On Jun 17, 2:17 pm, Al Thomason wrote:
Thank you all for your comments.

Yes, we are in very fresh water here. The zincs do not show the White
oxidization mentioned, but they are also not shinny. They are more a
dull gray and slightly ruff surface. There is some measure of wear
on them, but very little.

Though currently moored in fresh water, we do make trips to the coast,
and use in salt water. This will increase in the coming years.

I will look into the Magnesium anode for over the side. There was on
board a 'fish shaped' item which I remember was used in just such a
way. I assumed it was zinc, but perhaps it is actually magnesium.

Am going to call the Prop supplier Monday, will report what they say.
Again, to me the troubling thing is that only the Prop shows this
blackening... The rudder, cutlass tube, even the prop nut all are OK.

-al-

On Sun, 17 Jun 2007 13:31:24 +0100, "Alec"
wrote:



I agree with other posts, if you are in fresh water the Zinc anode will
become covered with a hard white deposit and consequently be ineffective .
I have a magnesium anode on a ss wire ( bonded to the engines) which I hang
over the side at the stern when I moor in freshwater for any length time e.g
a month or two and pull it in when I go to sea.

Many European sailors who venture into the canals and rivers do this.

Alec- Hide quoted text -

- Show quoted text -

Check that the prop is electrically connected to the anodes. You
should have bar anodes near the prop, not just a shaft anode. It's
possible that your shaft has not got a connection to the anodes and is
therefore corroding itself. Use a multimeter to check connections. On
your shaft coupler to your engine there should be an earthing strap,
if not then this will cause heavy corrosion on your prop - if you
don't have this then you need a springloaded stell connection to rub
against the shaft as it spins - like in an electric motor and that
should be electrically connected to your anodes and rudder. For fresh
and salt water, aluminium is recommended. Magnesium will not last five
minutes in sea water.

On Jun 16, 8:37?am, Bil wrote:

Magnesium is better than zinc as a sacrificial anode in sweet water.
Your prop manufacturer will be able to advise further.

Cheers

Bil

Did I somehow miss a mention of the boat's hull type? If Al's boat is
a wooden hull, magnesium would be entirely unsuitable regardless of
any fresh water considerations.

As mentioned above, the zinc's are not a good choice in freshwater
cause of the forming of zinc-patina.
Magnesium will probably be gone too fast. Better would be aluminum-
anodes.

Fair winds,
Len.