Re: [Amel Yacht Owners] Re: prop shaft bonding
Bill,toggle quoted messageShow quoted text
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