Tank testing for amateurs

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

  1. Manfred.pech
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    Manfred.pech Senior Member

    Better is abebooks: Basic Ship Theory: Ship Dynamics and Design von Rawson Tupper, Gebraucht - AbeBooks https://www.abebooks.de/servlet/SearchResults?sts=t&an=Rawson+Tupper&tn=Basic+Ship+Theory%3A+Ship+Dynamics+and+Design&kn=&n=100121503
     
  2. tspeer
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    tspeer Senior Member

    I'm surprised nobody has mentioned the move, "Wind," yet. :)
     
  3. rnlock
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    rnlock Junior Member

    Maybe I missed it, but did anyone discuss how the method of towing might skew the results? For instance, I'm sure towing by a string from the foredeck will give different results than pushing where the propeller would be.
     
  4. rxcomposite
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    rxcomposite Senior Member

    Not exactly. Towing with a "clean hull" will only give you resistance to be scaled whether push or pulled. "Pushing" is done with the attachment inside the hull at a chosen point but not where the prop should be as it means the mechanism will be immersed and skew the result. You must be thinking about how the model airplanes are tested where the boom is attached after the model.

    Adding appendages will increase error as it is too small to model accurately. It becomes more complex if you add a model propeller, whether it is just drag or spinning because then the flow to the prop is taken into account. Sophisticated tanks can test propellers and its associated afterbody lines. Been done for years but not on a simple tank.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    So long as the tow line is inline with the shaft/prop line....then you're ok. If not...you will get incorrect readings unless you account for the couple between the misaligned tow/thrust lines.
    Since you tow from the same line of action where the prop and shaft would be...just in reverse, i.e towing, rather than thrusting.
     
  6. gonzo
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    gonzo Senior Member

    Tension or compression at a point, as long as it is in the same direction and same magnitude will make no difference. Think of when you open a door. From one side you pull from the knob, and from the other pull. However, the door moves the same both ways.
     
  7. baeckmo
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    baeckmo Hydrodynamics

    If we are dealing with surface piercing props, the thrust line is (generally) not in the line of shaft direction. This effect can have a major influence on trim and consequently on resulting resistance. For example, DTMB tank tests concerning Hickman hulls should be viewed with some caution due to this phenomenon.
     
  8. BlueBell
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    BlueBell Ahhhhh...

    "Wind"?
     
  9. gonzo
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    gonzo Senior Member

    Sailboat models usually have a stubby mast to pull from the sails' center of effort.
     
  10. rnlock
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    rnlock Junior Member

    Problem is you don't know what angle the hull will assume at any particular speed when pushed by its own prop or by sails, as opposed to being pulled from some guesstimated location.

    I think it's beginning to get beyond "amateur testing", but if you attach a telescoping rod to a point in line with the thrust line, and that rod is counterweighted so as it doesn't push up or down on the hull, you might be getting close. OTOH, if there is significant change in the attitude of the boat, it would be best to cant the rod so it's perpendicular to the thrust line. Even then, a real prop interacts with the hull in a way that will be hard to reproduce. So it's not really all that much better than the string.

    If the thrust line is actually below the hull, then neither method will be all that great.
     
  11. gonzo
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    gonzo Senior Member

    There is no "guestimating". The thrust on a shaft is along its axis.
     
  12. rnlock
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    rnlock Junior Member

    Lots of guesstimating unless you have some sort of feedback mechanism that moves the "fixed" tow point to keep it on the thrust line as the model pitches up or down at different speeds. I'll admit there's not much guesstimating on the boat end of the tow line.
     
  13. John Perry
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    John Perry Senior Member

    We have been putting together some equipment for hull drag measurement by towing. There is a bit about this in the meeting report at the link below, about two thirds the way down the page.

    https://www.ayrs.org/ayrs-meeting-at-thorpe-surrey-uk-3rd-nov-2019/

    I am a bit pessimistic about the value of this kind of testing as a way to optimise hull design since the difference in drag between comparable hulls that are both 'good' designs are likely to be tiny and hard (or impossible) to determine even with large indoor purpose built towing tanks. However, it was a good project to add a bit of interest to our AYRS meetings this summer and with a bit more careful setting up next time I think we might at least be able to get an approximate validation of theoretical methods such as the Michlet software, or indeed CFD work.
     
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  14. Waterwitch
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    Waterwitch Junior Member

    Considering the OP is trying to test a paddleboard design, the thrust is more variable than a shaft on center line.
     

  15. portacruise
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    portacruise Senior Member

    My observations are that a boat towed with a single line, swings back and forth horizontally ie. it does not follow a completely straight thrust line behind. This occurs even under optimally controlled conditions of line length, calm conditions, different attachment points, Etc. I suspect that even with two separated tow lines, there would be some swinging of the stern. So I wonder if that introduces significant error to the measurements taken?
     
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