Anode advice please

pheran

pheran

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The large bar anodes on my GRP hull seem in excellent condition. Not wasted at all, no bits missing and the MGDUFF name is clearly visible. But the anode is covered completely with a hard, white, very rough coating. So, could I restore the anode to full health by grinding off this coating, back to shiny metal or has its protective effect been compromised? With replacements at more than £100 a piece, its an expense I would happily avoid as long as I'm not lining up other problems. Thank you.
 
pheran said:
The large bar anodes on my GRP hull seem in excellent condition. Not wasted at all, no bits missing and the MGDUFF name is clearly visible. But the anode is covered completely with a hard, white, very rough coating. So, could I restore the anode to full health by grinding off this coating, back to shiny metal or has its protective effect been compromised? With replacements at more than £100 a piece, its an expense I would happily avoid as long as I'm not lining up other problems. Thank you.
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You should be able to restore the anodes to normal operating condition by grinding off the white deposit

BUt you need to investigate why this has happened. Maybe they are not properly bonded to whatthey are supposed to be protecting

What have they been fitted to protect ?

How are they bonded?

Are you in fresh, brackish or salt water ?

What is the state of what they should be protecting?

Clearly not doing anything .... are they necessary?
 
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sailorman said:
what water are you in & is the anode zinc or aluminium
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Fresh water. Magnesium???

VicS said:
You should be able to restore the anodes to normal operating condition by grinding off the white deposit

BUt you need to investigate why this has happened. Maybe they are not properly bonded to whatthey are supposed to be protecting see below

What have they been fitted to protect ? Propellor, rudder, shaft, seacocks

How are they bonded? Internal cabling from fittings to anodes (but one such cable has become detached)

Are you in fresh, brackish or salt water ? Nearly always fresh these days

What is the state of what they should be protecting? No visible signs of pitting or any other damage

Clearly not doing anything .... are they necessary? Original builder thought so and I would like to keep to the original spec.
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If they are not wasting at all then I would think there is a problem with the bonding. You need to check that there is a good low resistance electrical connection between them and the items they should be protecting.

The seacocks should not the bonded to the anodes though!


Are you sure that they are magnesium..What you are observing sort of suggests that they might be zinc.


If they are not wasting at all they are not doing anything useful but if nothing is showing signs of corrosion then the obvious deduction is that they are not necessary anyway!
 
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Many thanks for that. I'm only guessing at the material the anodes are made of. Is there an easy way to tell? The boat was built in Norfolk and spent the first couple of years of its life on the Broads. Now on the non-tidal Thames with only the rare trip out into the estuary.

Could you explain why the anode and the seacocks shouldn't be connected please? The Owner's Manual shows such a connection (but typically, doesn't specify the anode material, presumably because its the inland/coastal version of the marque)
 
pheran said:
Many thanks for that. I'm only guessing at the material the anodes are made of. Is there an easy way to tell? The boat was built in Norfolk and spent the first couple of years of its life on the Broads. Now on the non-tidal Thames with only the rare trip out into the estuary.
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Very probably not magnesium then. what you describe fits with them being zinc. You could tell from their weight if removed but insitu perhaps not so easy. relative hardness perhaps or by doing something clever with a digital voltmeter and a reference electrode. I expect an experienced fitter of anodes would be able to tell them apart though
For you aluminum might be the best choice. Ok for saltwater, brackish water and polluted freshwater. If you consider your part of the river to be non polluted then magnesium perhaps but that is too reactive for more than occasional forays into salt water.

Whatever though check all the bonding

Could you explain why the anode and the seacocks shouldn't be connected please? The Owner's Manual shows such a connection (but typically, doesn't specify the anode material, presumably because its the inland/coastal version of the marque)
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At one time the advice was to bond everything underwater to the anodes. this is not now considered to be the best approach. Skin fittings and seacocks should be made of corrosion resistant materials which should not need the protection of anodes anyway. Bonding the seacocks was considered to be one of the factors contributing to the near loss of the FV Random Harvest a few years ago although not a cause perhaps taken in isolation apart from the other contributing defects in the vessel.
A forumite's boat probably would not have sunk a couple of years or so ago due to electrolysis of a skin fitting had there been no electrical connections to it.
 
Many people clean the surface of the anode not realizing that they accomplish nothing if there is not a good electrical bond between the anode and it's mounting bolts or the metal to which the anode is secured. Get a multi-meter, set it on ohms, touch one probe to the anode and the other to whatever metal the anode is supposed to be protecting. You should see something less than 1ohm, any more and its not accomplishing what it should.. If you see OL (open line) there is no continuity and your anode is doing nothing.
 
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