Re: Diameter vs pitch
Unlike cars and trucks where you can swap the 16 inchers that came on the vehicle with 20 inchers and rubber band tires, you can't do that with a boat. You first locate a series of props that fit your motor. This is not in a range (for example) from 10 inch diameter to 20 inches. Anything other than the prop series designed for your engine will either be way to small or it will hit the AV plate. Once you have a series of props for your xxxx Model xxxx outboard, you chose the pitch you need based on engine rpm. Studying the series of props you will see that as pitch goes up, diameter comes down slightly. As pitch comes down, diameter goes up slightly. The 75-90 HP series of Merc engines takes a 13-1/4 x 19P prop which is pretty standard. You can't put a 15 inch prop on that motor. Nor would you put a 12 inch on it. Going to a 21P would probably put the diameter at 13 inches. Going to a 17 might leave diameter at 13-1/4 or possibly 13.5. So diameter changes but only slightly and it is a function of prop design. You don't shop for diameter. You shop for application, then pitch based on WOT rpm.
