Just out of curiousity, is there any reason that would prevent a solid hub prop from being run on the big outboard motors, rather than having to run a rubber hub prop? If so, what????? thanks...

Why would you want to? The motors don't generate enough power to make it a requirement, so you're just asking for trouble if you hit something.

I have run soild hubs on outboards for years with no ill effect. If you a new merc with the 1 1/4'' prop shaft there is no other choice.