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TOPIC: Anodes...encore

Anodes...encore 02 Jul 2022 12:09 #130430

  • John & Joanna Ascough
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As you know Johanna has a very similar hull design to the Stanton Rose. We have 6 aluminium anodes each side plus the bow-thruster and propeller bosses. This includes an anode within each sea-chest (port & stb). We replaced them in 2016 after 7 years in, mainly, fresh water but they didn't really need changing. however, the boat was out so it made sense to do them. We surveyed the hull Feb 2022 (6 years) and the anodes looked fine with limited erosion. A couple of photos attached from 2016, and yes, the protective paper was removed before going back in the water.
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Anodes...encore 02 Jul 2022 10:18 #130425

  • Balliol Fowden
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17 zinc anodes in sea water would probably not be excessive. Bear in mind that anodes work line of sight, a bit like VHF radio. On a chined flat bottom hull you need to think separately about the bottom plate and the sides. So four anodes evenly spaced along one side would translate to say four each side + 4 along the centre line of the bottom = 12 anodes minimum, with perhaps some extra protection for the propeller, rudder and bow prop (if bronze). Bear in mind that I do not deal with a lot of steel sea boats so my personal experience is limited but any anode manufacturer such as M.G. Duff will publish recommendations, which may or may not accord with my gut instinct but are likely to be reliable. Salt water causes serious corrosion and nothing that I say below diminishes the necessity to have good catholic protection on a steel hull in salt water.

Now I do have a lot of experience with fresh water anodes. Fresh water is a very weak electrolyte. My estimation is that magnesium anodes (which are of course rather more potent than aluminium) only tend to protect an area of about 1.5 to 3 metres radius around them depending on whether they are mounted on a convex or flat surface respectively. So the calculation might suggest at least 20 anodes, 6 each side and six along the bottom plus perhaps a few extra at the rudder and prop.

That is a lot of anodes to buy!

The question I always ask is “Do you have a corrosion problem to fix?”

If on pressure washing the hull you see lots of shiny areas of metal which then corrodes rapidly as the hull dries off then you have at the least a galvanic problem, possibly an electrolytic problem, and you need to think about it and investigate the cause. The silvery stuff is not, as one magazine journalist ludicrously once stated, magnesium (zinc or aluminium) deposited from the anode. It is bare virgin metal exposed by the corrosion process.

If your pressure washing reveals good paintwork and minimal corrosion (possibly only at points of abrasion) then you could consider fitting anodes adjacent to these defined areas where abrasion and/or corrosion has proved itself to be an issue. Bear in mind though that in reasonably fresh still water the rates of corrosion are in practice (if not in theory) very low.

If you have good paintwork and no significant corrosion then you have no need for anodes.

There is one other pertinent comment I will add after over fifty professional years in the industry as a boat builder, restorer, boat operator and surveyor. Steel hulls very often suffer more from internal corrosion than from external corrosion. I have seen many many boats that, although perfectly painted externally and peppered with anodes, have had hull failures due to excessive internal corrosion. So direct some energies to internal preservation before busting the budget on anodes, which are of dubious benefit unless you have an active corrosion problem.

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Anodes...encore 01 Jul 2022 20:58 #130420

  • Richard Pentreath
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Thanks to Rob Davison, Andy Soper and Balliol Fowden for your resonses to my post a little while ago concerning mixing of aluminium and zinc anodes. For some reason I cannot access 'My Topics' to get int ano my original post.

However, the advice is somewhat conflicting...Perhaps 17 (actually16.5) x 2kg original zinc anodes on a 20 x 4.8 m barge is indeed excessive, even counter-productive? What is the norm for an anode count? There are 8 on the rudder alone...


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