When it’s time to buy a new propeller, diameter and pitch are the watchwords. To be able to select or upgrade to a new propeller, you must understand these terms and how they translate to real-world performance.
Propellers are described in numbers: the first number is the diameter, and the second number is the pitch.
Diameter If you could draw a circle around a propeller’s blades, diameter would be the distance, (in inches) across this circle. The bigger diameter prop will have more bite per revolution (this is thrust, basically you’re moving more air).
Pitch is the theoretical distance, in inches, that a propeller moves the boat forward during one revolution. So, a 14 X 19 propeller has a diameter of fourteen inches, and a pitch of nineteen inches. In conversation, most people would describe this propeller as a 19- inch pitch, or simply a 19.
The more pitch, the more forward motion per one revolution of the prop.
How to Select Prop Size?
It depends and it’s also all a compromise. Depends on the model and what your goal is.
All things being equal, and I’m just pulling numbers out the air here, but a 10×6 vs 11×5. The 10 will be faster because of pitch, plus it will be a little faster because of size. It will use more battery compared to the 11×5. You might also say that the 11×5 would be more efficient. So prop selection really depends on your goals. Lots of this would depend on the motor and airframe and (wing loading).
Generically, every inch of pitch is worth about 150-200 RPM
• Increase pitch an inch and the full throttle RPM should drop around 150- 200 RPM
• Decrease the pitch an inch and the RPM ought to go up roughly 150-200 RPM at full throttle
• Too few RPM at full throttle? You might need a prop with less pitch
• Too many RPM at full throttle? A propeller with more pitch could help.