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

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:
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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One more quick thought. If you know the
approximate surface area of the prop and
the weight of the zinc lost in say 2
months, it is fairly easy to figure out
whether that is in the normal range. Let
us know and we'll try to tell you.

Chuck

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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
..
You mention a Galvanic Isolator. Not sure what that is i.e. does it
isolate the shore power earth/ground from the ocean earth/ground of the
boat?

Am no expert on this but when we had a boat survey done, many moons ago
the surveyor seemed most interested in whether some slight corrosion
could be due to electric current from the shore supply going to
ground/earth via our boat.

In other words not 'galvanic' corrosion but that caused by electrical
supply 'leakage' through the metallic parts of our fiberglass boat that
were in the water?

Another way of putting it might be that our boat could providea better
'ground' through its immersed metallics into the seawater than a five
foot ground rod some distance away driven into dryish granular soil!

One answer is apparently to use an 'Isolating transformer'. Each side
of which has its own ground/earth. An isolating transformer can be
quite small if it supplies just a small amount of power for the long
periods that a boat is unattended/unlived in to operate say a small
battery 'refresher' trickle charger and/or one tenth of horsepower tiny
bilge pump. But nothing 'heavy' though, such as a fridge etc..

Suggestion anyway.

chuck wrote:
One more quick thought. If you know the approximate surface area of the
prop and the weight of the zinc lost in say 2 months, it is fairly easy
to figure out whether that is in the normal range. Let us know and we'll
try to tell you.

Chuck

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News==----
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Yet another thought.

If your larger zincs really do look
"new" (i.e., they are shiny) then they
are probably active. An inactive zinc
usually looks dark and tends to acquire
a film characteristic of submerged matter.

Chuck

"claus" wrote in news:1YOdnS4xRIdBtHLZnZ2dnUVZ_q-
:

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.



I'd bet the screw is electrically isolated from the saildrive, but I don't
see how. The little zincs are then consumed protecting, by themselves, the
metal of the screw, with no help from the big zincs on the shaft/drive
itself.

It would be impossible to test this without hauling the boat out and taking
a meter between screw and shaft.

If the marina or that derelict next door were the problem, it would eat
away ALL the zincs, not just the little ones on the screw.

There has got to be some way the screw is isolated, electrically, from the
other zincs. Does the saildrive have a rubber damper in the hub of the
screw? That would isolate it, right there.



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

I installed a galvanic isolator to protect my saildrives/zincs, and I
believe it works.
I have heard of someone w/ a saildrive who installed a battery disconnect
switch in the ground cable to the engine , which he switched off when docked
or even anchored. Seems to me this is a good idea, but I'd like to hear what
others think of this.

"claus" wrote in message
...
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

"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?