Fyi ... here's one link I found to testing ...
http://home.comcast.net/~dockklub/marinemechanic.htm ... and here's a link
to some high level info on galvanic isolators (a few paragraphs down) ...
http://www.boatsurveyor.com/corrosion.htm. Disclaimer: I'm a trailer boater
(for now) with electrical engineering experience. It seems to me that with
an appropriate galvanic isolator (the article suggests to me there are some
out there that could be hazardous to your boat's health), an appropriate
size/material (8 gauge copper ?) onboard negative bus with clean connections
to all equipments, batteries, etc that terminates at both stern drive(s),
and the appropriate anodes, there "should be" no problem. It is very
unfortunate what happened ... I too would like to know what went wrong.

"Brian Saucier" wrote in message
...
Hello All,

It has been awhile since I posted or subscribed to this newsgroup. I
hope everyone is well and enjoying their boating season.

My season just took a setback ... as my boat (2000 SeaRay Sundancer
290)
is sitting on blocks, awaiting delivery of 2 new lower units for my Bravo
II
drives. Severe corrosion, despite changing all sacrificial anodes, every
9
months, caused major cracks along the 'bullet' casing. There are two
cracks
on each side of the twin I/Os that are severe. Generally the drives'
lower
ends (only 4 years old have pretty much disintegrated. So .... estimated
~$8000 later, I'm wondering what the heck happened ?!!?

Some facts: I have a reverse polarity indicator that never shows a
problem from my shore power.
However, the marina was flooded last year after a hurricane and the
'light
and GFI doesn't work' on the shore power box ... and I suspect stray
current - but don't know how to check it (mostly fresh bayou water).

My neighbor boats (only 2 others nearby) seem to not even use their
shore power much.

How does one 'check' a Mercathode system on my boat to make sure it is
working correctly ? I really want to rule-out anything going on in my
boat
and the slip before I return with new drives.

Thanks in advance to all that provide me advice !!!

Greatest Regards,
Brian