Mercruiser Bravo 3 Problem! Suggestions??

[Alondra]

The boat is a bayliner 32 with twin Bravo 3 Mercruiser diesels with counter rotating props.
Does anyone ever had a broken Pignon Wheel (the crown on top of the drive shaft going into the tail, there where it goes down in a 90 degree angle)??
After a strange nocking sound we opened up the shaft to find out that the teeth of the wheel had been broken over a 3 inch distance.
What could have caused this failure? Does anyone had this before to? What needs to be done to prevent it from happening again??
There is a small damage on the prop but I can ahrdly imagine that hitting something would have caused it to break. The damage on the prop would have been much bigger i would think.
Any suggestions??