In answer to comments.
I believe I followed the instructions from PYI on being sure the prop was snug on the taper of the shaft. It has lasted 12 years and 3000 hours motoring. Perhaps I screwed up, but only by a little, so it took a long time to show.

Fred Hutchison, PYI tech support replied very promptly to an email query similar to my post, stating that this is an issue, and that where the hub is not snug on the taper, the key will eventually shear.

Needless to say, I will be VERY careful on reinstallation.

Would you guys recommend some blue Loctite on the shaft?