That would be a hard question to answer. Are you asking what pitch? Or you asking "aluminum or stainless"? If it's pitch, you'd want one to keep your engine rpm's in the optimum operating range, which should be between 5300 and 5600 rpm at full throttle. Too much pitch would lug your engine down and not develop 40 hp. Not enough pitch would cause you to go over the power curve and lose hp. You'd need a tachometer to find out which prop works best for you. As far as what kind of prop, I'd go with a stainless any day over aluminum.