Thanks for all the help on this, guys! That makes alot of sense: stick to the powerband where the proper amount of torque is applied in order to achieve your desired result. Too small of a prop with not enough overall mass could over-rev the engine and basically turn fuel into noise without achieving any significant results. Too big of a prop and your engine can't swing it around fast enough to give you any thrust. I was curious about the small " - " between the size of the prop and the other number. I assumed it was in tenths of inches instead of the more obvious "pitch" identifier (I gotta start thinking about things in terms of "aerospace engineering" instead of "industrial engineering"!!!)

It sounds like the smart money is on checking out the engine mfg specs and it's effective operating range and listening to what they say. Use that spec as a "baseline" for experimenting (within reason....) to get different results. Ex: What would be the effect of keeping an 11" prop on a .61 engine but increasing the pitch to spin slower but bite harder? or decreasing the pitch to cause it to spin faster but bite less? and how both situations would compare against the baseline spec. Instinctively, I'd bet money that they pay a couple of aerospace engineers quite a bit of money to crunch the numbers that would give the consumer the best "bang for the buck" in prop specs. Economically, if you supply the consumer non-optimal prop specs, your engine "appears" to run poorly and you get a bad rap for building and selling junk engines instead of just screwing up on the prop specs

I can't help it.... I have an engineering degree from Ga Tech...... I like to monkey around with stuff