Pink prop

[vyv_cox]

Nickel aluminium bronze is the material of choice when it comes to propeller manufacture. Virtually all ships' propellers are made from it, as it has excellent resistance to both corrosion and cavitation erosion.

I do recall a problem that I looked at very many years ago, in which one phase of a NiAl bronze formed a galvanic cell with another. This was in a propeller. It was a very long time ago and I don't have any records of it but I think it was a problem of cooling rate that allowed the formation of a gamma 2 phase. This reacted with the alpha/beta phase in seawater.

Some information suggests that low nickel content contributes to gamma 2 formation and also that pitting can result in seawater if its sulphide content is high. Any sewage or industrial outlets near the mooring?

If your prop that is made from a NiAl bronze has pitted in a fairly short time I suggest you speak to its manufacturers, as it may indicate a problem with the material.

 

[AndrewD]

Thanks everybody, I seem to have sparked (sorry!) off an interesting discussion here. I won't reply to each individual 'cos it would take too long, but:

1. Shaft anode - no chance, there is NO exposed shaft outside the boat, the shaft passes through the skeg, it's not a P-bracket setup.

2. Bridging the coupling would make no difference, the point is that the gearbox doesn't connect electrically to the shaft. In fact there are already bridging wires in place, which I maintained meticulously until I decided to check the continuity all the way through.

So it's going to have to be the brushy business I guess. Not cheap, as someone said, but cheaper than buying a new prop every few years.

Thanks again all!

 

[VicS]

[ QUOTE ]
Any sewage or industrial outlets near the mooring?

[/ QUOTE ] I dont know how much it has improved but when I worked on the banks of the Orwell there was a shi t-farm discharging into it in Ipswich. It was pretty foul at times and you could smell it over a large part of the town in the summer!

Google earth shows something a lot more modern now (between the Orwell bridge and the old power station site)

 

[Tranona]

You might find a prop nut anode will do the job. Available from MG Duff

 

[Sy-Revolution]

It was definately stinking a couple of years ago, maybe that's it then.....

 

[vyv_cox]

Does your antifouling go black? That's one of the best indicators of sulphides that I know. At Astakos in the Ionian, where raw sewage is discharged directly into the harbour, my white boot-topping went completely black in one night!

 

[Sy-Revolution]

My AF is black, so couldn't tell.