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