Re: Prop diamiter vs RPM
Agreed! Pick any prop and then look at the diameter as pitch changes from 13, to 15 to 17 to 19 etc. You will notice that as pitch goes up, diameter generally comes down a bit and vice versa. Diameter is a function of the prop design. You can buy a 10 inch diameter 15P prop and you can buy a 13-1/4 inch diameter 15P prop (not for the same engine obviously you get the point. Large diameter props go on bigger engines while smaller diameter props are on smaller engines. The lower unit gear ratios also change accordingly. Smaller engines tend to have higher gear ratios than bigger engines. (A little off topic but pertinent -- Don't confuse gear ratio with higher or lower gear. 1:1 is a lower ratio but higher gear than a 2.34:1 which is a higher ratio but a lower gear.) Theoretically, at the same prop rpm both props would deliver the same boat speed. Engine speed however would be very different because of lower unit gear ratios. There is enough prop design info on the net to keep you busy reading for months.
