Electrolysis is not your friend

This is what happens when one puts 12 volt power to the ground on your boat - galvanic corrosion (electrolysis).

Old and new prop, wee bit of difference.

Over the summer Ruediger's  wind generator was supposed to be charging the batteries but was instead putting power through the engine to the propeller shaft and propeller. The combination of this and being in salt water turned the canal into a giant battery.  Apparently different metals disintegrate at different rates so you put sacrificial zincs on boats (e.g. on propeller shafts) so the zinc disintegrates instead of your precious metal boat parts.  Hopefully his metal thru hulls are not in the same condition.

I fully admit that I still (despite it being explained numerous times) do not understand how this happens.

So here's something else that I don't in any shape or form understand - the other boats were not affected because Ruediger's boat, unlike the rest was not plugged into shore power so the "infection" wasn't transmitted.  This is giving me a headache.