember where I bought the last one - it may be of poor quality. I'll replace it and check that both the new anode and the prop are making electrical contact with the shaft, relaunch and forget about it:)
Thanks again.
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With the possibility that Scotland might become independent you might like to consider the Defender range from Aberdeen Foundries. Made to the US military spec which Vyv has drawn attention to

http://www.aberdeenfoundries.co.uk/marine.htm
 
A mate had a similar phenomenon caused by attaching a 1" shaft anode to a 25mm shaft. It was tight enough not to be loose but not tight enough for continuity.

Today's "teach your granny to suck eggs" tip - apply a blob of paint to the s/s screws and nuts - makes the anode last a lot longer.
 
awol said:
Today's "teach your granny to suck eggs" tip - apply a blob of paint to the s/s screws and nuts - makes the anode last a lot longer.
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Why would you want to make it last longer? Isn't it the anode's job to corrode (or whatever the technical term is) and protect everything attached to the shaft? And what does the paint do?
 
Sea_Spray said:
Why would you want to make it last longer? Isn't it the anode's job to corrode (or whatever the technical term is) and protect everything attached to the shaft? And what does the paint do?
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Some anodes corrode quickly around the fastenings and fall off even though there is plenty of meat left in the anode to do the main job of protecting the propeller. The paint isolates the fastening from the zinc so slowing down this process, so the anode does its job for longer.
 
Sea_Spray said:
Why would you want to make it last longer? Isn't it the anode's job to corrode (or whatever the technical term is) and protect everything attached to the shaft? And what does the paint do?
Click to expand...

Prevents interaction between the anode and the stainless steel fixings which causes accelerated rate of loss of zinc.
 
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