Drag coefficient test on the cheap?

Discussion in 'Hydrodynamics and Aerodynamics' started by Expandacraft, Dec 31, 2011.

  1. Expandacraft
    Joined: Dec 2011
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    Expandacraft Expandacraft

    Hello members,
    I'm new here and hope this is the right place to pose this question. I'm the designer and manufacturer of Expandacraft modular boats and wanted to know if there is an inexpensive way to determine drag coefficient. My bows are wave piercing and it is my theory that while they may not be as slick as some kayaks in flat water, they are in fact more efficient into the waves.
    My goal is to actually measure and document the data, not simply to calculate a number. I'm thinking of some sort of devise which I can attach to a motor boat and test in (real world) situations.
     
  2. Submarine Tom

    Submarine Tom Previous Member

    Expandacraft,

    Welcome.

    Mount a pole extending out the side of a power vessel, hang a large fish scale and attach it to a tow line from your test boat. Do some trials at varying speeds and plot a curve.

    Voila! A velocity/drag curve.

    -Tom
     
  3. Expandacraft
    Joined: Dec 2011
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    Expandacraft Expandacraft

    Submarine Tom, Thanks. That's kinda what I was thinking. I'm looking for some sort of computer program and devise to atach to such an apparatus which would create a chart I could post with some degree of validity. Perhaps I could use a kayak with well documented stats as a control. The issue with that is if I use another manufactures product as a control, I may piss them off and they may call out the legal team. So my new question would be, What else could I use as a control?
     
  4. Submarine Tom

    Submarine Tom Previous Member

    A rubber tire? I admire your desire for a control but I don't think it necessary.

    You are taking actual readings. So long as your pole is long enough to get in "clean" water and your scale reasonably accurate, you're away. If you use GPS for your speed and heading, be sure to do opposite runs immediately and average your results.

    -Tom
     
  5. Alik
    Joined: Jul 2003
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    Alik Senior Member

    1. With this method of 'pole testing', look where force is applied. Normally towing line should coincide with shaft extension, otherwise there is no real trim and results of measurements are waste;
    2. It might happen that the model will yaw, especially if tested without rudder or skeg, and forefoot is deep. Taking measures to stabilise the model will decrease accuracy of measurements.
     
  6. Expandacraft
    Joined: Dec 2011
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    Expandacraft Expandacraft

    Now that you mention it, I should drag the control and my Expandacraft side by side during the rough water test so as to record data from the impact of the same wave (which is the variable). I could video the scale and blur the image of the other manufactures kayak. I think that would serve as reasonable and acceptable data to your average youtube viewer.
     
  7. Expandacraft
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    Expandacraft Expandacraft

    Alik,
    I'll be testing full size production boats not models. Good point about the rod. I'm defiantly going to use that idea. Thanks.
     
  8. Alik
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    Alik Senior Member

  9. Submarine Tom

    Submarine Tom Previous Member

    Why would you do a drag test without a rudder or skeg?

    You wouldn't have a fair representation of the drag force.

    -Tom
     
  10. Expandacraft
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    Expandacraft Expandacraft

    Sorry, I should have made that clear. I do plan on using a skeg. However. being a catamaran I could tie off both bows with a bridal to control yaw. I plan on testing the 12 and 16 ft cat and maybe a tri hull 16. Will defiantly post the results here.
     
  11. Alik
    Joined: Jul 2003
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    Alik Senior Member

    This is basics: normally models are tested without appendages. Why? Because Reynolds number (Rn) on small scale appendages are low, so they can not be modelled that way. Usually appendages are tested separately at higher Rn or added as result of calculation.

    For testing real size kayak it might work to test with skeg.
     
  12. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    Tow testing full size

    Frank Bethwaite did extensive tow testing of some of his and his sons designs and he documents the testing in his book "Higher Performance Sailing"
    pages 153-162.
     
  13. philSweet
    Joined: May 2008
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    philSweet Senior Member

    Since you have a real world boat, not a model, why not a direct propulsion test? trolling motor/power meter/GPS. You could make instant adjustments in trim and make rapid consecutive runs. Don't know why you would need a high data acquisition rate. Average values over a few minutes would seem sufficient. Anyway, you will know what you needed to have done after you've done it. That's the nature of ad hoc experiments. Go get some stuff wet.

    I definitely think you should compare to a reference boat at the earliest possible opportunity. Day one testing if possible.
     
  14. ancient kayaker
    Joined: Aug 2006
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    ancient kayaker aka Terry Haines

    Kayaks are notoriously difficult to tow. You can improve control of yaw with a fore-and-aft pole on the bow of the kayak to put the towing location further forward. Should act similarly to a bridle. Alternatively a crossbeam with an actual bridle to the towing pole off the beam of the power boat, which will provide even better control.
     

  15. Alik
    Joined: Jul 2003
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    Alik Senior Member

    Yes, this works, we did and it works.
    The question is how high should be towing point on kayak.
     
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