Re: prop diameter
When comparing two props from two different manufacturers you cannot say it was the diameter that made the difference. Chances are good it wasn't. Props are manufactured with different rake, cupping, blade thickness, blade shape, blade area, etc. So unless you are comparing two props from the same manufacturer and from the same series, you don't know exactly why there was an increase in speed, lower rpm, better hole shot, worse hole shot, etc. Generally speaking, on any given prop in any series from a given manufacturer, as prop pitch goes up, diameter comes down 1/4 to 1/2 inch. Conversely, as pitch goes down, diameter goes up 1/4 to 1/2 inch. Its a function of design. When you buy a prop, the selection criteria is 1) it must be a prop for your specific engine, 2) pitch selection must allow the engine to operate near or at the manufacturers wide open throttle rpm 3) speed will be whatever that prop allows, 4) you can select plastic, stainless, or aluminum. You do not shop for props specifically by diameter except for #1 above which gets you the proper diameter from the git-go. Prop diameters are factored into the design for specific series of motors (low HP, mid range, or high HP). In the end, the ultimate answer to your question is yes -- prop diameter does affect rpm but not nearly as much as pitch.
