Build up on stainless propellor.

[Danf]

I am seeing a greyish build up on a polished stainless propeller not long after salt water use.
I believe that this is coming from the sacrificial anode which is getting eaten away quite quickly.
The boat is aluminium and is now on its eighth motor. This problem is worse than ever before and no other changes have been performed on the boat.
A small piece of polished stainless was bolted onto the cavitation plate and this too had a greyish coating on it just after a few hours of immersion.
The motor is a Mercury 60 4 stroke.
Just wondering if anyone can share any ideas on this.

[Noelm]

you are getting electrolysis from something, fix it now, how are your accessories earthed?

[Danf]

you are getting electrolysis from something, fix it now, how are your accessories earthed?

I am pretty sure that all accessories are earthed back to a common link. This link is then connected to the negative pole of the battery.
That may not necessarily rule out any stray currents which may be present. There may be a problem with an accessary taking a shorter path, through the boat back to earth.

[Fed]

How many wires are on the battery negative & where do they go to?

8 Motors? Did all the previous motors suffer from corrosion problems or was that just a passing comment? That's a lot of motors.

100% agree with Noel, "Fix it Now".

[Noelm]

Yep, it needs to fixed right away, I have seen this exact thing before, and if not fixed, you will be looking for another motor to replace this one, and possibly a hull too, the couple I have seen that were really bad, were new motors, and all (and I am sure this is coincidence) had a twin battery VSR setup.....to find the problem might require someone with some good electrical knowledge, and a good meter.

[Danf]

How many wires are on the battery negative & where do they go to?

8 Motors? Did all the previous motors suffer from corrosion problems or was that just a passing comment? That's a lot of motors.

100% agree with Noel, "Fix it Now".

The hull is 21 years old and is on its eighth motor.
The other motors never had this happen.

[DAVE_S]

Have a read of this link http://www.protroll.com/books/?id=5&p_id=4
Helped a guy that had the same problem as yours , turned out he had fitted a new stereo which had a earth wire but it was also earthing through the case . All he did was fit it in a plastic case to insulate it from the hull . It was quite interesting with the boat in the water if you turned the stereo on you could see the water bubbling around the outboard leg .

[Fed]

ProTroll are trying to sell flux capacitors for boats Dave, lost me in the first paragraph when they indicated galvanic corrosion & electrolysis were the same thing.
I did read on for some amusement and it went downhill from there.