What size 3-blade prop should I use for a .61 sized engine. I figured that since the third blade would give me 33% more surface to "cut" the air, I should use a around a 9" diameter.
I have a ground clearance problem with the standard 12x6.

Did I figure wrong???

Mike

Don't use a 9" prop unless it has a pitch of 10. You should use an 11x6 or 10x8 on that engine. Generally you drop 1" in diameter and keep the same pitch when going from 2 to 3 blades.

I took off a 2 bladed 13x5 Zinger, and installed 3 bladed 12X6 Master Airscrew,and turned a great pulling engine with good speed to a average engine with average pulling power and speed

You didnt mention which 61 you are running! That is also part of the equation. An OS 61 with a standard muffler likes an 11x6/11x7 so go to a 10x6/10x7 three blade. Most others besides YS need something smaller like a 9x7 or 10x5.

Just as a point of intrest the more blades a prop has the less efficient it becomes. From what I understand about it it's because the next blade arrives before "clean" air has arrived. I don't know if you are familiar with control line flying , but there was a time when the speed planes all used props with only one blade! The primary reason for multi blade props is that in real aircraft a high horsepower two blade prop would have to be so large,it would not allow enough ground clearance. They do however look cool, and I think they may be a little less noisey because at the same RPM a larger two blade would have a higher tip speed than the smaller three blade.

Correct, in general they are quite a bit less noisy. My buddy went from 90 db on a 17x12 2 blade to a 15x11 4 blade and dropped the noise to 86 db, although it didnt have anywhere near the power of the 17x12