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[ksurf] Re: Fwd: KitePhysics 102

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  • KiteBoard@aol.com
    I d asked Dave Culp: Welcome to this discussion. Are you POSITIVE that this statement is ... their ... To which he replied:
    Message 1 of 1 , Oct 2, 1999
      I'd asked Dave Culp:

      << >Welcome to this discussion. Are you POSITIVE that this statement is
      >incorrect?: For "land-sailing or ice-sailing vehicle...a beam reach
      >--...happens to be the course where apparent-wind speed and pull are at
      >highest possible levels while sailing" >>

      To which he replied:

      << Well, it's not all that black and white, Tom. It depends on the boat.
      A slow monohull, incapable of sailing faster than the wind, will see
      it's highest apparent wind when sailing upwind--likely even hard on
      the wind (40 degrees, true), if it's a powerful boat, able to make
      its maximum hullspeed on that course. OTOH, a land yacht, or iceboat,
      capable of sailing at 4-5 times the true windspeed, will see its max
      apparent wind on the same course they get their maximum speed--about
      100 degrees off true.

      For fast boats which are *not* iceboats, like kitesurfers, the point
      of max apparent wind likely depends on the specific multiple of true
      windspeed the craft is capable of--I've never run the numbers, as max
      apparent wind isn't really of interest. It might be interesting to
      chart this, for various different types of boats, just to see what
      the curve looks like. Doing so would need a lot of data points, for
      many different boats.

      Any sailcraft's max speed (and perhaps its max apparent wind speed?)
      is attained at 90 degrees, plus the craft's total drag angle. An
      iceboat has a minimum drag angle of maybe 7 degrees, a landyacht
      nearer 12-17; a really hot catamaran (Cogito--C-class cat) about 24
      degrees, a kitesurfer nearer 35-40 degrees. Thus a kitesurfer might
      hit his max speed around 125-130 degrees. I'd need to run a number of
      examples, from real-life kitesurfers' performance, to tell you
      whether they see their max apparent wind at that course, or something
      closer to wind.

      I can tell you, definitively, that the statement above *might* be
      true, for some boat (or boats), and is certainly untrue, for others.
      No help, huh? Almost without question, the authors made some (many?)
      presumptions, then ran the numbers, and now believe they have a
      "general" case. They are mistaken. However, they *might* have a good
      number, for some, or even most kitesurfers. I really don't know,
      without knowing what presuppositions they made. >>
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