I have a steel boat with several zincs attached. I know why they are there, but as the zinc anodes get smaller, where does the metal go?
Same with steel dock pilings protected with zincs.
Is there something in the water that gets zinc plated?
Some chemist will probably answer better than me but;
The water is an electrolite, zinc is a less noble metal than your prop or hull, the zinc is electricaly connected to the hull or prop. The electrolite completes a circuit between the dissimilar metals zinc to steel hull or bronze prop, and the zinc becomes an anode to the more noble metal cathode. Material is lost from the anode to the electrolite and increases by some miniscule amount the zinc in surrounding water.
Galvanic corrosion is greater in salt water but is also present in fresh water.