Prop De Zincification

[affinite]

When I bought my current boat 2 years ago the survey said that there was evidence of dezincification on the prop.
What is it and can it be cured ?
The boat is on the hard at present and the surface of prop looks rough but does it matter?
Thanks

 

[Roy]

Pitted prop could be due to erosion caused by insufficient anode protection - anodes used up or/and cavitation burn due to incorrect positioning and angle of prop at speed.

 

[john_morris_uk]

Dezinctification is the loss of zinc in the alloy that the propellor is made of by electrolytic action. There is no cure and advanced states lead to failure of the propellor.

You can prevent it by providing adequate cathodic protection with the caveat that for many wooden boats, cathodic protection leads to damage to the wood and its cheaper to buy a new prop every few years than enter into major surgery on the hull.

What sort of boat do you have?

A surveyor would tap the prop with a small hammer and listen for the ringing note.

A badly eroded prop has a dull sound and if its really really bad will fall to bits even when tapped gently.

 

[affinite]

John
My boat is a GRP hulled Feeling 345. I suspect that poor cathodic protection was the cause as most earthing straps were perished and needed to be replaced. I guess I'll do a search in here for advice on cathodic protection to ensure I got it right but sounds like I should consider a new prop as well.

 

[PeteMcK]

Dezincified bronze can be unbelievably weak and brittle. If you've a few pits around the tips, you might be ok, but if the pitting's widespread and it rings dull, or not at all, I would chuck it away.

Fit a shaft anode with the new one and change it every time you get a chance - you're lucky to get 6 months out of them.

 

[affinite]

There isnt a shat anode at present but there is one on the hull about 1m away. This is bonded to the engine etc.
Does the shaft anode simply bolt onto the prop shaft ?

 

[wagenaar]

Bronze is an alloy of copper and tin and therefore can not show dezincification. There is still a problem with the definition of bronze!! What is sold is marine bronze is in fact a brass. Brass is an alloy of copper and zinc and can show dezincification. What is often, incorrectly, called managanese bronze is an alloy of copper and zinc with an addition of managanese.

 

[tcm]

yep, bolt on to the propshaft. There's bound be a size that fits, like a collar. You can't really have too many, and there should be one on every bit of metal - some boats seem to skimp (or mebbe it's fallen off?)