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I've got a 15470 brushless 35kw motor from China
How do I find the smallest 3-blade propeller I can use for this motor?

Here are it's specs,

Max Power: 35KW
Max Amp: 400A
Max RPM: 8500
Max Volt: 120v
Torque: 75Nm
Resistance: .94

I've looked into eCalc but it's for determining power / thrust, etc and not finding prop size.
Any help would be appreciated!

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    $\begingroup$ "Because 120 * 35 our maximum RPM would be 4,200" - Come again? You can't get the maximum RPM by multiplying the voltage by the power. $\endgroup$ – Terran Swett May 9 '19 at 20:35
  • $\begingroup$ Since you're looking for "smallest prop", how much thrust do you need out of it and how much torque can your motor supply at 4200 rpm? $\endgroup$ – Dan Pichelman May 9 '19 at 21:36
  • $\begingroup$ @DanPichelman I've added more detail to the question, the manufacturer recommends a 36-40 inch 2-blade propeller, so I was wondering how to calculate an equivalent 3-blade propeller (or do I just take off an inch as is the "rule of thumb"?) $\endgroup$ – jscomino May 9 '19 at 23:06
  • $\begingroup$ Seems like the pitch of the propeller would matter quite a bit here as well. $\endgroup$ – Ralph J May 10 '19 at 5:42
  • $\begingroup$ I seriously doubt that thing is rated for life safety. You would need to use it in a jurisdiction that has lax safety regs. $\endgroup$ – Harper - Reinstate Monica May 11 '19 at 14:50
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You need the largest prop that keeps the tip speed below about .9 Mach, roughly 1000 ft/sec, so you want the largest propeller than can operate at that RPM at .9 mach. Here's a tip speed calculator for larger propellers that should suffice.

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