If your zincs look like new and your prop is
disintegrating, your zincs are not working, for many possible reasons, some covered by Dave.I recommend you do some research on electrolysis in boats. It's a complicated enough subject that entire books have been written on it and it would hurt our little fingers retyping all the stuff that's already right here on this website. Try a forum archive search on the topic and read away. Books, Google, other boating websites, Google "zincs", and a visit to your local chandlery. The C36 Association website (www.catalina36.org) is a tremendous resource and you should join there, too.