Tank testing for amateurs

Discussion in 'Boat Design' started by Alwaysthinking., Oct 30, 2019.

  1. TANSL
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    TANSL Senior Member

    When you talk about having control over speed, you mean controlling the speed of the model, right?. If the speed to be controlled were different, could you explain which and for what reason and how it would be combined with the speed of the model?
     
  2. Blueknarr
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    Blueknarr Senior Member

    Always thinking

    As you can see, the stream has a whole lot of variables which induce inaccuracies. It should provide a rough guide but won't be laboratory accurate.

    Try try try and don't be discouraged by the naysayers.

    The current crop of flight engineers would be sceptical of Wilbur and Orville testing air foils attached to a bicycle's handle bar.
     
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  3. Doug Halsey
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    Doug Halsey Senior Member

    I've thought about doing something like that to help with the foil settings on an RC foiler. Here's someone who's done more than just thinking about it:
    RCMultihull-2.jpg
    For those who use Facebook, he posts frequently to this group: Log into Facebook | Facebook https://www.facebook.com/groups/1647999775518228/
     
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  4. Squidly-Diddly
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    Squidly-Diddly Senior Member

    I'd use an irrigation canal. We got them all over CA farm lands. Many are dead straight for a mile or more, and they come in all shapes and sizes from about 1 yard across to small river size.
    The water is quite calm when flowing, but often just sitting. The banks will be also nice and uniform. I've NEVER seen anyone playing in them even on the hottest days, so maybe its a big taboo but I've never heard about it. Maybe just lack of people and lack of interest. I've only noticed a few signs about "no swimming" but only at major road crossings with lots of foreign tourists on their way to Yosemite.
     

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  5. Blueknarr
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    Blueknarr Senior Member


    Swimming in them is a major NO NO. Several drownings occur annually. The flow is far faster than it appears and the banks are very difficult to climb out. Check out Modesto Irrigation District's "Splasher the frog"

    content://media/external/file/9967
     
  6. gonzo
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    gonzo Senior Member

    Any canal or stream gives him no control over the parameters of the test. Basically, it is not of much use. If you scale down hull speeds, by their square root, velocities are very small. Therefore, they require a fair amount of precision to give meaningful results. The random turbulence is another major factor he would have no control over.
     
  7. TANSL
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    TANSL Senior Member

    You do not scale the speeds, what should be done is to keep the same Froude number, among other parameters.
     
  8. rxcomposite
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    rxcomposite Senior Member

    The tank is diagrammed as attached. I suspect the impeller is a high volume/low velocity. It does not need a lot of speed. Speeding up the water would need a long neck to reduce eddies and turbulence.
     

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  9. rxcomposite
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    rxcomposite Senior Member

    From an on line supplier. Digital flowmeter. If it is good for kerosene, it is good in water. Digital Flow meter.jpg
     
  10. rxcomposite
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    rxcomposite Senior Member

    Hall effect flowmeter. Hall Effect Sensor.jpg
     
  11. rxcomposite
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    rxcomposite Senior Member

    Here is from my textbook. How to do the Test. From Eric Tupper Introduction to Naval Architecture. I believe Eric is from Westlawn.
     

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  12. rxcomposite
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    rxcomposite Senior Member

    What Gonzo probably meant "Scaled Speed" meaning the model water speed is not the same as the full size version. You are correct in Froude number though as it is speed to length ratio. Another factor comes in. The Reynolds number when scaling up.
     
  13. DCockey
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    DCockey Senior Member

    The speed needs to be scaled by the square root of the model scale factor to keep the same Froude number.
     
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  14. TANSL
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    TANSL Senior Member

    Are you sure?. Check your concepts that, perhaps, are not up to date.
     

  15. DCockey
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    DCockey Senior Member

    Froude Number Fn = U / Sqrt ( g * L)
    For same Froude number of two different size boats:
    U1 / Sqrt ( g * L1) = U2 / Sqrt ( g * L2)
    U1 / U2 = Sqrt (L1 / L2)
    U1 = U2 * Sqrt (L1 / L2)
    L1 / L2 is the scale factor.

    Reynolds number scale is different. For same Reynolds number speed needs to be inversely proportional to the scale factor.
     
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