Possible reason for electrolysis on Prop?

[Bob & Sue]

When I had the boat pulled for repairs the marina found a small amount of electrolysis on the prop, last June the outdrive & prop were both painted with new Aluminum anodes. Marina installed new prop because of log damage & partially spun hub and put Zinc anodes on this time.

While greasing the coupler I found the ground wire from the transom assembly to the engine or maybe the steering lever not sure yet haven't been able to trace it back to the source yet it had been cut about 3" from the transom assembly. It was cut probably 5 years ago when the coupler was changed (lazy mechanic). I had the unit checked 2 years ago with a mercruiser test probe while the boat was in the water at a different marina before moving to this marina the readings were right in spec. I'm going to replace the wire this weekend.

I'm sure I know the answer that this caused the tips of the prop to have minor electrolysis where the paint had worn it's an aluminum prop.

My question is should I check with the test probe prior to hooking up the broken wire then check after the wire is replaced, would it make any change in the reading?

I can't find any info about this online or in the manual, I would think the readings should be the same because of the other grounding wires going to the outdrive.

Here's a link to daisy chaining of the components, I'm sure it's the wire in fig #1 inner gimbal is connected to the outer gimbal and steering lever.

http://www.mercstuff.com/bravo3corrosion.htm

Thanks any thoughts would be appreciated.

Sorry for the long explanation trying to make the question clear.

 

[BillK2632]

Definitely replace the cable without it the grounding "chain" is broken and the drive is not fully protected by the anodes. I still am not sure that is why you are seeing corrosion on your aluminum propeller - are any other parts of the drive corroding? Reason I say this is that most outdrive corrosion is usually from galvanism due to dissimilar metals on the drive (ie SS propeller and aluminum drive housing). If you have an aluminum propeller, there is not nearly as much dissimilar metal on your drive - SS nuts, driveshaft, trim pins, that's about it. Not like a SS propeller hanging there. I Bravo or Alpha drive with an aluminum propeller is pretty inactive, corrosion is not usually a problem. I'm wondering if the damage on the propeller blades is from ventilation and not galvanic corrosion. If you are seeing any other corrosion on the drive, then I would blame it on the missing bonding cable, look for corrosion around any edges and bolts where the paint would be compromised.

 

[Bob & Sue]

Thanks for the replies. That's what has myself & the marina stumped there is no sign of corrosion on the drive or any other parts at all just the tips of the prop had slight electrolysis. I am going down later to replace the wire, I would have done it when I found the broken wire but all my tools and spare parts were at home being reorganized while the boat was on land being repaired.