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[ksurf] Re: KitePhysics 102

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  • Ken Winner
    From: ... fact wrong, but if ... beam reach at ... to true wind speed. ... the apparent wind, ... boat speed. In ... automatically ... any
    Message 1 of 5 , Sep 30, 1999
      From: <KiteBoard@...>
      > I would also tend to not take them seriously if that is in
      fact wrong, but if
      > it IS wrong, it isn't so simple: When travelling below a
      beam reach at
      > multiple wind speed, VMG downwind is pretty high relative
      to true wind speed.
      > This at least somewhat reduces the effect of true wind on
      the apparent wind,
      > possibly as much as the increase caused by the increased
      boat speed. In
      > other words, just because boatspeed is higher, it doesn't
      automatically
      > increase apparent wind speed if your course is downwind to
      any degree.

      It's true that the angle can subtract from apparent wind
      velocity, but most often and for fast craft, that doesn't
      happen until the craft is sailing pretty broad -- at least
      below 100 degrees.

      The attached sketch doesn't reflect the speed polar of some
      craft in particular, and is actually a bit of an
      exageration, but it illustrates the idea. By the way, it's
      not there for your edification, as I'm sure you know what
      I'm talking about, but for anyone who hasn't been following
      this carefully (and I wouldn't have attached it were it not
      quite small).

      KW
    • Dave Culp
      ... I know you know what you are talking about, Ken, but your drawing is inaccurate. If one draws the remaining two bounding lines, from the ends of the
      Message 2 of 5 , Sep 30, 1999
        >The attached sketch doesn't reflect the speed polar of some
        >craft in particular, and is actually a bit of an
        >exageration, but it illustrates the idea. By the way, it's
        >not there for your edification, as I'm sure you know what
        >I'm talking about, but for anyone who hasn't been following
        >this carefully (and I wouldn't have attached it were it not
        >quite small).

        I know you know what you are talking about, Ken, but your drawing is inaccurate. If one draws the remaining two "bounding lines," from the ends of the vectors shown, they must define a parallelogram, but your drawing doesn't. As drawn, the vector geometry won't work.

        While it's true that apparent wind is greatest, for very high Vb/Vt sailcraft, on broad reaching courses, this is only due to the great craft speed, as you mention. However, the apparent wind will almost always be *less* than craft speed, on any course much over 100 degrees true, and certainly on the craft's fastest course, which will be nearer 115-120 degrees. This is true of all sailcraft, fast or slow.

        FWIW, for a given, fixed craft speed, apparent wind will be highest on close reaching (windward) courses, not broad reaching.

        Dave
      • Ken Winner
        From: Dave Culp ... drawing is inaccurate. If one draws the remaining two bounding lines, from the ends of the vectors shown, they must
        Message 3 of 5 , Oct 1, 1999
          From: Dave Culp <dave@...>
          > I know you know what you are talking about, Ken, but your
          drawing is inaccurate. If one draws the remaining two
          "bounding lines," from the ends of the vectors shown, they
          must define a parallelogram, but your drawing doesn't.

          Yeh, you're right about the sketch.

          KW
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