Does a boat's glide (deceleration) give a reasonable measure of resistance?

Discussion in 'Hydrodynamics and Aerodynamics' started by Paddlelite, Mar 5, 2013.

  1. Petros
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    Petros Senior Member

    a better way to measure hull resistance is to get a watch with a heart meter, and either a gps or some other way to measure speed (a little impeller driven speedo perhaps). Paddle the different kayak/canoe hulls at the same speed over a distance and see which one has the lowest heart rate. This will also include the efficiency of the sitting position for the paddler, and give you a direct measure of muscle power/imput required.

    The higher the heart rate, the more power it takes to drive the hull at the same speed.
  2. Paddlelite
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    Paddlelite Junior Member

    Thanks for all the suggestions on refining the glide test, and on alternative tests. Now I'm tempted to carve some different models out of foam and pull them on a balance beam.
  3. johneck
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    johneck Senior Member

    How well does the boat track when decelerating? Any yaw will have a huge effect on the result. That could be what is causing the scatter.
    In theory the method is workable, but as the others above have indicated, there are many potential sources of error and practical problems that make it difficult. The direct approach of measurement with a scale is probably the best.
  4. haribo
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    haribo Junior Member

    you use a interesting simple method for speed mesurement!

    I found this "Ship resistance versus ship speed for a slender mono-hull type" curve on Topics/hull_design_approach_2-1.html

    mathematical it looks as if the resistance is more or less a function of " a*speed² " up to the begin of planing in this case [5m/s], than a traverse part, and over [7m/s] also a funktion of speed² maybe like " b*speed² " with b<a

    in your speed test with the dominator 14´ it looks as if you start with a bit gliding for the first sec.??? and than the traverse part until t= 2.5 ???
    did you surfe a little on your bow wave between t=1 and 2.5 ???

    I think it is not a 4th order curve, maybe better you divide the curve in different parts for the different flow parts...

    between 2.5 sec and 7.5 sec your dominator deceleration-curve seems to be like 1/(a* speed²)

    I am not realy shure with the gliding, and I am not skillfull with your customary units,

    (my little SI question: how many feets is your mile long, the mile in the [mph] ??? )

    for this thesis I would test the same boat with more mass (+20%.....+25%), you need truly more power to reach the start speed, but will the curve of the deceleration be realy steeper???
  5. Paddlelite
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    Paddlelite Junior Member

    The boats are on a glide the entire time of the graph, if that was what you were asking. Your curve is resistance vs speed like the one I've attached which roughly models the Dominator. And yes, you can see that the early part of the curve is dominated by skin friction which goes about as V^2, but then residual resistance kicks in, which oscillates around the 4th power of V, I believe. In my first graph, miles are 5,280 feet, and in this graph knots are 1.151 miles.

    Attached Files:

  6. haribo
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    haribo Junior Member

    glide on the entire time? ok than I used the wrong english word, I mean that your decelerationstest seems as if you start with planing (or semi-planing), planing down to the speed of 4,1 mph (4,7kn or 2,4 m/s...)

    my apologies if planing is also the wrong discription for the change of the flow under this slender boat in the moment the bow wave catches your boat again on the rear tail, and gave you back some energie (or lift?) in this moment

    with just empirical playing around with your second curve, we could easy discrib the speed[kn] resistance[lbs] curve
    from 0,0kn to 4,7kn with total=0,25 V^2
    and from 4,7kn to 8,0kn with total=0,40 V^2 -2

    IMO it is easyer to split the drag forces at this speed and use only 2nd order of V for such a smal boat with not so much relevant residual resistance
  7. Michael Y
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    Michael Y Junior Member

    If you have a smartphone with GPS, you can get an app that will record speed versus time. GPS speed is calculated via Doppler from the satellites, so it's instantaneous accuracy is in the centimeters per second range. The key is the app needs to not compute speed by dividing distance by time. Some do, as either the Phone location code does not support a subroutine call for accessing speed directly, or the programmer didn't think to use it.

    I used GPSessentials for the droid. I've gathered accurate aircraft takeoff flight data with it.
  8. nimblemotors
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    nimblemotors Senior Member

    The glide down tests look pretty clever, well done!

    I would think a simple speed test would do, using a trolling motor to push the boat with constant power and simply measure the top speed (and maybe acceleration) of each boat, keeping the weight constant. I was thinking of doing just this for my 1/3 scale model.

  9. Anders B
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    Anders B New Member

    Also make sure the boat follows a straight path, any coarse instability would bias the result.
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