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

James Alton


  I think that you understand metals and corrosion pretty well.   In my experience, a really good bronze can survive the Marine environment for the life of the boat unprotected but those types of bronze are not strong enough to make a thin prop blade out of.  Props as I understand things are made of stronger but more corrosion prone alloys, hence the need for protection.  I am quite sure that you are correct that the bronze prop on my Loki is protecting the stainless shaft galvanically.

  We took care of a customers Gulfstar 54 over the period of 20+ years and tried various coatings for the twin PYI feathering props and even Prop Speed failed after 6 months by bubbling off.  While the coating tended to fail first on the back side of the leading edge of the prop, bubbles would form over all of the surfaces with the boat just sitting.  I think that the zincs for the PYI Max Prop might have some magnesium in the alloy to increase the level of protection but at any rate the anodes did corrode away on those props much more quickly than the common shaft zincs.   Imo there is a definite  connection between the rapid corrosion of the anodes and the lifting of coatings on underwater metals such as a prop.   

  I have had to replace a lot of wood in boats damaged by the Alkali buildup and have seen a few nice wooden boats completely destroyed from serious over zincing.  The amount of damage to the wood is as you allude a function of how heavily protected the metals are.   While our  fibreglass hulls are not affected themselves, you can sometimes see some (generally minor)  alkali damage to wooden backing blocks under protected bronze seacocks.  For some reason, bronze alloys are the worst about building up the Alkali when protected than stainless or monel.  

SV Sueno,  Maramu #220

On Nov 24, 2017, at 12:13 AM, greatketch@... [amelyachtowners] <amelyachtowners@...> wrote:

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 al kali 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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