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?

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?

"claus" wrote
The (folding) prop on our boat is made of Bronze.

Are you trying to protect the prop or are you trying to protect something
else from being corroded by the prop? Bronze is way up on the cathodic end
of the galvanic scale, meaning it *should* corrode zinc fairly rapidly. And
of course it will also corrode other anodic metals like aluminum or iron if
there's no zinc handy. What other metals are involved? Is there a problem
other than the prop zincs eroding?

No - nothing else is corroding. The bronze sea-cocks are all healthy - they
are painted over and the diver checks them every 2 months - whenever the 3
small prop zincs are replaced.

After reading all the very helpful responses to my initial post I am now
leaning towards the theory that the prop is somehow electrically isolated
from the engine block (and the large zinc)- as unlikely as that seems.
However, I won't know until the next haul-out when it can be checked with a
meter.

BTW the large zinc had some sea growth on the surface at the last dive
inspection - so it is not protecting anything...



"Ernest Scribbler" wrote in message
...
"claus" wrote
The (folding) prop on our boat is made of Bronze.

Are you trying to protect the prop or are you trying to protect something
else from being corroded by the prop? Bronze is way up on the cathodic end
of the galvanic scale, meaning it *should* corrode zinc fairly rapidly.
And of course it will also corrode other anodic metals like aluminum or
iron if there's no zinc handy. What other metals are involved? Is there a
problem other than the prop zincs eroding?

"claus" wrote in
:

After reading all the very helpful responses to my initial post I am
now leaning towards the theory that the prop is somehow electrically
isolated from the engine block (and the large zinc)- as unlikely as
that seems. However, I won't know until the next haul-out when it can
be checked with a meter.



All it needs is a rubber damper to isolate it, electrically......



--
There's amazing intelligence in the Universe.
You can tell because none of them ever called Earth.

"claus" wrote
I am now leaning towards the theory that the prop is somehow electrically
isolated from the engine block (and the large zinc)-

That may well be the case, but it's not necessarily a bad thing. If you
haven't done so already, you might want to try to find out if it's that way
by design. Perhaps frequent relpacement of prop zincs is the price of
admission for your particular unit? There generally are no easy solutions to
galvanic corrosion, only trade-offs. For example, if you electrically
connect your prop to your drive unit, you might be encouraging its galvanic
currents to attack a nearby aluminum housing rather than a more distant
sacrificial anode.