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From: Robin (no email)
Date: Tue May 02 2006 - 19:05:47 EDT
To bond or not bond (keel coolers or other isolated underwater metals) and
whether zincs are needed on isolated underwater components not bonded is
mostly art, some science.
The short answer is: use military spec zincs and isolate the keel coolers.
A long answer follows:
Any zincs may disappear due to water turbulance rather than galvanic
corrosion protection or they may be an inferior quality.
The first thing to do is find out what military spec the zincs meet. I forget
the number, but quality marine zincs for saltwater use should meet a military
spec number. It basically limits impurities which would other wise cause the
zincs to self destruct (similar to crevice corrosion in poor quality bronze
fittings). Camp brand, for example, meets the spec last time I checked. If
nobody knows, change supplier.
The next simple thing to do is use larger zincs. You can also consider
shielding them from water flow. Maybe put a small block in front if feasible.
Rudder and shaft zincs, for example, likely lose more than half their mass due
to water turburlance, probably 75%!!!! If you had been using larger ones all
along, they would last as long as your others. Maybe they are a bit small
compared the cooler metal mass or composition.
Next, I would ask Walter what material the keel cooler is made of and what
it's postion is on the electrochemical (Noble) scale..that is, what other
metal is closest to it. And while you are at it, ask how long the zincs are
calculated/expected to last. Do they recommend a brand?See how the cooler
metal composition compares with your shafts and props, likely the two largest
underwater metal masses nearby. If they are within 10 or 20 millivolts all
should be well (thats close in electrical activity). If far apart, say 50mv or
more, that may be the source of some zinc use if the cooler metal is the more
active.
Absent any other effects, my guess would be that the stern end zinc of the
keel cooler would disappear first because that's closest to props, usually
manganeze bronze. With stainless shafts and props (less ac tive metals) this
effect might be negligible. Distance,prop size and material all affect any
interaction. But if the forward zinc is most exposed to water flow, that one
could be the faster to dissolve.
For a quality underwater material, like silicon bronze seacocks, isolating
them (not bonding) is my preference. Once connected to a bonding system, they
are subject to stray current ("electrolysis") from many potential sources. So
if the coolers are at least that inactive on the noble scale isolating them
even without zincs is likely ok. But ABYC still recommends bonding, likely
because it's traditional.
Once you bond coolers to your boat bonding system they become electrically
connected to an underwater antenna...all your underwater metal, and stray
currents can accelerate zinc use. All those unerwater parts can pick up
induced voltages in the water, most likely to occur (if at all) in marinas.
Since the keel cooler company doesn't want to be responsible for that kind of
failure, I can understand why they want them isolated. In addtion, a lightning
strike might blow them up.
If your keel cooler IS bonded, when inactive you can hang a guppy type zinc
near where your area of concern may be and attach the on board connection end
to your bonding system. For example, I hang one astern (scrounged from
commercial shipyards and attached to a battery cable) near my rudders and
props and double the life of my underwater zincs to almost two years. I put a
wing nut on a stud in my cockpit connected to my bonding system.
You may also be able to get practical suggestions from lobstermen....they run
high annual hours mostly with fiberglass boats and keel coolers...see what
they do. Or query manufacturers of such boats. There are many in Maine, for
example...practical experience trumps theory here because of the wide variety
of variables.
Rob Brueckner
Hatteras YF
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