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Thread: How to select motor kv value combined with prop size

  1. #1
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    Default How to select motor kv value combined with prop size

    How do the experts here compare motor kv value and prop size.

    I just watched a video of someone who swapped a Leopard 1050 kv motor and a 54mm prop into a sonicwake and claimed 14 minutes run time and no heating on a 6s 5000mah.

    I assume power is very little different if the speed calculator shows the same value.

    1600 kv with a 40mm prop versus a 1050 kv using a 61mm prop should perform about the same. Is there any advantage of using 1 combo versus the other?

  2. #2
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    Prop size depends upon the following factors:

    KV
    Motor size (diameter and length)
    Type of hull (mono, rigger, hydro cat)
    Size and weight of hull
    Speed controller (amp rating)



    Sent from my iPad using Tapatalk

  3. #3
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    Optimizing for the hull is clearly required. I am just curious if at the same speed value does a lower kv motor have an advantage such as longer run time?

  4. #4
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    lower kv lets you run a bigger prop .lower kv can handle 6s better as well and with less heat then a higher kv motor .

    claimed 14 minute ,was there a video? was he putting around? i find it highly unlikely he got 14 minutes unless he was really babying it around .

    post this video please.
    volantex vector pro ,proboat veles 29 , traxxas spartan, hobbyking/tfl pursuit ,ft009 with rescue rigging

  5. #5
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    I'm following this thread with great interest since I'm seriously considering trying the same approach, a bigger motor with lower Kv giving me lower rpm but more torque driving a bigger prop still giving me decent speed, this would be applied to my 1m aluminum rigger (to be designed) and need to figure out how this all relates for a given battery (in my case 4s).

  6. #6
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    yes. i am testing my new 6s sport hydro which has a low kv motor swinging a good sized prop and it seems to be working very well. i have good speed and after a good 6 test laps at full speed there is minimal heat to motor / esc and battery pack. will be good to compare at our next race meet just to see how mine does compare in top end speeds against the other guys running higher kv motors etc.

  7. #7
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    I have briefly played with this in the past, mostly on first maiden runs where I was just checking things out (ie:6s to start on 12s setup). And on those runs I feel Great runtimes can be accomplished if moderate speeds are the goal. I too would like to try this big prop to boat ratio thing. Fluid has been preaching this for some time and it’s time to investigate, lol.

  8. #8
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    I have thought about this, and read up on it a bit, but I definitely need to learn more, here's where I'm at right now (it may change) if it helps anyone.

    Hull : 1m Aluminum rigger (glued 0.5mm thickness)
    Motor : TFL SSS 4092-1650Kv or 1800Kv, aiming for 20k-25k rpm under load.
    Drivetrain : 3/16" (I understand the upper prop size limit for 3/16" is about 65mm)
    Prop : to be tested, I'm hoping a motor as described above can cope with a 60 - 65mm decent pitch prop.
    ESC : Flier 250A (considered seaking 180A but I believe this will be on the limit).
    Battery : 4S with a capacity about 200A, meaning 4000mAh at 50C or 5000mAh at 40C etc.

    This is roughly how I have balanced it for now, nothing is purchased as of yet, the hull is to be designed.

  9. #9
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    Amps is where things peak up a bit with the lower voltages. Though in theory the should steady out and NOT be so ‘peaky’. Basically it is going to take X amount of watts to get to a given speed. One should be able to dig around and find someone’s amp draw/speed ratio. Then you can go from there.

  10. #10
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    Ok so even assuming this guy ran out 100% of his 5000 mah battery, he would have to have an average current draw of around 20A. So because it is a video I call BS on the claim of a 14 minute run! You MIGHT do that on 10000 mah if you are not running full throttle and you have a very well prepared and suitable prop.
    NZMPBA 2013, 2016 Open Electric Champion. NZMPBA 2016 P Offshore Champion.
    2016 SUHA Q Sport Hydro Hi Points Champion.
    BOPMPBC Open Mono, Open Electric Champion.

  11. #11
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    https://www.youtube.com/watch?v=4UZD0yJjy9E

    I rewatched the video. He is not running very hard at the end when he claimed 14 minutes of run times. It also appears significantly slower with the low kv motor.

  12. #12
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    And that folks is why you do not base anything on you tube videos! "I must have been out there for about 15 mins". Really time guessing! You just cannot get 15 mins run time out of 5000 mah, I don't care how much of a you tube expert you think you are! And this guy is no expert, he has 4mm bullets on his batteries, and the original motor setup,and he wonders why it all melted!
    IMO the Sonicwake has the wrong motor, kv is too high for 6s (for a rtr and inexperienced boaters), too low for 4s and being a 36mm can is undersized for the boat. If they had used the same motor as the Voracity it would have worked better.
    NZMPBA 2013, 2016 Open Electric Champion. NZMPBA 2016 P Offshore Champion.
    2016 SUHA Q Sport Hydro Hi Points Champion.
    BOPMPBC Open Mono, Open Electric Champion.

  13. #13
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    Quote Originally Posted by Peter A View Post
    IMO the Sonicwake has the wrong motor, kv is too high for 6s (for a rtr and inexperienced boaters), too low for 4s and being a 36mm can is undersized for the boat. If they had used the same motor as the Voracity it would have worked better.
    Based on my boat, I agree with you. It ran way to hot. Again, I can prop down for 6s and prop up for 4s and optimize things. I am realizing that the root of my question is how did the 25,000 to 30,000 rpm range get picked as optimal. There should be many rpm ranges that can be made to work based on prop rpm and total prop pitch.

    I am impressed that the gentleman on youtube got that large motor in the boat.

  14. #14
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    Higher rpm tends to get the amps kinda high when peaking. And in all honesty it is just harder on things. From strut bushings to having everything Uber balanced to run the rpm. I know as I do have one boat running in the upper 40k range and doing just shy of 120.

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