Re: [Amel Yacht Owners] Re: prop shaft bonding


I haven't seen problems with propspeed being bubbled off my prop. (A MaxProp that I keep a zinc on).  What I do see is the coating fail (after about a year) at those parts of the blades that tend to cavitate--on the back side, near the root of the flat, uncupped, blades.  Maybe in the absence of zinc that might not happen, but that's not an experiment I intend to run!

I wouldn't expect zinc to protect the stainless in a cutlass bearing or under the packing.  The problem there is low O2 content of the water, resulting in incomplete formation of the protective oxide coating. It's not the kind of galvanic corrosion that zinc prevents.  

In most boat's running gear the stainless is the most noble of the connected metals, so it sits there happy while the bronze dissolves.

I have little first hand experience with wood boats.  I know of the problems with alkali attack on wood, but I thought that was only a problem when the metal was significantly overprotected by excess zinc.  If I ever lose my mind and buy a wood boat, I'll remember what you say here!

Bill Kinney
SM160, Harmonie
Fort Lauderdale, FL

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