claus wrote:
We have a Volvo MD22L engine equipped with a sail drive.

The hull and prop are "supposed to be" protected with:

1) A Large Zinc block attached to the hull just forward of the sail drive
and electrically connected to the engine block through the hull.

2) Round Zinc around the propellor shaft - attached to the sail drive

3) 3 small Zincs attached directly to the propellor

The problem is that the 3 small zincs erode rapidly - sometimes within 2
months - and sometimes within 4 months...no regular pattern.

The other zincs (1 and 2) look brand new after 2 years in the water and
obviously do not protect the propellor at all.

When the all the zincs were installed at the last haul-out 2 years ago I
made sure they were free of any surface oxidation.

I have also installed a Galvanic Isolator in an attempt to alleviate the
problem - and the boat has been inspected for any stray currents and none
found by the Marina staff.

The boat next two ours is a derilict without any maintenance whatsoever -
but is is also hoohed up to the same shorepower circuit as ours. The Marina
staff dioes not think that is the culprit.

Any suggestions?

cvj


Hello Claus,

I'm on my way out, so just a brief post
for now.

First, it would not be surprising to
find that the small zincs mounted on the
prop go first. After all, they're right
there. If everything is OK, then the
other zincs will begin to work when the
little ones are depleted.

At this point, you can easily verify
that the larger zincs are indeed
connected electrically to the prop shaft
and prop. Sometimes good contact is not
made through shaft couplings, etc. and
that would prevent the larger zincs from
working. Of course the small ones on the
prop are bonded electrically by definition.

You can check the voltage between the
engine ground and the prop side of the
shaft. If you see nothing, even on the
lowest DC voltage setting, chances are
good they're bonded. Using an ohmmeter
might be troublesome given the galvanic
potentials that might be floating around.

Good luck.


Chuck

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