Chuck wrote in news:1179189498_23605
@sp12lax.superfeed.net:

I'm still leaning toward the idea that
one of your 12 volt appliances is
causing +12 volts (more or less) to be
set up between the engine/prop and
something like a through-hull. It could
be a short or incorrect wiring to the
appliance. The effect would be to
accelerate the corrosion of the zinc.


We had one eating zincs something awful on an adjacent dock. That was
eating everyone's zincs from the stray currents in the supercharged
water. I had a hand in it after someone mentioned it to me. Noone had
switched off one breaker at a time on the DC panel because everyone was
blaming the marina's AC power system, which was in plastic conduit under
the docks.

We used some Family Radio walkies and setup some meters monitoring zinc
currents to some boats and dock post grounds. It was hilarious when I
flipped off the offending boat's water pump. Everyone keyed their radios
at once to report the current just stopped.

What was wrong was the fresh water pump was plumbed to a metal
(stainless?) water tank in a wet bilge that had contact somehow to the
boat's lead keel...into the ocean. Closer inspection showed there was
12VDC on the old metal diaphram pump's frame when the breaker was on!
One of the brushes was contacting the ungrounded case of the motor, which
was bolted to the metal housing of the old pump's parts.....which hooked
it to the fresh water. I supposed the fresh water had to conduct the
current down the plastic pipes into the metal tankage....to the keel.

Putting +12V, or something less, on a big metal keel weight didn't trip
the breaker (nothing down there was grounded to the battery (-) post,
including the motor's frame). The breaker, of course, was 20 amps so the
current leakage was much less than that trip point. If the pump frame
had been grounded, it would have shorted the 12V and tripped the breaker.

I forget the pump name but it's that one that has a little cog belt
driving a concentric that moves a metal pushrod up and down on a rubber
diaphram that pumps the water. Its name escapes me after midnight. He
got a new plastic pump from Jabsco and everyone's zincs quit fizzing
away. I don't think galvanic isolators can stand a good 12V house
battery's potential.

Larry
--
Grade School Physics Factoid:
A building cannot freefall into its own footprint without
skilled demolition.