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Thread: New guy's motor questions for a non-racing 1/8 hydro

  1. #1
    Join Date
    Aug 2019
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    WA
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    Default New guy's motor questions for a non-racing 1/8 hydro

    I'm building a 1/8 hydro for just fun running around my lake, so I don't have to fit any specific motor requirements to fit any rules. Since that opens up pretty much any set-up that I could ever want, I started shopping around, and getting confused. Even thought I won't be racing others, I still want a fast boat.

    KV ratings vs motor diameter vs motor length - In real world applications, what would be the difference between a 56mm x 92mm (like a TP 5670) running 850kv, vs a 40mm x 102mm (like a NEU 1527) running 850kv? In theory, they would turn a prop at the same RPM, right? But I'm guessing the bigger motor would have more torque? So it could run a bigger prop? I get the bigger motor would be heavier, but would it be able to push a boat faster? Plus, the TP5670 is way less expensive than the NEU 1527.

    Motor KV ratings - I've read that it is good to think of them more like a transmission where a lower kv rating is like a lower gear that can turn a bigger prop slower instead of a higher kv rating can spin a smaller prop faster. I also see that a sport boat should run around 25k rpm and a racing boat around 30k rpm. I calculate a kv rating motor of 950kv on 8s or 10s would spin at around those goals. Since I'm not stuck with required motors, would this be a good kv rating for not being limited by rules? I'm sure there are lots of variables that I don't know about. I'm sure prop size is the big one that I don't completely understand.

    Sorry for the technical questions, but I'd really like to understand this instead of just blindly ordering a motor that looks good.

  2. #2
    Join Date
    Mar 2020
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    NH
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    Default

    Hi you can also look at TP 4070 940 or1060 it's an good motor for a hydro ,keep the voltage from the motor and your battery set in side the 10v

  3. #3
    Join Date
    Apr 2007
    Location
    AZ
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    7,720

    Cool

    For a hull this size, ?10v? just isn?t enough. Since this is a sport boat, I?d recommend using 8S (two easy-to-find 4S packs in series) and a motor with a Kv between 800-900. The NAMBA racing power rules for 1/8 scale are actually rather mild and would work well for the OP. Generally, with motors of the same quality, the bigger one will spin a larger prop faster. The large motors like the 56x92 are simply not needed, they weigh more, cost more, and are often difficult to fit into a scale hydro. A 40x102 or similar would work great, spinning props in the range of 55-60 mm. Speeds would not be slow, depending on the prop between 55 and 60 with a well set up boat, a bit higher with more aggressive props.

    One reason for the mild race setup is that with all the detail in an 1/8th scale hydro, a high speed flip means a lot of damage. Keeping the speeds down reduces the chances of an accudent.


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  4. #4
    Join Date
    Mar 2020
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    NH
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    Default

    what i mean is if you have a TP4070 860kv for instance it's a 60v motor a will not perform on 8 or 10s but a4070 1060kv will be perfect

  5. #5
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    AZ
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    Sorry, buy you are incorrect, for the OP’s needs the 850 Kv motors run best with 8S or 10S power. You clearly have little successful experience with high voltage sport boats. I do....and will not lead the OP astray.



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  6. #6
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    Aug 2019
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    WA
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    Fluid, I was hoping you would reply to this thread. I've seen your responses in other threads, and I've come to respect your opinion and appreciate your input. What you are saying in this thread also fills in the gaps of my knowledge quite well, and makes sense as I understand these motors so far. I still have a lot to learn, but that's why I'm here asking questions. Thanks for taking the time to help me out. I'll look for an 800-900kv motor around that size you are saying. I'm sure I'll be happy with working to get into speeds in the high 50s (and learning to drive one that fast), and if it is not enough, looks like I should move towards an outrigger for my next boat.

    Koen, thanks for taking the time to respond. What you are saying is different from what Fluid is saying, and while both of you are random guys on the internet, I've read enough of Fluid's posts to lean towards following his advice. I've also read about a fair amount of boats set-up close to what you say won't work well, and they seem to run fairly well. I'd love to see some evidence to the contrary, but for now, I think I'll go with Fluid's input.

  7. #7
    Join Date
    Oct 2015
    Location
    Ca
    Posts
    51

    Default

    You can't go wrong talking Fuids (Jay Turner) advice. I have been in this
    hobby for 33 years and I would venture to say that he knows more about
    fast model boating than anyone on the planet,and I am not exaggerating.
    I make this statement from years of reading his magazine articles in times
    past,and following the help he gives to new hobbiest like yourself now
    days. Nothing but respect for knowledge.

  8. #8
    Join Date
    Jul 2016
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    IL
    Posts
    3,087

    Default

    Some may say he wrote the book, or one of them anyway...
    Vac-U-Tug Jr (13mph)

  9. #9
    Join Date
    Aug 2019
    Location
    WA
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    I'm looking around at what motors are available, and I'm narrowing down the list. I'm hoping I can get specific input on a few choices here.

    Leopard 4092. Shorter than ideal, but close. It comes in 700kv and 930kv. Too slow, or too fast. Light and inexpensive.
    Or a TP 5660. I know, 56mm is wider than recommended. Also 92mm long, but it comes in an ideal 888kv. But it is more $$$ and heavy.

    The seemingly "standard" Neu 1527 1.5Y is out of stock, and 2x as expensive as the TP 5660.

    Thoughts? I'm guessing the 4092 in 700kv in 10s? I'm playing around with the OSE conversion/speed calculation page, and it seems that any of them would work, but I don't know enough to understand the other factors that aren't input to the calculation, like amps, benefits and disadvantages to different types of props, motor weights....

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