Here is a very general primer on stray current corrosion
A galvanic isolator, by design, will block galvanic current
up to a galvainc potential of about 1.4VDC. A galvanic isolator cannot block stray current
as stray current is produced by battery
Stray (electrolytic, not "electrolysis") current is caused by an inadvertent B+ connection to an underwater metal component which becomes the anode. The cathode is connected to the B- of the same DC source and is protected from corrosion
. Both the anode and cathode are immersed in the same electrolyte to create the stray current cell. With both anode and cathode in the same electrolyte, metallic ions leave the surface of the anode, which deteriorates, and migrate towards the cathode which is protected.
sail drives are a special case because aluminum
cannot be protected to a Ag/AgCl reference cell potential >1100mVDC. To go above this limit will "overprotect" the aluminum sail drive and the caustic water
created on the surface of the saildrive
will badly deteriorate it to the point of failure. I suggest you search the web for "aluminum sail drive stray current corrosion" as Yanmar
and others have references
Some corrosion factoids:
1. Galvanic corrosion is a very slow process; durations of months and years.
2. Stray current corrosion is a very fast and very destructive type of corrosion; durations of hours and days to see the effects are not uncommon.
> A sail drive can completely disintegrate in a few days.
> A 3" SS shaft looks like it has been eaten by worms in < 1 week.
3. In my experience, it is extremely rare for another boat
to cause stray current corrosion. The victim boat
is almost always the cause of the stray current that is destroying its underwater components.
Note: There is one galvanic isolator on the market that advertises that it blocks 2.2VDC but since the standard galvanic series shows magnesium (used in freshwater) with a galvanic potential of -1630mVDC referenced to a AgAgCl cell, this added range will do nothing to protect against stray current corrosion.
There is a lot of bad info available regarding this subject. I recommend that you engage the services of an ABYC Certified Corrosion Tech to help you determine and correct the problem on your boat.