Not saying you are wrong but maybe the magazine? Seems like the standard inbooard skiboat (DD) prop for the last 20 years was a 13x13? We are talking about direct drives and old but well proven American V-8 engines, whether it be the Ford Windsor 351 or the Chevy 350, they turn about the same RPMs. You would usually see that the top ends of all these boats was from 45 to 48mph.