[C38] zinks, shore power, and electrolysis of the prop and key

[Les]

On Mon, 2007-10-01 at 09:44 -0700, D. R. Strong wrote:
> showed me a fairly subtle pitting on the prop that I had missed
> during 
Zinc replacement is highly correlated to leakage current.  In the
marina, when I was surrounded by sail boats, I never seemed to have a
problem (except for one steel hull that was not well maintained).  When
the power boats were closer, my zincs deteriorated badly.  I don't
believe that sailboaters are necessarily better at boat maintenance, but
just that power boats have so much more metal underwater, like the steel
hulled sailboat.  The more metal in the water, the greater the leakage
surface, and the more you need your zincs, and the more they degrade
over time, even if everything is well done.  Part and parcel of that is
the marina ground, which you have apparently discovered, and "fixed" by
simply not connecting.  I reduced my losses with one of those Galvanic
isolators, which simply puts a double diode in the ground, thus reducing
your galvanic current.  A transformer is used on a lot of large cruising
boats to accomplish the same thing, and increases the isolation from
shore power (but these are massive and expensive).

	The types of metal below the waterline is also important in determining
your galvanic issues, so check the leakage to various bits in your boats
below the water line.  Note that on our boats, the DC ground is
typically tied to earth via the shaft.  I don't know how effective this
is, and I have seen some books recommending bonding to the keel.
However, our keel bolts are steel, and our keels lead, so there might be
some galvanic issue here that could be aggravated by the ground current,
and this is one place I hesitate to experiment.  You might look at some
websites and check out the marine certifying organizations for their
recommendations.  

	I notice that for RF grounding, most recent articles recommend a screen
inside the hull rather than some other form, also I suspect due to the
issues of another hole in the hull and additional galvanic issues.

Regards,
Les H