On Sun, 27 Jan 2008 18:22:55 -0800 (PST), Bil
wrote:

Thanks for your responses.

For Gordon: the RF ground/counterpoise is a copper foil which - I
think - is connected to the engine block.

For Chuck: I've not noticed the same erosion of the anode when in any
of three different marinas and connected to shore power, including for
months at a time. I've only noticed the erosion after passages that
have included daily Pactor modem work. I renewed the shaft anode 3
months ago, found it erosion free 6 weeks ago before leaving a marina,
and found significant pitting when I checked it two days ago after
entering a marina (with 6 weeks of passage making and no shore power
connection in between checks).

Bil

So we can rule out the shore power connection.

I'd be inclined to rule out the HF rig as the culprit since from the
perspective of galvanic currents it looks the same whether you are
operating it or not. The HF rig would be a player only if you had a
completely isolated DC ground system, in which your prop and shaft
were not connected galvanically (by wire) to your DC ground. Such a
system is rare on older boats but is beginning to appear on some new
boats. As far as I know, there are no known cases of 150 watt radios
accelerating zinc erosion via RF.

The interesting variable in your situation appears to be the boat's
motion. At rest in a marine, no problem. Underway, rapid erosion. Some
of that erosion is simply due to the motion of the water relative to
the prop. The faster you go, the faster the zinc will erode. Whether
this could explain the problem depends on how long your passages were
and how fast you were moving and past experience.

Another possibility is that while underway, some piece of electrical
equipment is energized that is off while you are at the marina. If
this "appliance" is faulty, say with the positive wire shorting to the
bilge water, you could be setting up a condition for accelerated
erosion of your zinc. Next question might be how does your recent
experience with zincs compare with previous passages?

Chuck

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