I bought a Michigan Wheel propeller off ebay for my bassboat.  Had the right diameter and pitch, but it doesn't fit on the shaft of my Mercury 115hp.  It doesn't look like the hub comes out.  Am I just looking at it wrong, or do some of them come with the hub permanently in the prop?

Not sure about that particular prop but my dads older stratos has a prop that the hub is pressed into it. All of the older boats I've had had this type of prop. Was surprised when I bought my new boat that it had the hub that was separate from the prop. Im sure someone will chime in on that particular prop. Good luck

Return the prop. 

Tom

My Michigan Wheel prop has a square rubber insert that is specific to the boat. I bought the hub kit separately from the prop.

 

Maybe post a picture?

A little more research suggests that I can put in a Mercury hub in the prop, but I have to get a prop-shop to press out the existing hub.

Yep, and that's probably another $100+.  Hope you got a great deal on the MW, I've always considered them and general purpose prop. 

Wow!  That much?  That's disappointing

 

Back in the late 50's we raced Michigan Wheel chopper 2 blade props, they have been around a long time.

The reason I recommend returning the prop is what is unkonwn; pitch, rake, cup and diameter all could be wrong for your boat and engine. Another question is has it been repaired? 

Putting a new hub with proper splices for your Merc prop shaft means you need the right hub kit from MW.

Good luck, hope everything works out for you.

Tom