RE: Small diameter props and large spinners
I have a WM Dago Red and that has a 3.25" spinner, and I'm using a 10 X 8 std. APC sport prop on a Webra .50 (slightly modified internally, and U-T muffler) and it hauls it along quite well (126 if you believe radar guns)
Everyone who sees it can't believe it performs so well with only 3 3/8" of prop sticking out either side of that big spinner. Main reason why is because that's the most effective part of the blade...the hub area, or probably the center third of the prop doesn't contribute all that much toward forward motion.