Drive shaft corrosion

[dillehay]

On hull #1576, a 2001 MK II, which I bought new, I have discovered extensive corrosion on the shaft between the strut and the cutlass bearing.  Has developed in the last 3 years, during which time I have attempted to maintain zincs by a 2-month inspection along with bottom cleaning.  When last hauled there was no evidence of any issue.  The strut is fine.  Anyone with similar experience?  Thoughts?

[Jon Schneider]

The easy answer is that something has changed in the environment.  I'm assuming that you're in a marina.  Either a power line problem has been fixed or a problematic neighbor has left or fixed his problem.  Given the severity of the problem that you "had" (if the past tense is truly applicable), you may still want to install a galvanic isolator.  The other answer is a question: are you sure that the zinc you installed on the shaft is making good contact with the shaft (i.e., what the shaft thoroughly cleaned and roughed up where the shaft was placed)?  If not, the zinc may simply not be able to perform its role, and, sorry to say, you may still have a problem that's just not evidenced by zinc dissolution.  BTW, in the past, had the zinc required replacement every two months?

[canuck]

Our practice is to drill through the solid strut and attach a #3, the sand dollar type. They will show wear after one year.  I also replace yearly the two on the shaft. Make sure you have at least 6" between cutlass bearing and the zinc to allow a good flow of water into the bearing. Give these zincs a good whack to ensure proper contact. There is also a tendency to over zinc which can create it's own problems.

[Ron Hill]

dille : As Jon said - I'd be suspicious of the neighboring boats and their "wiring" . 
When I purchase a Zn I make sure it's one with a "BB" on the inside. 
You have to remember to use a hammer when mounting your Zn.  Clean the shaft where you're going to mount the Zn with some 100 grit sand paper.  Mount the Zn and turn the key tight, then rotate the shaft 180 degrees and tighten.  When both screws are tight hit the Zn with the hammer, rotate 180 and hit it again and re tighten each time .  Now your Zn is mounted and should be in contact with the shaft.
A few thoughts

[Jon Schneider]

When I purchase a Zn I make sure it's one with a "BB" on the inside. 

Ron, what does "BB" indicate?

[Ted Pounds]

It's a little copper ball on the inside that that help ensure electrical contact between the zinc and the shaft.

[Bob K]

I also put a zinc on my strut, in the same fashion as Canuck mentioned above.  I started doing so after noticing a little pink discoloration which looked like corrosion.  Sure enough, the zinc needs replacement every year.  I wonder why, since the strut is isolated from the grounded shaft?.  On a similar note,  I have wondered if I need to add a bonding wire to my one bronze through-hull valve (AC intake).  I've read opinions/theories etc. both for and against.  I've left it unbonded, and I do check it for corrosion, and so far so good - but surface inspection probably doesn't tell the whole story, does it?  Maybe the best thing is to replace the through-hull with plastic. If anything, it would get rid of the worry.

[canuck]

BB = ball bearing

[Ron Hill]

The "BB" is the same as the BB (projectile) that you shoot out of a BB gun (ie. Red Ryder range rifle!).  It makes sure that you get a good electrical connection. 
Hitting the Zn with a hammer makes sure the Zn is seated and the "BB" makes a small dimple in the shaft. 

[Roger Blake]

I do the same as Ron and haven't had any problems...get the zinc with the BB, tighten, hammer, tighten, hammer...etc. Replace the zinc every year, but some years, only about 1/3 is gone. Maybe I just have good neighbors with no electrical issues... 

[Ron Hill]

Because I have a flexable coupling the shaft is electrical isolated.  I usually can easily get 2 years out of the shaft Zn.  A thought