"Jim Woodward" wrote in message
om...
Problem: When you connect an AC line to shore power, there is an
opportunity for low voltage DC to sneak aboard riding on the AC
connection. This usually happens because someone else in the marina
has electrical problems, but can happen even if you're the only boat
on the system. The DC causes electrolysis -- the reverse of
electro-plating -- and can remove metal from things (propellers,
shafts, thru hulls) that you'd like to keep intact. Zincs help
because they are less noble than other things and therefore go away
first.

This is generally less of a problem in freshwater than salt, because
fresh water doesn't conduct electricity anywhere near as well, but it
can happen anywhere. If you're losing zinc, it is happening.

Solutions:
1) As mentioned in a previous post, the best solution is an isolation
transformer -- this is absolutely standard on large vessels. Only the
AC can get through, so you create your own isolated electrical world
even though you're hooked up to shore power. Iso trannies are
bulletproof, old technology, with no moving parts and essentially
nothing to fail unless you vastly overload them.

Problem is, they're heavy and expensive (around 20 pounds and $150 per
kilowatt capacity {a kilowatt is around 8 amps at 120VAC}) They can
get hot (about 5% of the power passing through is lost as heat) and
can hum a little.


2) Next best is a galvanic isolator. This is basically a set of back
to back diodes that pass AC but block small DC voltages. If you keep
within the rating, they'll do fine, but they're more sensitive to
overloads and can, rarely, fail. $160 for 3.5KW from West Marine,
about 3 pounds (1/3 the price and 1/20 the weight of an iso tranny of
the same capacity).

3) Do nothing. Replace your zincs as required before they go away
completely. This will probably be all right, particularly if you keep
an eye on your zincs every week or so. If your marina has DC
problems, I wouldn't want to go for months without checking.


Jim:

I'm not real savvy when it comes to things electrical, especially of the AC
variety, so perhaps you can explain to me what is behind the observation I
made of my own situation, which formed the basis for my post. My boat has a
galvanic isolator, but not a Mercathode system. It is slipped in fresh
water. In its first two years of existence, it was my practice to turn off
all DC devices as well as the master battery switches. Bilge pumps are
wired direct to the batteries. I would always leave on the AC supply to the
fridge, the air conditioner, and the on board AC battery charger when
leaving the boat for the week. The only reason I left the battery charger
turned on was in the remote case of a large chronic leak that might develop
and could after time drain the batteries from extended bilge pump activity.
Thought it would be a good idea to have the charger online in case the bilge
pumps were to drain the batteries in this remote case. The charger is an
intelligent 3-stage rapid electronic switching charger from Charles Marine
(Series 5000). All electrical components are factory installed by Sea Ray.
After owning the boat for two years, the original flooded cell batteries
were cooked and in need of replacement. In addition, I went through zincs
at a rate that I thought was excessive. Probably replaced them 3-4 times in
the two year period. With two outdrives, that's a lot of zincs.

At the two year period, I replaced all three flooded group 27's with
Lifeline AGM group 27's. Love these batteries, but that's another story.
One of the benefits of the AGM's is a very low self-discharge rate. Based
on that, I decided to forego leaving the charger turned on while away from
the boat. Figured I'd take the chance of extended bilge pump activity
(which has never happened, my bilge is always dry as a bone), in favor of
not cooking these relatively expensive batteries. I still leave the AC
supply to the fridge as well as the air conditioner turned on while away.
Where I live, if I didn't leave the air turned on with the thermostat set to
80 degrees or so, the cabin temp would climb well into the 140+ degree
territory, making life a living hell for the fridge. All DC devices as well
as the main battery switches are turned off while away. I'm still in the
same slip, and my immediate neighbors are the same as well. It has been two
years since I made the change of not leaving the battery charger turned on
while away from the boat. The difference between the second two years and
the first two years is that the batteries are as strong today as when they
were new (partially due to those terrific AGM's no doubt), and I have yet to
notice any significant amount sacrificial activity from the zincs. They
have yet to need replacing in the last two years. The only thing I know to
attribute this difference in zinc activity to is the fact that I no longer
keep the battery charger turned on while away from the boat. I do keep the
charger turned I'm on while on the boat and in the slip, just as I always
have done.

Any comments as to the battery charger/zinc relationship?