Anode reaction

[Brickbat]

In pulling out my Hanse after the first years sailing in the Great Lakes, I found damage to the VC 17 bottom paint extending out about 12" from two anodes on the hull. These are zinc, and I will be replacing them with magnesium anodes this spring to suit the fresh water environment. But it is still hard to believe this amount of damage was done by galvanic reaction to the small amount of copper in the VC paint. Any similar experiences or more intelligent observations would be appreciated.