Zinc spacing on steel hulls

Discussion in 'Metal Boat Building' started by Steelboat, Feb 21, 2022.

  1. Steelboat
    Joined: Feb 2022
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    Steelboat Junior Member

    I am considering adding a couple streamlined zincs on my steel sailboat hull. Some coating damage left deep corrosion pits in the bow section, approximately 15 ft (5m) away from the big plate zincs on the keel. The keel and main sections of the hull never have pitting corrosion when scratched. The rudder and skeg both have small plate zincs that work very well to protect.

    The best I can find online is 3 meter maximum spacing, and "within sight of each other", so hull curvature does not diminish protection. Does anyone have a more detailed reference on this? The extra drag of a couple teardrop bow zincs seems worth it if damage occurs.
     
  2. bajansailor
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    bajansailor Marine Surveyor

    @Steelboat can you post a photo or two of your boat to illustrate what you are planning on doing please?
    And also mention the length / size of your boat, and what your full complement of existing zincs is, and where they are.
     
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  3. Steelboat
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    Steelboat Junior Member

    Hi Bajansailor, the boat is 13.7m (45 ft) 2.1m (6.9 ft) draft
    The pic shows where the two teardrop zincs might be placed, far enough back from leading edge that hopefully are not too likely to catch nets and flotsam.
     

    Attached Files:

  4. bajansailor
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    bajansailor Marine Surveyor

    That sounds like a reasonable proposal - however I am no expert on zinc anode placement.
    Do you intend to weld on studs for the teardrops, and bolt them on, or weld them on directly?

    These folk are very well known in Britain re cathodic protection - do you have an equivalent type of supplier in the USA?
    https://www.mgduff.co.uk/
    I remember contacting M G Duff in the past with a question, and they were helpful - you could try asking them a question by email, and include a link to this thread?
     
  5. jehardiman
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    jehardiman Senior Member

    The bow is more susceptible to flow erosion and object strikes which damage and thin the coating. That is why the pitting starts. Once the steel is exposed, it becomes the electro-potential of the steel to the zinc. If the zinc is more negative (in mVolts typically in seawater compared to a silver chloride cell) than the exposed steel, then the zinc is preferentially consumed (i.e. oxidized and gives up electrons). Otherwise, the steel is. A zinc anode is typically -1030 mVolts and steel is ~ -620 mVolts, however the resistivity of the steel and the physical connection of the zinc to the hull make up resistance to the electron flow, meaning there is a real defect size and distance away from the zinc that exposed steel is no longer protected. Exactly how far this is?...we took voltage probes to measure in situ to make sure we had enough zincs in the right places. Is it also possible to over-zinc? Yes, it is, and it can cause plating out of chloride and calcites on even the un-exposed portions of the hull, especially at corners and joints, setting up differential corrosion cells that the zinc's won't protect.
    Anyway, here is a anode calculation site.
    Anode Weight Calculation Formula https://www.boatzincs.com/anode_weight_calculation.html

    Edit:
    Did a little more poking around the web and found this site, a quick perusal seems that it covers the basic concepts.
    Levels of cathodic protection - Cathwell https://cathwell.com/level-of-cathodic-protection/

    I am not associated with either site.
     
  6. Will Gilmore
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    Will Gilmore Senior Member

    Without one of the three elements needed for cathodic corrosion, disparit metals, circuit connection to a ground, and electrolytic, the corrosion you see may not be cathodic corrosion. It seems likely, oxygen was introduced by the collision.
     
    Last edited: Mar 2, 2022
  7. Steelboat
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    Steelboat Junior Member

    Thanks for the link to calculation sites. I worked through these and tested with a reference anode a long time ago, all good.
     
  8. Steelboat
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    Steelboat Junior Member

    Traditional wisdom from coating experts has always said do not use zinc rich primers below the waterline, as the entire coating will become an anode theoretically. Lately I have seen offshore drilling structures, ships, and pilings extensively use zinc rich primers over bare steel in submerged service. They overcoat the zinc primer with thick epoxy primers, which should isolate the zinc until damage occurs. Then the zinc primer slows the spread of corrosion until repairs can be effected.

    Perhaps this practice is an evolution? Those guys spend a lot of money on their coatings, so it's hard to write them off as fools.
     
  9. jehardiman
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    jehardiman Senior Member

    Different horses for different courses. Most offshore structures are flame sprayed because they will never receive another coating. Most ships plating used to get a zinc-chromate wash before coating, today there are less toxic anti-corrosion options before coating. The wash is to prevent corrosion from spreading under the topcoat and just having your paint and antifouling fall off.
     
  10. Steelboat
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    Steelboat Junior Member

    Yes zinc dichromate based coatings can be great, but now largely restricted to the aviation sector. We still use it for aluminum spars but it is getting difficult due to legislation.

    Zinc rich epoxies and high zinc moisture cured urethanes are quite different, and in use today. The debate I mentioned is regarding compatibility with cathodic protection (zinc hull anodes). It seems some coating systems are very compatible.
     
  11. Rolf East
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    Rolf East Junior Member

    I also have a 44 Offshore. It has 5 annodes on each side; 2 on the hull, 2 on the keel and 1 on the rudder. They were half eaten after 8 years in the water in northern Sweden. There is no corrosion outside below the waterline and the epoxy barrier is intact. Very little corrosion inside, and that is caused by poor drainage from the longitude stringers.
    Off topic: Id like to send you a PM Steelboat, but I do see an option for that in this forum. Or is there?
     
  12. bajansailor
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    bajansailor Marine Surveyor

    Welcome to the Forum @Rolf East
    I think that the reason why you cannot send Steelboat a PM is because you are still 'new' - make a few more posts, and then you should be able to.
    There should be a button on Steelboat's page (when you are allowed) labelled 'Start a conversation'.
     
  13. Rolf East
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    Rolf East Junior Member

    Thank you bajansailor! It is a pleasure to be here. I have read the forum for some time and found a lot of valuable information.
    If I could, I would have liked your post! But I guess Ill be able to, with experience.
     
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  14. Steelboat
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    Steelboat Junior Member

    Hi Rolf
    It would be great to see where you put the hull anodes. Have you had any trouble with snagging lines?
    Have you modified the original skeg design? That was a big job for me, did it 15 years ago now.
    I will try to PM you
     

  15. Rolf East
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    Rolf East Junior Member

    Hi Steelboat.
    See attached picture for annode placement. They are alle drop shaped.
    I bought the boat last summer and is in front of a substancial refit, so I did not place the annodes . But since they seem to have done a good job, I do not plan to change the distribution, when i cut the old ones of and weld studs on for the new ones. The propeller shaft does not have annodes, but it might be because the shaft has cutless bearings (I guess. I do not know yet precisely what bearings etc I have) and are not in direct contact with the hull? Or neglect. Does not seem to be corrosion on the shaft and propeller.

    No, I have not modified the skeg. I am not sure if mine is the original BR design. What do you think, from the picture? I see you have more skeg than rudder, compared to mine.

    I am in many regards a novice and have a lot of questions about my boat, which is why I joined this forum and would appreciate to share knowledge about boats in general and in particular 44 Offshore.
    Hope the picture helps.

    EDIT: Not sure what you mean by snagging lines?
     

    Attached Files:

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