Doug Kanter wrote:
Even in fresh water???

(rolls eyes)
C'mon, Doug... if the penny is inside the hull, in the bilge, what
difference does it make what kind of water the boat is floating in??

The dissimilar metals is what does it. Different metals have different
valence bonds, meaning that they are more or less eager to give up an
electron to the metal atom next door. As they swap electrons, the bonds
break down and you have bunch of loose atoms.
Zinc is used for zinc because it is "least noble" ie the most eager to
give up it's electrons, and breaks down the fastest, which actually
offers some protection against galvanic corrosion by other metals in
contacts with it. That's why they use it for zincs!


...If yes, I'm shaking down all my passengers for spare
change from now on.


Shucks, I do that now, and I have a fiberglass boat. If asked why, I say
it's because the compass is so sensitive.

DSK