More Q's about props

[Anonymous]

Will a wide, short blade produce the same push @ the same RPM with less noise than a longer, narrower blade? Doesn't tip speed affect sound the most?

If so, is a 3 blade, shorter prop a better choice than a long two blade? I'm very concerned about noise factor

 

[Waterthunder]

A smaller prop must turn more RPM’s to push the same amount of air. So the tip speed end’s up being around the same per thrust. A 72’’ prop turning 3,200 RPM’s has the same tip speed as an 80’’ prop turning 2,880 both tip’s are traveling at 685.4 MPH.

 

[GWTW-000]

Waterthunder

You know how to make someones head hurt with that one. I was ok at 1370.9 mph. I had a hard time with the rest untill I divided by 2 for each tip. This makes a clearer picture for me on what everyone has been talking about on length of the prop, thrust and rpms. What you are saying is to match the max. prop speed rpm to the max. torque of your motor and do this using the longest prop your motor can turn? Am I close?

 

[Waterthunder]

That’s pretty much how I do it on a ride boat. Just remember some people like more snap some like to twist up their motor higher. Also in race boats everyone likes a shorter prop so the boat don’t torque roll as much. But for a ride but your pretty much dead on the money. Cracker said it best prop’s are kind of a black magic each ones a different spell. I made an Excel spread sheet utilizing what’s considered max tip speed (just under the speed of sound). You can put in a gear box ratio, prop diameter and it will tell you engine RPM, prop RMP and radial velocity of the tip in MPH and inches per second. I use it for every gear box boat I set up it’s almost dead on out of the box. Each time it has been within 300 or 400 RPM of what ran the best on the boat. I always prefer to run a little more pitch than what’s recommended and let the gear box do the work instead of twisting up the motor.