Dezincified is a term that never seems to agree with my computer's spellchecker, but it is fairly well accepted within marine surveying circles. To quote Nigel Warren's excellent book on Metal Corrosion in Boats, dezincification is the 'gradual dissolving of the zinc content of brass leaving a spongy mass of copper: although the shape of the article is maintained its strength is virtually nil'. Well you may say to yourself 'phew, I'm safe as my prop is made from Manganese Bronze'. Unfortunately this grade isn't a bronze, but a high-tensile brass of 60:40 Copper and Zinc and dezincifies readily. The photograph below shows a nine year old propeller off a Bavaria 49, kept in seawater, showing all of the signs of dezincification. I can't say for sure, but it is likely that it was made from Manganese Bronze.
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| Dezincification leaves the item with virtually no strength |
Don't panic just yet, in practical terms a manganese bronze propeller on a GRP, steel or timber boat is acceptable, just make sure that a sacrificial anode is fitted, wired up, reasonably close to the propeller and renewed regularly. The anode on this boat was heavily wasted and probably too far gone. This suggested that it had at some time been doing its intended job, but electrical continuity between the propeller and the anode couldn't be found with my multimeter. If you check your own, the resistance between the two should be 1 ohm or less. An additional anode attached directly to the propeller shaft would give further protection.
Replace anodes when about 50% wasted and never, ever paint them!
Contact www.fieldhouse-yacht-surveys.com if you want help with checking your boat's stern gear or skin fittings during this Winter's lay-up period.
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