The wrong propeller caused pitting in the gelcoat above the rudder!! It’s quite amazing what water pressure can do to a fiberglass hull when harmonics are concerned. In this case the wrong prop was fitted to this Spy ski boat which ate into the glass in two distinct areas. To repair these areas, the hollows were filled with a high density epoxy filler, blocked down, primed with an epoxy primer and finished in colour-matched 2 pack hardtop. Now running the correct prop, let’s hope no more pitting is experienced.