Updraft

Discussion in 'Boat Design' started by Velsia, Nov 4, 2020.

  1. Velsia
    Joined: Oct 2008
    Posts: 92
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    Location: Jersey C.I

    Velsia Floater

    I know this requires some serious CFD or empirical testing but does anyone have any rules of thumb or educated guesses on how high up an anemometer should be to not be influenced by the wind coming of the sails?

    I would just go for the biggest but I can't as I have a burgee above it (more turbulence!!).
     
  2. jehardiman
    Joined: Aug 2004
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    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    Not so much above, but upwind or in front of a sail. Remember that an anemometer reads apparent wind not true wind, so things like pitch, roll, and true wind direction are going to have much more affect on the reading than updraft. While chapter 11 in Applied Fluid Dynamics Handbook by Blevins has much data on blocked wind flow, Marchaj directly address the problems with wind instruments and the flow around the mast and sails in part 3C of his Aero-Hydrodynamics of Sailing.

    FWIW, I'd use a standard masthead fitting, so just in front and just above the masthead.
     
  3. Velsia
    Joined: Oct 2008
    Posts: 92
    Likes: 11, Points: 8, Legacy Rep: 15
    Location: Jersey C.I

    Velsia Floater

    Hi Jehardiman, thanks for your input and for the book references. Good excuse for me to by another Marchaj book is what I say!

    The sales blurb from the electronics company says

    "Higher altitude for clearer readings
    The vertical spar minimises the effect of sail-plan disturbance on wind measurement ensuring the best performance with minimum calibration. This means reliable True Wind Direction to make precise race calls when needed and better steer-to-wind performance in all conditions when on autopilot."

    As the statement is "higher altitude for clearer readings" I want to make sure by reducing the altitude we don't adversely effect the performance of the unit beyond what calibration can fix.

    These wands are costly items to change out and I want to make sure its right first time round.

    It seems high end race boats use the tallest wands they can get their hands on and sometimes even two types like below.

    upload_2020-11-5_12-45-0.png
     
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