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>> Home > Forum > Any Other Questions! > Prop Size
Prop Size
Author Message
(Fleet Admiral!)

Forum Posts: 240
2nd Jan 2018 17:35  
>> Permalink

Whenever I have asked the question, "I have this size motor to run this size boat, which prop size shall I use?", I am led into the dark science of thrust, pitch, voltage, RPM and many other mysterious forces. The discussion always seems to conclude with "Suck it and see."
I have a Turnigy D3536/9 910kv out runner which I want to use in an airboat length 750mm x 300mm beam.
The manufacturer recommends a 12x5 prop @ 7.4v and a 10x7 @ 14.8v.
I use 11.1v 3cell lipos and cannot find the recommended prop for this voltage.
Have any of you knowledgeable fellows any suggestions please?
(Apart from the one mentioned above of course.)

Thank you.

I used to be indecisive but now I'm not so sure?
Dave M
(Fleet Admiral!)

Forum Posts: 1527
2nd Jan 2018 18:03  
>> Permalink

Hi Steve
Stick your motor on a temp ramp holder and connect up to the ESC.
Get a selection of different sized props and pitches and fit one to the motor.
Connect the battery via a Wattmeter and run the motor at full whack. Record the results Amps and watts.
If either are above the motor or ESC ratings the prop is too big or too coarse pitch...
Try with different props until you find one that is within ratings spec
This is a very rough and ready method and the load will reduce when the model is in motion so will always be within spec.

Once you have the size and pitch it really is a question of trying slightly different sizes and pitches to get the best for your model.

Take great care when testing with air props, modern brushless motors pack a big punch and at full whack any contact with the blade is to be avoided.

Live long and prosper

(Fleet Admiral!)

Forum Posts: 240
2nd Jan 2018 20:51  
>> Permalink

Thanks Dave. I'll give it a 'whizz'.


I used to be indecisive but now I'm not so sure?
(Fleet Admiral!)

Forum Posts: 2527
2nd Jan 2018 22:19  
>> Permalink

Hi Steve, not really my field but looking at the figures you posted, and assuming a 'halfway linear relationship' logic says you need an 11 x 6 prop!
If I were you I'd start Dave's experiments and sea trials with that. Good luck👍, Doug 😎

Young at heart - slightly older in other places 😉 Cheers Doug