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.

Checked the below with the local Volvo Marina Diesel dealer - and his reply
confirms your "bet"

--------------------------quote----------------
I don't have a folding sailboat prop which I can check, but I did check one
of our outdrive props and there is no continuity between the center hub
which fits on the shaft and the actual prop due to a rubber "hub". I believe
the same is true of your prop but the only way to be certain is to check the
continuity of your prop the next time it is out of the water.

On the newer Volvo engines there is no continuity between the engine block
and the saildrive. The ring zinc just forward of the prop protects the
saildrive and only the saildrive. Any other underwater metal object need to
be protected with their own anodes,

-------------------------unquote ------------------------

"Larry" wrote in message
...
"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.