The (folding) prop on our boat is made of Bronze.
"Dennis Pogson" wrote in message
...
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

I had the same engine many years ago. At that time I was amazed to
discover
the prop was made of cast iron. Could that have anything to do with the
problem? I am no expert on galvanic corrosion, but could it be that zinc
and
cast iron should not be in direct contact with each other?