Which prop you choose depends on which redrive you have. A high ratio means the prop spins at a slower speed and needs more pitch–a bigger bite. At some point of increasing pitch, efficiency suffers and/or the motor will bog down and won’t get up to its power band (rpm with the highest horsepower). That’s one potential cause of power fade where you go to full power, it gets almost to full rpm then fades.

Starting from Scratch

The book covers this in more detail but basically you start with the largest prop you can tolerate. At our slow speeds we get more bang for the engine-displacement buck by swinging the largest possible diameter. But a bigger prop means the tips go faster which makes them louder as they exceed half the speed of sound (0.5 Mach). Max performance may be faster than that, up to 0.8 Mach, but with increasing noise.

Then you need to make sure the engine gets into its power band. For example, the 80 cc motors hit their max horsepower at around 9500 RPM. If you have a 48″ prop and want to keep the tips at a comfortable .6 that means a prop RPM of 3200. You’d need roughly a 3 to 1 redrive since 3* 3200 = 9600. But that’ll be pretty noisy so Top 80’s typically use a 4:1 redrive ratio making for a quiet prop RPM of 2375. 

With that calculation done, the only task is to find the right pitch. Increasing pitch too much decreases performance at which point you would add another blad for a 3-blade prop. In general a 2 blade prop is more efficient if it can be pitched properly.