Cathodic Protection

[davidhand]

I just had a prop. fail due to dezinctification (spelling?) the prop. had been on since 1991 so it had lasted long enough. However as far as I can see the only zinc on the boat is on the prop. shaft. The through hulls are not interconnected or as far as I can see connected to a zinc, is this unusual? The boat is a 1985 Beneteau First 375. Any comments welcome, thanks.

 

[ccscott49]

If you had a zinc on the shaft, I can't understand why youve lost a prop, you shouldn't.

 

[vyv_cox]

Completely unnecessary to connect bronze skin fittings to anodes. All of mine, dated 1985 in a grp boat, are original and virtually uncorroded.

 

[dickhicks]

Sounds like Beneteau fitted the cheapest props they could find - probably manganese bronze - not the best material for longevity.
Aah - just reread your post, the prop was not original - did you or the previous owner fit it and why? Props usually last longer than 6 years(assuming conventional drive and not saildrive)

 

[davehu]

I had a similar problem many years ago, I also found that the propshaft zinc kept corroding around the fixing bolts and the zinc would fall off long before the end of the season and this left the prop unprotected. I fitted a pear type hull anode connected to the engine and with a earth strap from engine to prop shaft and I never had any more problems with prop dezincification

 

[ccscott49]

How can manganese bronze de-zincify?

 

[vyv_cox]

Colin,

You're falling for it again. Manganese bronze is a 60/40 brass with a few bits and pieces, 1% tin, 1% iron, .25% manganese.

Vyv

 

[ccscott49]

Then it's not effing bronze!!! It's brass!! I wish they would stop all this bullshit, it's confusing the hell out of me. Manganese bronze!!! indeed! Thanks for putting me right. I guess I need to update my reference data, I don't have manganese bronze/brass on it. Only thing I can find with Mn, and Zn in it, is High tensile brass. Some Mn in silicon bronze aswell. I only ever have dealings with Silicon bronze, aluminium bronze and phosphour bronze, the real ones. IMHO.