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[kerk fisher]

I just purchased a pencil zinc for the heat exchanger and collar zinc for the shaft. (I assume our heat exchanger is 3" as we have an M35 on a 1992; if not I'll cut the zinc to size as Ron Hill suggested in the FAQ).  QUESTIONs: Where is the zinc on the shaft? Anything special about applying it?      We're in Lake Huron, fresh water; do we need a galvanic isolator? We're hardly ever at dock with shore power. We're staunch sailors on the hook. Thanks, Kerk

[Gary]

Kerk,

The shaft zinc fits around the shaft externally between the strut that supports the prop and the shaftlog, or where the shaft enters the hull.  We use two spaced about an inch apart in the Gulf of Maine because one can pretty much disappear in a 6 month season.  They are also a safety measure in the event the shaft should come loose from the transmission coupling.  The shaft is stopped by the zinc from leaving the boat. 

Have a great weekend sailing Lake Huron.

Gary

[isabel98]

Not to diminish the experience of another sailor, but I have changed my prop shaft and I can tell you the shaft will not leave the boat while there's a prop on the end and the rudder is still attached to the boat. Now lose your rudder, and you could lose your shaft, but with that kind of catastrophic failure, the last thing on your mind will be a zinc....

[Ron Hill]

Kerk : Isabel has it right.  NO way will the Zn save the shaft from leaving the boat.  If the connection at the transmission fails the entire shaft with prop would hit the rudder.
On your question about the shaft Zn:  Make sure that the egg shaped shaft Zn has a BB in the inside.  The BB indents into the shaft and make a better electrical contact that the Zn without a BB.  I use only one shaft Zn and have it about 6/8" from the strut.  That way the flux pattern protects the shaft, strut and prop.  Insure that the Zn is seated by hitting it with a hammer.  Then tighten the set screws.  Rotate the ZN 180 degrees and whack it again and retighten the set screws.  I do this about 3 or 4 time to make sure that they Zn is well seated and will NOT loosen and slide up and down the shaft - a VERY disconcerting sound!!

On the pencil Zn take a #2 pencil and stick it up in the hole and see how much of the new 3 inch Zn will go in.  Maybe all, but if not cut it to size.  DO NOT FORCE A 3" Zn in the hole if the #2 pencil won't go in 3 inches!!   

[Tom Glennon]

I like the Godfrey brand shaft zincs.... rather than a BB inside, they have a faily broad band of copper.  They do cost about 2.5 X's the regular type.... but the protection is worth it.   

[kerk fisher]

Thanks, all, for your help. Kerk