Checking Boat Zincs
Whether you are buying a used boat, or just maintaining a boat, the zincs – hull
anodes or sacrificial anodes – are an essential part of her protection. If boat zincs are not working or replaced
when they need to be, then that can lead to a catastrophic failure of hull fittings. For example, a brass seacock
may slowly dissolve and finally fail. Many, many boats have sunk at sea and on their moorings because the zincs
were not replaced.
How do boat zincs work?
The zincs (which may be magnesium in brackish waters), are metallic zinc – just lumps of metal which corrode.
Your hull fittings, propshaft and propeller (and outboard motors too) will naturally corrode in sea water as small
electric currents remove their metal and take it, atom by atom, to another metal which is more attractive (and acts
as a cathode). So, atom by atom, your precious fittings leach away, as part of a big ‘battery’. With a zinc, it is
the zinc itself which acts as the anode, the zinc being more ‘attractive’ to the cathode we mentioned (which could
be a metal pile at the dock, or some nearby mooring chains). So, the zinc gets eaten away, leaving your precious
hull fittings intact and safe.
The material of the hull fittings may be made of a variety of metals – brass, bronze (varying grades including
'gunmetal'), stainless steel and even high quality plastic (hopefully not low quality). Apart from plastic, these
metals have differing ‘electrochemical potentials’ and those which corrode more slowly are known more ‘noble’.
How to check boat zincs
First off, it’s important that the sacrificial anodes ‘work’ – that is that they are sacrificed and corrode.
That’s the reason for them being there. Their life will depend on many things. Most GRP smaller leisure boats will
have only one or two, perhaps 3, which can be changed inexpensively every year or two. One or two to protect the
propshaft and propeller, and one to protect the hull fittings. These will weigh typically 1/4-1/2 lb. Larger power
boats may have 4 or 5, and a 35’ steel cruising yacht may have 6 or 7, and the size of the anodes will be larger. A
larger boat with SSB or ham radio will have an expensive sintered copper earth plate to protect too.
When you inspect them on the hull, they should be clean and pitted (if not new), and once they have reached 50%
of their original weight, they should be replaced (you should be able to estimate this by eye). If they are covered
with a white carbonate deposit or are fouled, then they are not working properly.
Many owners replace them every annual haul out, though there is no need to do so if they are working and not
below 50% corroded. The boatyard, though, will always suggest it...
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