There is enough information in Morganstrn's comments to keep me busy for a while. This is not an easy subject, but the comments and article are enlightening. Bill, I can see you are doing as the Practical Sailor suggests. What troubles me is that I don't see that being done with other boats in our yard - excepting only the zincs on the propellor shaft.

There are two subjects now being discussed. The first is the corrosion caused by shore currents and galvonic corrosion. The second (at least so it seems to me) is the issue of gounding the mast, bow and stern pulpits to water ground - and a centrally grounded system.

The second one I can address, in as much as I have some confidence about my knowledge of the subject. The issue is not just lightning. Your mast head can be going through electrically charged air (even though the cloud is high above) and even though you may be at the dock. With a grounded mast, that cloud is being discharged through your grounding plate - and, voila, there is a current -and potentially a very large current. Without the grounding plate, the path of the current is unspecified, and the path taken may be dependent on the intensity of the voltage build up.

As to what can happen if everything internal is connected but not grounded? Your electronics and everything gets fried if the voltage build up is sudden and substantial. Better carry a hand held compass. Obviously, with an electrical storm, the problem can be acute, and the masthead can attract lightning if not adequately grounded with the ability to discharge immense amounts of current.

A zinc anode and a grounding plate are not synonomous, nor do they do the same thing. The zinc anode will not discharge enough current for a lightning/near lightning/charged cloud hit. It simply does not have the mandatory surface area.

The question was raised about the grounding plate on an OB. My OB is connected to the battery directly. It has an electric start so the ground is a solid #6ga wire. However, while pondering this, I realized that, with just the generator, as I had last year, the generator was likely isolated from the motor (Yamaha 8hp). With the electric start, the battery ground and the engine ground have to be commoned.

On the original Ariels, as I recall, the wire from the head inlet (There is a small #6 screw that fits into the fitting base) was commoned to wires from the battery/distribution panel and the chainplates (also the masthead) near the bottom front of the locker closet in the cabin. Note that for lightning protection purposes, the bow and stern pulpits should also be grounded. I use #6 guage wire for mast/bow pulpit grounding. Be careful not to have any sharp corners in any of these grounding wires (for lightning protection)

Back to corrosion caused by electrolysis and the sacrificial plates: I need to do further review the information mentioned above as to why multiple sacrificial plates are needed if the metal fittings on the boat are wired together to a single sacrificial plate. Why is not one plate adequate? Also, if there is a single plate system, from one end of the boat to another, with the totality being grounded, that should create a "shield" of a common voltage level, even in salt water.

For example, let us say that points A and C are 6 inches apart, underwarer, and bonded and grounded. An isolated metal point B, is between the two points and not bonded or connected to the other two. It is totally isolated. It would not develop a differential to cause corrosion according to my theory. Galvonic corrosion requires a current - no matter how you cut it. There is no way current could flow from either points A or C to B, even through the sea water is a conductor connecting all three because there is no path. Going further, assume there were a bunch of metal fittings, each being a point B between A&C, separated by a 1/64" gap. The only conductor connecting them was the seawater. Then lastly, assume A,B&C were points on a single copper plate where the points on the plate were connected both by the sea water, and the conductivity of the copper. I think all three situations, as for corrosion, are identical. But if that is the case, then every fitting need not have a zinc plate.

Unfortunately, at this time, for me, there are more questions raised than answered. Hope some of this makes sense.