Are the props interchangable from a Volvo-Penta SX drive and a Merc? Will the flow torq hub have to be switched? I've read somewhere that they will fit. My father-in-Law has a very beat up alum 21p on his SX drive. I am looking for a Stainless Steel 21p.

Yes, I believe all you have to do is change the flow torq hub to a volvo one. The volvo sx drive shaft is different. I found the Ballistic prop to work great on that drive.

Thanks for the info