WBH wrote:
"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
| |
| | ----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet
| News==----
| | http://www.newsfeeds.com The #1 Newsgroup Service in the World!
120,000+
| Newsgroups
| | ----= East and West-Coast Server Farms - Total Privacy via Encryption
| =----
|
|
| 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



Wout, there are three major types of
corrosion: galvanic, electrolytic, and
electrochemical.

Your comments apply to galvanic (two
dissimilar metals, etc.) and to
electrolytic (two separate metals, not
necessarily dissimilar, with a voltage
source between them).

I was referring to the third type of
corrosion known as electrochemical
corrosion. Electrochemical corrosion is
also called stray-current corrosion.

Electrochemical corrosion rates are
highly dependent on environment and it
is difficult to generalize. In addition,
there are numerous corrosion mechanisms
that full under this category.

Marine bronze, of course, stands out as
quite resistant to self-corrosion. Not
so with most irons and steels as the
military and commercial shipping
industry have learned.

Chuck

----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----