WBH wrote:
"Chuck" wrote in message
...
| WBH wrote:
|
| Hi,
| For lightning protection, your keel should be connected to the mast, not
to
| the boat's ground.
|
| These are not mutually exclusive
| options, of course. Typically, the keel
| is connected to the mast and to the
| boat's "ground".
|
| Isn't a fair amount of imagination
| required in order to derive a benefit
| from isolating the DC ground from the keel?

No, I don't think so. Isn't galvanic corrosion all about creating
"batteries" by putting dissimilar metals in seawater and by connecting them
galvanically to allow electron-flow? There will be no "battery action" if
the metals are not connected inside the boat.
For example, an iron keel and an alluminium saildrive can form a nice
battery if they are connected through the boat's ground. Of course there are
many factors that determine "how well" the battery works, such as the
distance between the 2 metals. Naturally, by galvanically connecting an even
less nobel metal such as zinc to the anode of the 'battery' (the saildrive),
this metal will be sacrificed first, but doesn't it make sense to disconnect
the dissimilar metals inside the boat in the first place, i.e. isolate the
keel from ground?
In a fibreglass boat like a Beneteau this should be no problem at all.
Most European boats do not have the thru-hull fittings bonded. For some
reason this is much more common practice in the USA. I've never understood
why. The theory is that the potential is equalized, but at the same time it
can form 'batteries' if the bonded metals are dissimilar.
On one of my previous boats I had mistakenly used the keel as a counterpoise
for my SSB installation, effectively connecting it to ground. As the Volvo
saildrive was also connected to ground, the zinc anode had been eaten away
in weeks and the galvanic corrosion process had started to eat away the
saildrive. This happened while moored and the batteries were switched off.
As soon as I disconnected the keel from ground and used a porous bronze
groundplate for the SSB instead, the zinc anode corroded at the normal rate
(1 every 2 years) again.

Wout

|
| Disconnect the keel from ground and measure again.
|
| If that can be easily done, it might
| confirm the diagnosis.
|
| Also, there are 2 schools
| of thought about bonding all bronze thru-hull fittings.
|
| There are some strange theories about
| bonding metal thru-hull fittings as part
| of a lightning protection strategy, but
| from a corrosion perspective, the
| science is clear.





|
| A quality bronze thru-hull, unbonded,
| and immersed in seawater may well
| outlast a fiberglass hull! Bonding
| (i.e., attaching) a bronze prop to a
| stainless shaft is what creates the need
| for a sacrificial anode in the first
| place. If you hang the bronze prop over
| the side on a rope, you will have to
| wait decades before you notice any
| corrosion. It won't propel the boat, of
| course, but think of what you'll save on
| zincs!
|
|
| Recommended reading:
| Nigel Calder.
| Do you have a saildrive or a shaft?
| Cheers,
| Wout
|
|
|
| Chuck
|
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Ah! Glad to see we're on the same
frequency regarding galvanic couples.
Isolation is a great deterrent to corrosion.

But the context of my comment was the
reference to lightning protection,
rather than to corrosion.

Chuck


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