Drag Coefficient versus Size

Discussion in 'Hydrodynamics and Aerodynamics' started by Inquisitor, Sep 15, 2020.

  1. Inquisitor
    Joined: Nov 2005
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    Location: North Carolina Mountains

    Inquisitor BIG ENGINES: Silos today... Barn Door tomorrow!

    I have a question that's kind of academic and kind of practical. Say I'm going dead down wind, wing on wing with a 155% Genoa. I have a telescoping whisker pole that can "near" flatten out the sail. Somewhere between...

    1) flatten (Cd = 1.0) with and maximized sail area
    2) parachute shape Cd (maximized) yet smaller cross-sectional area

    ... there is a maximum drag on the Genoa and thus thrust to the boat.

    Even if I had access to a 3D-CFD program, I don't really think I could calculate this. There are just too many variable... triangular shape, indeterminate cut, head-stay angle, influence of the main... etc, etc.

    So, I'm thinking historical experience is far better toward a practical, real-world answer. So... what percentage of the flattened length of the whisker-pole do you think would result in the most speed?
     
  2. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    100%
     
  3. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    I would agree with Bluebell, but I must admit that I do not know much about sail theory - 100% seems logical though.

    Rather than going dead downwind, I would sail at 140 degrees apparent to the wind, with the 155% genoa flying loose and the mainsail to leeward, and a much smaller high cut jib poled out to windward.
    We did this the whole way on a transatlantic passage from Tenerife to Antigua, gybing every couple of days or so, and it worked very well.
    This was on a 54' yawl; we took 18 days for perhaps 3,000 miles sailed through the water (a bit more than the rhumb line distance which I think is about 2,700 miles).
     
  4. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member

    I would treat it like a spinikar. Pole in until the luff breaks. Then pole out just a bit.

    Lift + drag will out run drag alone.
     
    Will Gilmore and bajansailor like this.
  5. Inquisitor
    Joined: Nov 2005
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    Location: North Carolina Mountains

    Inquisitor BIG ENGINES: Silos today... Barn Door tomorrow!

    I've been away from sailing for some time due to extenuating circumstances and am now arm-chair sailing why I get a boat back in order. I completely agree about the off-wind. That's one of the reasons I said this thread is kind-of academic. My inquisitive nature drilled down to China about the trade-off between larger area versus higher drag coefficient. Another reason I started this, I priced a 16' whisker pole and it hurt. Thing costs more than my boat is worth. I'm weighing attempts at making one versus really needing the full length of the genoa.

    Years ago, a buddy and I were in a race using McGregor motorsailers (imagine snails racing). We tried some questionable things back then... one was slightly off-wind flying the cruising spinnaker leeward, 155% Genoa poled windward and trying the main both leeward and windward using the soft boom-Vang tied to the rail windward to prevent crash jibes. In our youthful insanity, we even tried emptying the water ballast. This required us to stay more DDW for obvious reasons... kind-of like balancing a yardstick on your nose while riding a unicycle. We didn't have any modern electronics to weigh benefit for VMG. It was all seat-of-the-pants. I look forward to doing some trials in the future.
     
  6. Will Gilmore
    Joined: Aug 2017
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    Location: Littleton, nh

    Will Gilmore Senior Member

    [​IMG]
    This is the Alligator Amphibious Tank. It was originally developed to reach remote and stranded victims of hurricanes in the swampland of Florida. During its development, they originally tried flat tread flanges to propel her through the water. They couldn't get any reasonable speed. They then cupped the flanges, as you can see in the photograph, and the results, as I understand it, was the only patent to come out of the Alligator's development. They got more grip on the water with cupped tread than flat tread.

    I would assume the same for a sail. A flat board would have less pressure from the air than a cupped sail. In the case of DDW. A broad reach usually translates to a greater VMG than DDW. Boat and rig, of course, will have something to say about its reality.

    -Will (Dragonfly)
     

  7. Inquisitor
    Joined: Nov 2005
    Posts: 271
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    Location: North Carolina Mountains

    Inquisitor BIG ENGINES: Silos today... Barn Door tomorrow!

    A more fitting example could not be found! :D
     
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