Elder:
My guess is that you are experiencing galvanic corrosion. I suggest that
you check your thru-hull valves and other brass / bronze underwater fittings
as well. Also, take a good, hard look at your prop.
These concerns are more important if your boat is = or has been kept at a
dock, especially if you have shore power brought aboard.
I'm sure others will have some words on this.
Good Luck..............Ed