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What is the purpose of the anode on the prop shaft?
Essentially, when you have two dissimilar metals in a marine environment, you have the potential for a battery, the less "noble" metal being the anode or positive terminal and the other being the cathode or negative terminal. (The concept of nobility is ancient in that the more precious metals such as gold, sliver and platinum, which don't oxidize in air, were considered "noble" while those such as lead were considered "base".) This sets up an electrical current in which metal from the anode is deposited to the cathode, i.e. the anode becomes corroded. That's the case with steel propellor shafts and bronze propellors, especially in salt water where the battery effect is accelerated. A zinc anode, which is made of a less noble metal than the propellor, becomes the preferred anode and hence is "sacrificial", helping to preserve the other metal components. That's why it's critical to keep an eye on your anode and replace it as soon as you seen any deterioration other than mild discolouration. That said, the whole issue of electrolysis and isolation of various metal components such as engines, fuel tanks, keel bolts and through-hulls, is one which provokes lively debate. One could argue that by sacrificing itself to save the other metals, the zinc actually is more noble, but that's for Philosophy 101. I expect others may want to wade in and, if I've erred in any way, correct me.
Ken Pole
Santiva
Ottawa
Ken Pole, Ottawa
1975 Mark III Santiva
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That's sounds like a nice explanation to me....some of you might be interested in the two part series on corrosion in "Good Old Boat" magazine (last two issues) as well.
I just purchased a replacement zinc anode to go on the shaft, per the surveyor's recommendation (from last spring). My MK IV is in salt water. However, the diver who takes care all things below the water line said he usually only replaces these when they are down to about 15%. So, does anyone recommend replacing shaft zincs earlier? Also, if anyone has advice for checking/replacing zincs in a Yanmar 2GM, I would welcome helpful hints.
Jim Andrews
"Over the Yardarm"
Palm Harbor, FL
The Good Old Boat articles were very informative.
When I pulled my boat out to replace the stuffinig box, etc., the prop was in such bad shape from electrolysis that it had to be replaced.
The zinc on my Yanmar 2GMF is a simple plug affair that screws into the block. I check it every year when flushing and replacing the coolant. (OK, I live down south too and understand that some years are 730 days long when it comes to changing coolant).
Mike M
SV Wind Horse
#375
Galveston Bay
Mike M
SV Wind Horse
#375
Galveston, Tx
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