posted 12-21-2014 06:35 PM ET (US)
Bob, in order to fit a particular motor, the prop needs to have a hub diameter which matches the gearcase diameter of the motor's lower unit, and have the correct spline configuration to fit the prop shaft. Overall blade circle diameter also matters, but it's less of a concern - generally, once you match the gearcase size, the overall diameter is going to work.Nowadays, the aftermarket prop manufacturers build generic props; you get a prop for the diameter gearcase you have, and then you buy a "hubkit" to fit your particular motor. In years past, this wasn't the case, and the hub was not interchangeable, so you bought a whole prop that would fit for the gearcase size and spline configuration you had.
To get a sense of how this all works, you may want to browse the "Dan's Discount Props" site at www.dansdiscountprops.com.
Of course, getting a prop to fit the motor is the easy part - you then need to get the right pitch for your application, which is as much art as it is science. If that's what you're looking for, as again and you'll get some good help from this site.
Hope that helps!
Tim