Your post didn't comment on the status of your shaft zincs. Those are intended to protect other components like props. If you didn't have any in place, there's your problem. If you did and they eroded away, start using an extra zinc hung into the water off the stern, connected to th engine. There a model shaped like a fish at most stores. If you have this problem with the zincs in place, there probably some poor quality material in the prop or shaft.
To prep the prop, use a zinc prop paint such as: http://www.pyiinc.com/index.php?sect...u=mppaint&sn=2
This stuff has worked Ok for us, keeps growth off the prop, but not much to do about the problem of corrosion.