How much pitting of a prop is acceptable?

[NPMR]

I have the 3-bladed fixed propellor off the boat at the moment.

Having scrubbed and polished it, there are 'stains' (like pitting - they're darker and don't polish out but aren't very pink) on the blades and around the centre boss, there are vaguely pinkisk stains as well.

But overall, the prop is 'brassy' and 'sings' when you tap it (i.e. not a dull sound) so I suspect it has some de-zincification (new anode and wiring put in last year, so can't be sure how good it was before that) but wanted opinions as to how far 'gone' it has to be before it should be binned.

Or who would/could look at it and give a sensible opinion, in deepest, darkest Cornwall!

 

[Norman_E]

If it still rings when rapped with a stick it is probably OK, but if there is deep pitting near the root of a blade that could be a problem. A lot of propellers are brass rather than bronze, and as such suffer some degree of dezincification, but when it gets really bad the dull tone when hit is a giveaway. Check for any signs of cracks in the metal, as any cracks should be considered serious. You could try posting photos on here, but your description does not indicate too big a problem. My old 3 blade prop was quite pink, but still rang when suspended on a piece of rope and struck with a piece of wood.

 

[rafiki_]

Sounds like a bit of corrosion. What sort of state are your anodes in? Are the working correctly or do they need replacing? Steel Developments in London will do a very good prop repair at excellent value.

 

[PeterGibbs]

I have the 3-bladed fixed propellor off the boat at the moment.

Having scrubbed and polished it, there are 'stains' (like pitting - they're darker and don't polish out but aren't very pink) on the blades and around the centre boss, there are vaguely pinkisk stains as well.

But overall, the prop is 'brassy' and 'sings' when you tap it (i.e. not a dull sound) so I suspect it has some de-zincification (new anode and wiring put in last year, so can't be sure how good it was before that) but wanted opinions as to how far 'gone' it has to be before it should be binned.

Or who would/could look at it and give a sensible opinion, in deepest, darkest Cornwall!

I think you will find a lot of props have some damage or corrosion but are nowhere near failing. As you know, it's just another piece of gear that has a limited life and needs watching.

When corrosion and pitting begin to affect the balance of the prop then collateral damage to bearings will be apparent along with vibration. This is an "action point" ie renovation by a prop company or replacement.

Any cracks however small are serious and could lead to prop failure at any time, especially if you strike debris.

Colour changes as you describe are, I suggest, not an immediate call to action but anode protection should be maintained at a high level.

Fixed props generally have a lot of built-in reserve.

PWG