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Thread: Power output does not reflect to power input.

  1. #1

    Default Power output does not reflect to power input.

    I was curious as to why sometimes an electric boat is faster than a nitro boat. For example I saw that very often I would notice a 3.5cc nitro boat doing a certain speed then a 4s electric boat could rival and sometimes beat the nitro boat. And many other instances like that. Then looking at the power rating for that giving 3.5cc I would see something like 2.8hp. (2088watts output) And an electric motor with less input than 2088 w would be faster than the 3.5cc nitro boat of a similar hull.

    And if the electric motor is only 85 or 90 percent efficient how is it faster.

    Then after studying more closely I realize some motors put out horse power greater than the input horse power. Example I have done research of the aquacraft 1800kv 3656 spinning a 45mm prop anywhere from 24000 to 25000 rpm underload. Well my 1730kv leopard 4092 is 8 mph faster than the aquacraft with the same 45mm prop on the same 4s and pulling close to the same amps. So imagine what the leopard is reving heavier motor which takes away from prop efficiency. And a 1730 kV on 4s.

    But hear is where it gets interesting the ampdraw for me is around 42 amps on 4s with the 45mm prop. Voltage underload for me is around 16.1volts at the 42amps. That's is 676w input.

    Now tell me if a 1 horsepower nitro engine can turn an octura m545 at 28000 rpm.

    1 horsepower is said to be 746w. That's more than my input.

    In fact a 3.5cc rated close to 3 horsepower will often require props around 40 to 42mm.

    So power input in an electric motor does not always reflect power output.

  2. #2

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    isn't one a 2 pole and the other 4 pole?

    that would explain it, i would imagine

  3. #3

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    The aquacraft is a 6 pole and the leopard is a 4 pole.

  4. #4

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    Really, what 3.5cc engine puts out 3hp?
    IC engines generate maximum torque at x rpm. Electric motors generate maximum torque at stall which is why they will turn the props.
    Also there are many other variables when racing. A slower boat can beat a faster boat by being driven better, I do that often against bigger faster gas boats. FE boats have the advantage of being able to accelerate pull out of corners faster due to the torque generated, those little bits of extra go can be enough to win.
    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.

  5. #5

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    Horesepower claims are unsubstanciated BS, that they can get away with because very few people have a dyno, somone here did dyno a bunch in the late 90s and found them all to be so far out from the claim that it is now illegal to advertise power claims here unless you have proof, which none of them do.

    The broarder torque curve would also help electrics even if the power was equal.
    Paul "tug Killer" Upton-Taylor, Cat lover.
    FastElectricBoats.co.uk

  6. #6

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    I have seen a 56 inches areomarine apache with a full mod twin which I think would be rated around 13 to 14hp. That apache did a little over 70.

    On the other hand I have seen a castle 2028 push a 60 inch areomarine apache faster than the twin full mod gas was able to push the 56 inch apache.

  7. #7

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    I have gathered data for amp draw in my location with different diameter props with one particular motor and on a similar battery full charge. With one object to learn to predict amp draw by knowing a prop dimensions.

    I used a carbon filled 32mm, a carbon filled 34mm, a carbon filled 36mm 3 blade and a graupner k42. The k42 pulled 2 times more amps than the 32mm same motor same cell count. The k42 also pulled 1.41 times more amps than the 36mm. Then I experiment with some numbers to draw a connection with the results.

    This is what I came up with for every 2mm increase in diameter the amp draw increase by a factor of 1.15.

    For every 1mm increase the amp goes up by 1.069 times what the smaller prop is.

    These numbers connected my results from the gathered data. Bear in mind certain variables can throw off the calculation slightly.

    A 3 blade can multiply by 1.08 to 1.15.

    For pitch ration take the higher ratio divide it by the lower ratio and find the square root of the result to give you the multiplayer for the pitch ratio increase.

    Example a v937 vs a x437. You who have a formula like this. √1.9÷1.4=1.1649 as the multiplayer.

    Bear in mind some props do not have the pitch they are advertised to have. Some props have more pitch some have less. So predicting can be deceptive going off of the manufacture rating.

    I did formulas for cell increase and motor kV too.

    I tested my formulas on many other gathered data on different setups and find it works. For all I have seen.

    Here is the main point. In context to this thread.

    If the increase in amps is proportional to torque increase for a given motor.

    I think I can conclude that electrics are more powerful than gas and nitro. And if the formula 746w equal to 1 horsepower.

    I can tell you power out is greater than power in, for many setups.

  8. #8

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    Quote Originally Posted by Speed3 View Post
    I can tell you power out is greater than power in, for many setups.
    Congratulations on your discovery, I look forward to seeing you sitting back and raking in the trillions from the power companies. I couldn't borrow a couple of billion could I?
    Paul "tug Killer" Upton-Taylor, Cat lover.
    FastElectricBoats.co.uk

  9. #9

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    I wish, that would be something.

  10. #10

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    YOu are mixing up soooo many variables here.
    First of all, you cant break the laws of physics. For an any machine, power in is always greater than power out.
    Period.
    Now, once installed in a boat, there is a lot of factors that affect speed.
    I have seen it documented here in this forrum, that simply changing props increased the speed of a boat, while decreasing the amp draw. I believe it was an Octra to an ABC.
    They had data logging, but did not compute input power, only saw a decrease in amp draw. Don't know the voltage difference, so you don't know the input power.
    Cheetah, Super Rio, (Mod) Starship (Mod and sold),

  11. #11

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    The majority of the power an electric motor puts out comes from the metal in the stator and the magnets on the rotor. And most being the rotor magnets. The input power only creates an electromagnet to create a field that the rotor repels against. The rotor could have multiple times more energy than the stator. For that reason the input energy cannot reflect the output energy.

    Because it only serves as a catalyst to unleash the real energy.

  12. #12

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    And this can explain why my 4092 size motors are so much more faster than smaller motors of similar kV will using the same input or around.

    I have tried 4074, 3674, 3660, 3656, 2860, 2858. The smaller motor even with a higher kV comes up short.

    The bigger motor need a prop that can generate adequate thrust though.

  13. #13

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    To clarify when I refer to electrics being more power than gas I am refer to motors like 4092 against single gas engines and castle 2028 size against twin.

    The castle 2028 has to be on 12s though with the right size prop to beat a twin highly modded gas engine in a boat of equal length.

    And the 4092 need 6s to beat the single engine gas boat.

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