Bronze Prop De-Zincification

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bobm

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Can some one tell me what this is, what does it look like and what are the implications please. i have cleaned my prop but it has some brownish wavet discolouration lines accross the blades. The anode is well worn, 14 months old, the hull anode, same age has some slight wear. Thanks for any replies.
 
"Bronze" props are usually managnese bronze which is actually a brass!

Dezincification is loss of the zinc from the alloy leaving the copper behind which has a pink colour and no strength.

(true bronzes are copper tin alloys)

An anode connected to big chunk of copper alloy is going to waste away whether it is doing any good or not. If it doesn't then it is probably not effectively connected.

What is the hull anode bonded to? Anything or just sitting there.
 
To test your prop give the blades a tap with a little hammer. If the resulting noise has a ring to it then it's fine. If you just get a bit of a dull thud then it has de-zinced. If the blade breaks then it's proper f*cked.
 
Vic S

Thanks for the info, the hull anode is bonded to the engine, what else it is bonded to I don't readily know but the boat was built to a high standard so I have some confidence it will be correct based on the general standard of the electrical installation.

Bob
 
Bonding to the engine is generally good - with the previso that an insulated drive coupling may have been fitted at a later date and if so the bonding path to the prop will have been broken. Have a look at the drive coupling and if it has a flexible plastic disc linking the two halves, then you'll need a copper braid to join a bolt head on each side to connect the propshaft and prop to the engine electrically. Of course, the braid will need to be long enough to flex easily as it rotates, but not project too far so it doesn't catch on anything.

Rob.
 
bobm said:
Can some one tell me what this is, what does it look like and what are the implications please. i have cleaned my prop but it has some brownish wavet discolouration lines accross the blades. The anode is well worn, 14 months old, the hull anode, same age has some slight wear. Thanks for any replies.
Click to expand...
Bronze props dont de zincify, bronze is a mixture of copper and tin, no zinc in it!
http://en.wikipedia.org/wiki/Bronze
Stu
 
Bronze props dont de zincify, bronze is a mixture of copper and tin, no zinc in it!
Click to expand...

But manganese bronze is a common alloy used for props. A material with good resistance to seawater corrosion.
As said earlier though it is a copper zinc alloy and therefore technically a brass with just smaller % ages of other alloying elements

In adverse conditions it can dezincify although normally it does not.

Also from Wiki:
Manganese bronze (ASTM 138) Cu 58.5, Zn 39.2, Fe 1.0, Sn 1.0, Mn 0.3
 
Our prop lost one of its two blades some years back, shortly after we bought the boat. On examination the remains of the prop were pink and visibly porous. We had previously suffered rapid anode loss (needed replacing at least every 6 months).

The cure has been to fit a galvanic isolator in the shore supply system - the boat is, and I think had always been, permanently connected when in the marina, surrounded by other boats similarly plugged in. Since then anode wear has been sensible - about 12 to 15 months for both shaft and prop anodes.
 
I had a prob with my present boat, the prop. was getting slightly pink. I tested between prop shaft and engine and got a reading of a few m/amps this meant that there was a break between prop shaft and engine.The coupling was linked across ,so it could only be the oil in the g/box bearings insulating the connection.I had an old Black & Decker drill took out one of the brush holders and fixed it so that a brush could run on the coupling.I made a brush out of a starter motor brush,(used this as they are high in copper so less resistance).This cured the problem.What was the electrical current,tho low,doing to the g/box bearingsI dread to think.
 
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