A different anode question

[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Parker 21 has a Merc 5 4-stroke in a well which lives in position all season (the sailing prop is too big to be easily withdrawn each time I leave the boat and I didn't like the hernia I got last time I tried).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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[snowleopard]

for salt water, anodes are zinc, for fresh water magnesium is used. zinc will do nothing in fresh water and magnesium will disappear very quickly in salt.

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[geoffatstanpit]

Aha!! The Merc ones are very light, I'll measure some weights and volumes:----

Merc anodes density 2.75
Button anodes now so pitted my geometry isn't up to measuring the vol. but they feel a lot heavier.
Magnesium should be 1.74
Zinc should be 7.14

So I guess we have some kind of alloy here and I haven't got access to a chemmy lab any more.

I think I'll melt down some old anodes and cast them into Merc shaped ones and see what happens next season.

Watch this space (if anyone's interested!)

Geoff

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[LadyInBed]

What would you suggest for a river estuary where it is probably 40% salt then 60% fresh?

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[Keith]

Ah yes reminds me of the Wessex helicopter, half of that was magnesium,couldn't keep up with the corrosion on it after it had been wet winching........keith

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[BrendanS]

Aluminium is normally what is recommended for brackish waters. Interestingly these are now recommended and fitted as standard for seawater use by Mercruiser outdrives now

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[duncan]

yep - get through loads of them on my BIII ! Still the drive has stopped melting............/forums/images/icons/smile.gif

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[geoffatstanpit]

The density of the merc ones (2.75) would be close for Al (2.702) maybe that's what they are.

Geoff

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[BrendanS]

If they warn of dire consequences if near sea water, I would suspect they are magnesium.

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