Anodes

Discussion in 'Metal Boat Building' started by sailrjim, Nov 29, 2005.

  1. sailrjim
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    sailrjim Junior Member

    Searched the www including this forum for clear and complete recommendations on the specific DETAILS OF MOUNTING of zinc anodes on steel hulls (spacing off the hull plating).

    I have Nigel Warren's "Metal Corrosion in Boats". It contains much useful info, but not on spacing off the hull surface.

    Anyone have a reference source or info on this point.

    As important as the subject of cathodic protection is to metal boat owners, perhaps we should have a "permanent thread" or a separate forum for it.

    Around my area (south Chesapeake Bay), there is much building and repairing of ships and barges; not much small steel vessels. It is surprising how difficult it is to get clear and practical info on this subject. I had expected this area would be awash in info on the subject. Have seen many barges and small metal boats with a variety of arrangements (placement grids) and mounting methods. Some show forethought while others seem haphazzard.

    Someday, I will own the proper electronic equipment Ag/AgCl cell and microvolt meter.

    In the meantime, can anyone advise?

    My specifics: steel hull, approx. 300 square feet wetted surface, full keel sailboat, pettit 60% CuO ablative antifouling paint.

    BTW, Nigel Warren recommends that copper based antifoul paint should never be used "on any hull if sacrificial anodes are fitted" (page 134 of "Metal Corrosion in Boats"). Such paint is all I see used in these parts, except for military vessels and racers who dry sail.

    Jim
     
  2. Grant Nelson
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    Grant Nelson Senior Member

    Geer, in 'the nature of boats' (1992) says that the zinc should be directly in contact with the metal it is protecting. Thus I would say with your hull (the steel, not the paint). So I am thus also curious why the ships you see space them away from the hull. It could be a maintenance issue, in that they have them on bolts connected to the steel hull, assuring good contact, which might, if the zincs where against the hull, be hindered by someone not removing all the paint where its attached, and then carefully painting around it again, or something like that.

    As far as no zincs on a copper painted hull: I can only guess that the zinc will reduce the effectiveness of the copper 'poison', prehaps by actually holding the copper better in the paint etc, so that it is not free to repell the bio-growth. I can not see that it would increase corrosion, as copper is low on the galvanic series, steel in the middle, and zinc above, thus the zinc will do the corroding. Without the zinc, it seems that any exposed parts of your hull will start doing the protecting for your bottom paint...

    But, these are only my guesses, and I would like to hear what some greater experts have to say on this topic.

    Cheers,
     
  3. MikeJohns
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    MikeJohns Senior Member

    Anode size location etc

    Jim
    Here is my handout on the subject. I hope this will tell you what you want to know.
     

    Attached Files:

  4. sailrjim
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    sailrjim Junior Member

    Thank you for that informative document.

    I also note that from "The 12 volt Doctor's Practical Handbook........", it is suggested that zincs should be rated as 50% efficient in service. From this and other reading, including your paper, I infer that I should begin with twice the weight in zincs as would be theoretically required. However, and as you suggest in your paper, the weight and surface area of the anodes must be in some balance so as to function properly; protecting over multiple years while not over protecting.

    Also, I am curious to know whether there is a practical limit to the number of years service for which an system of anodes can be designed to properly protect. I have now seen mention of two and three years for systems.

    It seems to me that attempting a longevity record would cause over protection initially (primarily affecting the paint film nearest the anode.

    So, My plan: wetted surface approx. 300 square ft (28 square m), WL is approx. 28 ft. (8.5m) Draft is 5 ft. (1.5m), beam at WL is approx. 9 ft. (2.7m): 4 anodes per side, two on the full keel vertical sides, two on the bottom plating, all to be spaced roughly evenly according to wetted surface area of the hull.

    BTW, for those in the U.S.A., found this site as purveyor of zincs (in U.S.A.): http://www.boatzincs.com/
    BoatZincs.com (978-841-9978) - Discount Zinc Anodes

    Thanks again for all the help.

    Jim
     
  5. gonzo
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    gonzo Senior Member

    There doesn't need to be any spacing. Anodes in metal boats are usually fastened to studs welded to the plate. You can weld stainless studs for ease of maintenance.
     
  6. sailrjim
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    sailrjim Junior Member

    Yes, I us stainless steel machine screws; weld the heads to the hull. I mount the anode so that there is enough distance between it and the hull to scrape off oxides and marine growth occasionally. As service time goes, this is probably of limited value, as it is difficult to clean the hidden surface as mounted, and in the water. The idea is to ensure that all surfaces of the anode are available for effect. I was curious whether there was an ideal distance or minimum recommended for other reasons; have not heard nor read of any.

    Jim
     
  7. riggertroy
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    riggertroy Senior Member

    Just been working on newbuilds in South East Asia - The ships had the anodes fastened onto studs, but between the anode and the hull is a rubber sheet (no gaps) - asked about it and was told that the rubber was to prevent damage to the protective coating by the decay of the anode or marine growth inbetween the anode and hull
     
  8. sailrjim
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    sailrjim Junior Member

    Dangling anodes

    It occurs to me that sailboat owners (of metal boats) may be averse to having many anodes attached to the wetted surface of their vessels. As an alternative, it seems that it would be as well to have perhaps one anode per side attached near the keel (it takes a beating) and to also have some anodes dangling over the side. These dangling anodes are often seen as the "grouper" or fish shaped anode. The spring loaded squeeze clamp at opposite end of the cable from the dangling anode is attached to some conductive item on deck that is in direct contact with the hull (chain plates, mooring bit, etc.).

    As I see it, with the hull coated with a good paint scheme and having minimal dissimilar metals below the water line, there is little risk of corrosion when at sea or at anchor (far enough from other boats). Most problems will be when in a dock and near other boats or debris, fastenings, etc. that are often found there, as well as shore power.

    Do any of you have thoughts or experience using this concept?

    TIA

    Jim
     
  9. Grant Nelson
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    Grant Nelson Senior Member

    Just to double check on this: stainless steel welded to alloy (mild) steel. These are quite far apart on the galvanic series, and would suggest corrosion risks... I assume since they are right next to the Zinc, this danger is a moot point, and the close proximity will not create anykind of micro corrosion environment. It will nail down my rought understanding of galvanic corrosion if you can confirm this is not an issue for this, or similar cases where far separate metals are bonded underwater but a zinc is in the neighborhood...
     

  10. sailrjim
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    sailrjim Junior Member

    Well, here is what I did and why: Replaced the mild steel shaft tube (approx. 1.3 m along) with type 316L (passive) stainless steel. The old tube was corroded through. Hull is mild steel and, until now, was the only metal in the water. Note that the hull is hot zinc coated. My thinking: Could have used type 304 (active) ss for the tube. It is quite near mild steel on the electromotive scale. However, I was concerned about other possible forms of corrosion in the tube (crevic corrosion, electrolysis, etc., and replacing the tube is such an unpleasant task that I decided to choose a material that was high on the scale (more noble) and to protect the much less noble hull material if necessary. However, as the amount of material in the shaft tube is much less than the mild steel in the hull, any wastage from hull to tube would likely be small (IIRC). If the reverse were true, the mild steel could be at great risk. So, it will be interesting to see the results in a year or two. I think good arguments could be made for using either the active (low nobility) or passive (high nobility) stainless steel for the shaft tube.

    I have changed my mind on the arrangement of zincs: Attach one or two to the hull, low near the keel bottom. Attach a single zinc to the rudder. Use two or more zincs to be hung over the sides and attached via cable and clamp to the hull. This should accomplish the two requirements of sufficient mass and sufficient placement of anodes. In addition, the hanging anodes will be easier to examine frequently.

    Cheers,

    Jim
     
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