Outboard Anodes

[geoffatstanpit]

My Parker 21 has a Merc 5 4-stroke in a well which lives in position all season (the sailing prop is too big to be easily withdrawn each time I leave the boat and I didn't like the hernia I got last time I tried). When the boat dries out each tide the water level is about half-way up the prop.

The anode on the leg is above the cavitation plate and therefore out of the water some of the time.

So I got a pair of those round button-shaped ones that you bolt through and fitted them to the skeg below the prop - always in the water/mud.

I find that the top anode wastes away much faster than the bottom one - even though it's only in the water part of the time and I suspect that the bottom one doesn't start to waste till the top one has gone.

Is the Mercury anode (top one) made of something different for fresh water (the instructions for the motor warn of dire consequences if you get salt water near it). Should I use a different sort for the skeg, or better, can I get Merc-shaped ones made of button-stuff?

Whatever, the leg is nicely free of any evidence of corrosion so the system is working. Maybe I could save the cost of the little Merc ones (exorbitant) if I could get them that shape in the other stuff.

Geoff



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[tugboat]

Anodes for freshwater use are magnesium, while those for saltwater are zinc. It could be that your engine was originally destined for a freshwater environment. It sounds as though you need to get a zinc anode to replace the upper wasted one.

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[geoffatstanpit]

Yes, thanks. I ran the same question on Scuttlebut and the concensus is that the Merc anodes are Aluminium for fresh water.

I shall melt down some old zinc anodes and cast them to fit the top position.

Geoff

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[BrendanS]

No, Magnesium for freshwater. Aluminium are sort of inbetween, for brackish water, though Mercruiser are now fitting and recommending Aluminium for their outdrives in sea water, after corrosion problems with zinc in some circumstances.

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