Kmot

My Feedback: (24)

My buddy has built a 29" swamp boat/air boat. It will be powered by a brushless motor that has a Kv of 3900. I have looked at the APC pylon props, the Master Airscrew Formula One props, and the Graupner Cam speed props. I figure something in the 7-8" diameter is about max for this motor? Probably need more like a 6" prop?

I'm concerned about how much rpm these props are safe up to.

Are there other props available to purchase besides the three listed above?

Thank you.

cncswiss1

at 3900 kv a 8" prop is going to draw a LOT of amps! how much is the motor rated for? that and KV will tell you prop diameter.

air boats need thrust, you may be better served with a lower kv motor swinging a bigger prop.

Kmot

My Feedback: (24)

He has already ordered this setup and I am just trying to help him get it sorted. Thanks!

evan-RCU

A 3900KV motor is going to be in the 4.75X4.75 range on 3S. A APC 5X3 would be a good choice. If you use a 2S pack you can run a 5.5-6" prop, APC has a 6X4 that would be good on 2S.

The thing is to match the prop to the motor and not draw more than about 120% of the amps the motor is rated for.

Is this the United Hobby inrunner?

cncswiss1

a [link=http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=2082]gearbox[/link] may be a very nice possiblility for this motor, outrunner or inrunner? with a 5 to 1 gearbox on 6s you could spin a 10-11 prop with the average 540 size inrunner


I agree with Evan, a 5" (maby 6") prop is probably max on 3s unless it can handle a few hundred amps.

Kmot

My Feedback: (24)

I suggested that this motor would need a gearbox but his airboat does not have the provision to mount it wiuthout some sort of surgery to the setup. I am going to suggest a 2S and 6" prop. Thanks!

Here is the motor in question:

airraptor

My Feedback: (66)

we are all just guessing with out the type and size of the motor. you could have a 60mm inrunner with that KV or a 12mm one we need more detail to give the best advice but the KV no matter the size is way to high for this.

Kmot

My Feedback: (24)

It is designed as a replacement for r/c cars so I presume it is a 540 sized motor, but in brushless terms I have no clue. Thanks for the help, we will see what we can do with it.

airraptor

My Feedback: (66)

i would try a apc 7x3 cut down to a 6" prop then try it on two cells if running cool then try the 3 cells you need lots of thrust to move an airboat

Troy-RCU

My Feedback: (1)

kmot, any of the small pylon props are good to high RPMs. APC rates theirs modestly and most are well over 30K. Same with the CAMs. Start with a smaller prop and measure the current. So many guys run motors and never check the current. I use a Sears clamp on style meter but there are also in-line plug in style meters that will tell you watts (amps, volts). If you over prop you risk frying the motor, esc, or battery.

Kmot

My Feedback: (24)

Thanks for the advice Troy. I have a WattsUp meter and I will use that to check his amps.