A problem wth zincs ... should I bond?

[AndrewB]

Dived down yesterday ... my propshaft zincs have almost gone after just 6 weeks since they were refitted! This happened last time too.

I have a stainless shaft with an AB2 prop, which was new last year. The shaft is electrically isolated from the rest of the yacht, which has a steel hull, to which everything else aboard is connected. The zincs on the hull erode at a slow-to-normal rate, i.e. need replacing every three years.

Should I electrically connect the shaft to the engine and hull? Leave it as it is and just stop wasting zincs on it? Work out a way of electrically isolating the engine from the hull (this would be difficult)? Or what?

 

[FullCircle]

Andrew, cant see it being a good idea not to bother - after all if the current is flowing down through the anodes, it must be protecting something!
Would suggest setting up metal brush pickup on shaft, and connecting back to engine ground and seeing if that makes a difference.
What teype of anodes are you using? Are they different in composition to the other hull anodes?

I am fitting a 2.3kg pear drop dangler for when I am away from the boat next year, connected to engine/earth.

Give John a ring at www.zincsmart.com, he may have an answer.

 

[DaveS]

Just for clarity - you say the shaft is electrically insulated from the rest of the boat. Does that include its connection with the engine?

 

[tcm]

are you on shorepower mostly full-time?

edit- ah, probably not, i see you're in carib

 

[DeeGee]

Andrew, my shaft was isolated from the boat, and the prop had no anodes (no room) when I bought it. The prop was de-zinc'd to the extent bits were falling off it! I put brushes on the shaft and the new prop stills pings nicely after nearly six years. I made a bracket to hold the brush kit. I can't find a ref to this off-the-shelf kit, which was quite cheap, praps someone else...

The practice was to put a bonding wire across the coupling (with rubber inserts) but the gearbox oil sets up a big resistance which can defeat this trick.

 

[AndrewB]

Thanks for that. I can't understand the logic of why bonding would help but it did in your case, so I guess its worth a try.

I can put a bonding wire across the nylon flexible coupling easily enough, and the gearbox coupling seems to have a good enough electrical connection to the hull (Yes, I have checked there is enough oil in the gearbox!). This would be a lot cheaper and easier to install than a brush.

My old prop (also AB2) lasted 10 years and then suddenly became badly pitted which is why I fitted a new one. Which suggests something must have changed but I can't think what. Haven't fitted any new electrical gizmos since that Navtex you sold me umpteen years ago!

 

[AndrewB]

Yes ... there is a nylon flexible coupling between the shaft and transmission. My multimeter confirms no connection.

 

[smb]

ASAP sell them. I believe they are called 'electro eliminators' or some such. The replacement I bought after a bent shaft vibrated the old one to death cost about £75.

Steve

 

[DeeGee]

The story I got sounds plausible - the alloy of which the prop is made forms zillions of micro-cells from the dissimilar component metals, and with the noble shaft. With no lower galvanic element around to take the heat, my prop (your shaft anodes) got zapped.

ELECTRO-ELIMINATOR - yes, that's the thing. It has been taken over by MGDuff, but is pretty much the same as before, with a mounting I couldn't use. It took about 2hrs to knock up a mounting out of some 1/4" acrylic I had around. Go the jumper link if you can undo something on each side, but mine threw off after some weeks, so I put on the EE thing. /forums/images/graemlins/smirk.gif

Pictures here

Hope Navtex still working!