Welp... Just found out I have a bent prop shaft.  My outboard started knocking so I took it in to the shop.  The strange thing about it is that I was in open water when it started knocking.  I never hit anything (that I know of).  So now I'm looking at a huge bill to fix it.  Any thoughts on what could have caused this besides hitting something?

I expect an out of balance prop could cause a bent prop shaft eventually. I expect it gets to a critical point, then just goes. If the prop wasn't in perfect condition that might cause it.

A  prop often displays some kind of tell tale signs that something is amiss.

However - if your's is routinely "shined" from running aground, is a veteran of some previous touch & go's or even has a few battle scars, it would be less help here figuring out where & or when something occurred that could have bent it out.

Often associated this with impact or at least some kind of prop strike, a prolonged or hard grounding (especially through sand) can bend of a shaft pretty good. 

Good Luck

A-Jay

Those are some great replies.  I do have a few small dings on the prop....... I run shallow water all the time.  Sounds about right. Thanks guys.

What size motor is this. If you boat shallow water rivers like I did in PA. we used to have custom prop shields made. Some looked like a pitchfork and others had to metal circular rings (one in the front of the lower unit and one larger one surrounding the prop) connected by metal to protect the prop from impact on the Susquehanna River rocks.  They even sell some commercially. I had mine on a 9.9 bu I have seen some on engines all the way up to 25 HP.