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.