"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.

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.