"Chuck" wrote in message
...
| 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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|
| An additional thought:
|
| If your iron keel was exposed enough to
| form a significant galvanic couple with
| the saildrive, isolating it protects the
| saildrive, as you indicated. But the
| iron keel may now be pitting and
| corroding if there are no sacrificial
| anodes on the keel itself.
|
| Chuck
|

No Chuck, because if it is not connected to anything inside the
(fibreglass)boat, there is no 'battery circuit' with the keel as anode and a
more noble metal as cathode.
My present boat has a cast iron keel without any zinc anodes. It is not
connected to ground, just to the mast to provide a lightning path.

For the steel to get pitted, there would need to be a cathode of a more
noble metal. Surely if a zinc anode is bolted to a cast iron keel, it may
(slowly) be eaten away, but not necessarily because it protects the keel. I
have often seen zinc anodes on steel keels that had been on it for years.

The corrosion problem I had due to the SSB counterpoise was actually with a
lead keel, but that is irrelevant, because both cast iron and lead are more
noble (cathodic) than aluminium.

Wout