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  1. #1
    Join Date
    Aug 2001
    Location
    Scotland
    Posts
    1,689

    Default Looking for advice re Zinc Anodes, to bond or not to bond.

    I am looking to fit a Zinc anode to my 40 year old GRP motor sailer as I noticed a bit of corrosion on my stainless steel rudder when boat was lifted out for the winter. I have no idea why this has suddenly started and it may be an inherent flaw in the plate. The boat has been in a Marina on the Clyde for last 7 years but before that was on Loch Lomond for unknown period. My pontoon in the Marina is some distance from the larger motor yachts so I do not think it is a locality problem to third party stray currents.

    The full inventory of metallic items immersed is as follows.

    Stainless steel rudder (Spec not known 40 years old)
    Bronze propeller (unknown SH off Moody fitted 4 years ago)
    Bronze shaft (Stuart)
    Bronze engine seacock (Stuart)
    Bronze stern tube (Stuart)
    DZR Toilet Seacocks.
    A4 stainless steel bilge keel bolts (flanged GRP bilge keels)

    At the moment there is no bonding whatsoever, the engine is indirectly cooled, the drive to the prop shaft is via a "Carden Shaft" with two Hardy Spicer couplings so there will be some electrical continuity between the engine and the prop shaft.

    I am proposing to fit a sandwich anode to either side of the Rudder but am not at all sure if I should bond the rudder stock to the engine and seacocks.

    Can anyone advise, thanks.

  2. #2
    Join Date
    Jan 2010
    Posts
    386

    Default Re: Looking for advice re Zinc Anodes, to bond or not to bond.

    If you just want to protect the rudder then there is no need to bond to anything else BUT have you considered using aluminium anodes? Is the Clyde salt water or is there a chance it could be brackish? If so than definitely fit ali anodes.
    Fair wins
    John

  3. #3
    Join Date
    May 2016
    Location
    Suffolk
    Posts
    6,368

    Default Re: Looking for advice re Zinc Anodes, to bond or not to bond.

    Anything that is electrically isolated cannot be effected by galvanic corrosion. So if your rudder and rudder stock are not connected to anything other than the GRP hull the corrosion cannot be galvanic and fitting an annode will be pointless. Likewise the seacocks, isolated they cannot suffer galvanic corrosion, add them to a full bonding circuit and there then exists the possibility that they could (although technically they shouldn't as the annode should protect them).

  4. #4
    Join Date
    Aug 2001
    Location
    Scotland
    Posts
    1,689

    Default Re: Looking for advice re Zinc Anodes, to bond or not to bond.

    Quote Originally Posted by zambant View Post
    If you just want to protect the rudder then there is no need to bond to anything else BUT have you considered using aluminium anodes? Is the Clyde salt water or is there a chance it could be brackish? If so than definitely fit ali anodes.
    Fair wins
    John
    Yes, the Clyde is salt and the marina is open to rise and fall of ~3m tide so plenty of salt water flushing through the pontoons.

  5. #5
    Join Date
    Oct 2007
    Location
    Central Scotland
    Posts
    2,195

    Default Re: Looking for advice re Zinc Anodes, to bond or not to bond.

    I have no bonding at all.
    Volvo MD2030 engine, has bronze intake valve for cooling. There's a stainless propshaft a bronze prop, various bronze seacocks, stainless rudder shaft, none bonded together.
    There is insulation between engine and gearbox - a standard VP thing of that day I think
    It's taken 24 years to slightly corrode the prop nut, but rest is 42 years without visible corrosion.
    And that's mostly also in the Clyde.
    Geoff

  6. #6
    Join Date
    Nov 2007
    Posts
    31,979

    Default Re: Looking for advice re Zinc Anodes, to bond or not to bond.

    Any corrosion on stainless is likely to be crevice corrosion so an anode wont help. However you should check any fastenings, brackets, bearings etc to check that they are all the same grade stainless. If not replace them. If that is not practical then an anode on the plate might help. Does not matter if it is zinc, magnesium or aluminium as the choice depends on the water, not on the metals you are trying to protect.

  7. #7
    Join Date
    May 2001
    Location
    North Wales, sailing Aegean Sea or Menai Strait
    Posts
    20,872

    Default Re: Looking for advice re Zinc Anodes, to bond or not to bond.

    Quote Originally Posted by PaulRainbow View Post
    Anything that is electrically isolated cannot be effected by galvanic corrosion. So if your rudder and rudder stock are not connected to anything other than the GRP hull the corrosion cannot be galvanic and fitting an annode will be pointless.
    Sorry, not true. An anode will protect any metal more noble than itself. Steel offshore structures are plastered with aluminium or zinc anodes, nothing to do with galvanic couples.
    Answers to some technical queries at new website http://coxeng.co.uk

  8. #8
    Join Date
    May 2016
    Location
    Suffolk
    Posts
    6,368

    Default Re: Looking for advice re Zinc Anodes, to bond or not to bond.

    Quote Originally Posted by vyv_cox View Post
    Sorry, not true. An anode will protect any metal more noble than itself. Steel offshore structures are plastered with aluminium or zinc anodes, nothing to do with galvanic couples.
    So you can get galvanic corrosion without an electrical circuit ?

  9. #9
    Join Date
    Jul 2002
    Location
    Home: Kent. Boat: Chichester
    Posts
    42,280

    Default Re: Looking for advice re Zinc Anodes, to bond or not to bond.

    Quote Originally Posted by PaulRainbow View Post
    So you can get galvanic corrosion without an electrical circuit ?
    Galvanic corrosion is by definition corrosion between two dissimilar metals or alloys immersed in the same electrolyte and in contact or electrically connected to each other.

    Not all corrosion involves two different metals or alloys but in seawater almost invariably proceeds by an electrochemical mechanism between cathodic and anodic sites on the same metal or alloy. Corrosion of iron and steel, dezincification of brass and crevice corrosion are examples.

    The corrosion can be prevented by cathodic protection, except perhaps crevice corrosion, using impressed current or sacrificial anodes, which makes the entire surface cathodic with respect to the anodes.

  10. #10
    Join Date
    May 2016
    Location
    Suffolk
    Posts
    6,368

    Default Re: Looking for advice re Zinc Anodes, to bond or not to bond.

    Quote Originally Posted by VicS View Post
    Galvanic corrosion is by definition corrosion between two dissimilar metals or alloys immersed in the same electrolyte and in contact or electrically connected to each other..
    So that's a no then.

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