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  1. #1
    Join Date
    Aug 2013
    Location
    SW Leicestershire
    Posts
    1,169

    Default Positioning of shaft anode

    Earlier this year after finding the propellor anode on my then new Featherstream was pretty well eaten away after 8 months, I fitted two shaft anodes to try to mitigate this. In fact there may be no scientific basis for this at all as the prop anode deals, I think, with the difference between the bronze boss and the stainless steel blades. They were positioned as so:

    IMG_0906.jpg

    This is after 6 months so noticeable improvement in prop anode degradation. In avoiding placing them in the middle of the shaft (section from P bracket to hull is fairly short) the aim was to mitigate any vibration. I recall that Vyv Cox has said shaft anodes should be close to prop they are meant to protect so would it be better to place both together about 2.5cm from the P bracket - or would putting weight at one end of shaft cause vibration?

  2. #2
    Join Date
    Sep 2017
    Posts
    2,048

    Default Re: Positioning of shaft anode

    These MG Duff MGD range are much better due the the steel reinforcing band cast in so dont fall off


  3. #3
    Join Date
    Aug 2013
    Location
    SW Leicestershire
    Posts
    1,169

    Default Re: Positioning of shaft anode

    Quote Originally Posted by Old Harry View Post
    These MG Duff MGD range are much better due the the steel reinforcing band cast in so dont fall off

    I haven't yet had one fall off.

  4. #4
    Join Date
    May 2001
    Location
    North Wales, sailing Aegean Sea or Menai Strait
    Posts
    22,133

    Default Re: Positioning of shaft anode

    The worst place for rotordynamic reasons is in the middle between bearings. Near the cutless bearing is better from this POV and also better for protection. Painting the prop with a good paint system is better for corrosion and would reduce fouling considerably.
    Answers to some technical queries at new website http://coxeng.co.uk

  5. #5
    Join Date
    Nov 2013
    Location
    Perth WA
    Posts
    2,441

    Default Re: Positioning of shaft anode

    It just crossed my mind that maybe I should be hanging on to all the zinc I come across.

    I am currently melting down wheel weights for ballast but a lot of the wheel weights are zinc which is easily melted. Is there any point in holding on to some of it?

  6. #6
    Join Date
    Jul 2002
    Location
    Home: Kent. Boat: Chichester
    Posts
    43,825

    Default Re: Positioning of shaft anode

    Quote Originally Posted by coopec View Post
    It just crossed my mind that maybe I should be hanging on to all the zinc I come across.

    I am currently melting down wheel weights for ballast but a lot of the wheel weights are zinc which is easily melted. Is there any point in holding on to some of it?
    No
    The zinc used for anodes is a special high purity grade, with very low lead, copper and critically iron content but with the addition of small amounts of cadmium and aluminium

    It is usually to the US military specification MIL-A-18001K

  7. #7
    Join Date
    Nov 2013
    Location
    Perth WA
    Posts
    2,441

    Default Re: Positioning of shaft anode

    Quote Originally Posted by VicS View Post
    No
    The zinc used for anodes is a special high purity grade, with very low lead, copper and critically iron content but with the addition of small amounts of cadmium and aluminium

    It is usually to the US military specification MIL-A-18001K
    Thanks for that. I will heed your advice but there wouldn't be any iron or copper but there could be lead though. (I was starting to curse the fact that I must have turfed out 20kg or so but now I can relax )

  8. #8
    Join Date
    Nov 2013
    Location
    Perth WA
    Posts
    2,441

    Default Re: Positioning of shaft anode

    My boat will be in sea water but I didn't know that magnesium anodes should be used in freshwater.
    " An alloy much more active than zinc or aluminum is needed. Magnesium is the answer. A magnesium anode is a super activated metal which means it will protect more efficiently in fresh water. Traditional zinc or aluminum anodes are just not effective as they do not produce the voltage necessary to work properly.

  9. #9
    Join Date
    Jan 2011
    Posts
    5,862

    Default Re: Positioning of shaft anode

    Quote Originally Posted by Old Harry View Post
    These MG Duff MGD range are much better due the the steel reinforcing band cast in so dont fall off

    I use these now. They are also about twice the size of the ones you're using and I find that one of these does the job of 2 of the little ones.

  10. #10
    Join Date
    Jul 2002
    Location
    Home: Kent. Boat: Chichester
    Posts
    43,825

    Default Re: Positioning of shaft anode

    Quote Originally Posted by coopec View Post
    Thanks for that. I will heed your advice but there wouldn't be any iron or copper but there could be lead though. (I was starting to curse the fact that I must have turfed out 20kg or so but now I can relax )
    This is the spec

    Cadmium . . . . 0.025% - 0.07%
    Copper . . . . . . .0.005% Max.
    Iron . . . . . . . . . 0.005% Max.
    Lead . . . . . . . . .0.006% Max.
    Aluminium . . . . 0.1% - 0.5%
    Zinc . . . . . . Remainder (special high grade 99.995%)

    I understand that 0.005% iron is only tolerated by the addition of cadmium.

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