blisters around zincs on steel boat

Discussion in 'Materials' started by lfrancis, Aug 21, 2008.

  1. lfrancis
    Joined: Apr 2008
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    Location: key west,fl

    lfrancis Junior Member

    Anyone out there know why there are blisters and corrosion around the zincs on my steel boats bottom? I have over 50 lbs and the zincs are bolted on directly to the bare steel . There is no paint between!!! i have a 32 foot full keel tahitiana and the manuals told me i needed this much zinc for my size and displacement of 10 tons. The zincs have been on for two years. thankyou!
  2. lazeyjack

    lazeyjack Guest

    god almighty i used one small zinc on a 54 foot alloy BOAT
    and how much percentage of those zincs are left, i mean on each?
  3. eurosurv
    Joined: Aug 2008
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    eurosurv eurosurv

    Hi there
    if there is still a large percentage of your zincs left after two years then as the previous person states you probably dont need all of them. Next time you look at the hull out of the water just have a look at what you are trying to protect, there would need to be a disimilar metal within 2 yards of the zinc to really need it.
  4. Meanz Beanz
    Joined: Jun 2007
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    Meanz Beanz Boom Doom Gloom Boom

    One thing I have discovered (ya I'm slow) with electrolysis is that it can very much matter where your boat is. I have seen a boat have a devil of a time with corrosion, destroying anodes quick smart where nothing has been obviously wrong. Then after the boat moved the problem disappeared, turns out the old neighbouring boat was a bit of a disaster area.

    I can't help much with your issue directly (it don't seem right to me) other than to say if nothing else makes sense look to your neighbours...
  5. MikeJohns
    Joined: Aug 2004
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    Location: Australia

    MikeJohns Senior Member

    Far from protecting the vessel, too much zinc surface area literally blows the paint off.

    Every compromised patch of paint passes more concentrated current from the hull-anode battery and hydrogen bubbles form on the hull metal surface under the paint coating.

    The bubbles lift and crack the paint often causing a patch to disbond and break away. The exposed metal quickly gets a deposition of metal salts and the bubbles form on the cleaner paint/hull interface slowly stripping the coating. This gives the classic pock marked paint and blisters particularly around the anodes. Often in the case of poor paint jobs this blistering/pocking is extensive in the bottom paint. The anodes then expire even faster. Poor understanding usually leads to installing even more anodes and the poor owner thinks he has a “problem boat “. I have often found an abysmal level of understanding of anodes amongst would be specialists who should know better.

    Note that these patches don’t corrode while the anode material remains active and the metal in the centre of the patches is usually dull with a blue-green tinge. Often bare of marine growth.

    Touch up the blistered areas with a few coats of high build epoxy and reduce the anode area, in reality a well epoxied steel hull can effectively protected with minimal anodes, some have none with no problems but then any accidental breach in the paint needs sorting sooner.

    It’s often a good idea to paint the backs and immediate edges of anodes with epoxy so that the active face is away from the hull and the total area reduced along with the rate of depletion.

    We calculate the mass and exposed area of anode relative to the time between dry docking and the protected area and the geographic location/usage .
  6. MikeJohns
    Joined: Aug 2004
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    MikeJohns Senior Member

    Here's something to go on from a guide I wrote a while back. This is the 'ideal' protection regime. If the paint job is a good thick epoxy job and the boat is hauled a least annually then trying fewer anodes won't hurt. If you are in warmer water then go and look at the anodes and you'll see what they are doing. Bolt on anodes are easily replaced in-water with breathing gear.

    Guppy anodes larger anodes hung externally when tied up or moored long term are good practice since the anode is well away from the hull.


    To calculate hull anode requirements
    The level of protection required is quoted as a current density (Amps per square metre) and can vary from 10 to 40 mA per square metre (a milliAmp is 1/1000 of an amp)
    coastal vessels need a slightly higher protection than ocean going ships and should be provided in clean coastal waters with around 20 mA/m^2.

    To calculate the number and weight of anodes on a new vessel:

    1: Work out the area to be protected, ie the area of your hull/structure under the water in square metres
    2: Multiply this area by the current density of 0.02 to get the total continuous current required (use other values only if previous experience dictates or on professional advice).

    3: Multiply the continuous current by the number of hours in a year (8760 hours) to get an amp-hour requirement for the period you want protection (usually one year, otherwise whatever period is required in hours).

    4: The total anode weight required in kilograms is now easily found by dividing the amp-hour figure from step 3 by the amp-hours per kilogram potential of the anode material ( Zinc 730Ahrs/kg, Magnesium 1250Ahrs/kg, Aluminium 2500Ahrs/kg ).

    5: The number of anodes required is found from the total weight calculated from step 4 divided by the individual net weight of the anodes to be used. This will be co-dictated by the spacing required (see anode location below).

    Anode location
    Evenly around the boat, each anode gives a protective radius of around 3m. Space the anodes up to 6m apart either welded or bolted to the hull and keel. Best sited on the lower curve of the hull in protected positions (from collision) and on the keel sides. The anodes must be continually immersed to work. Tend towards greater protection with the dissimilar metals at the stern. If you are unsure of how to arrange your spacing, start at the stern and work forwards.
    Note that streamlined anodes should have the blunter end be closest to the bow for least resistance.
    When calculating the mass for 2 or 3 year protection; rather than using lots of small anodes it is better to use fewer anodes of greater mass to reduce the surface area and avoid problems from over-protection.
    1 person likes this.
  7. Meanz Beanz
    Joined: Jun 2007
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    Meanz Beanz Boom Doom Gloom Boom

    Thanks Mike

  8. lazeyjack

    lazeyjack Guest

    diito, , I never knew all that, I just know what works and have only ever gone for one, with one on the shaft
    I have never has Issues, with any of them, but it is very suprising how many mix up the issues of galvanic action, with those of electrical wiring
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