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    Re: True vs Magnetic
    From: Dan Hogan
    Date: 1999 Jul 18, 2:19 PM

    >Why?  I think we'd all agree that the less pressure we put ourselves under
    >while underway the better.  The more of one's navigational work one
    >pre-calculates while at anchor or underway in fair conditions the better.
    >When I'm underway under rough conditions or have been sleepless, the last
    >thing I want to go through is TVMDC.  I don't care how long I've been
    >doing it, how instinctive it is, it's still math which is prone to error.
    >(And the errors can be subtle, not in-your-face like many celestial
    Agreed. What ever *you* can do to eliminate steps in plotting or
    navigating that minimize error are good.
    >So when I sail, all my course lines on coastal charts are marked in
    >magnetic (to be sure, I do put an "M" after the course in good USPS style
    >to make sure anyone else who reads the chart knows the direction is a
    >magnetic one).  That takes care of variation, there's still deviation.
    >I've found, on my boat at least, that I can adjust my compass to get
    >deviation to be no more than a degree.  At that point it's good but not
    >essential to correct for it -- being bounced around (or how did Dan put it
    >-- "buffeted") by weather or pushed by an irregular current or making
    >leeway can all affect course made good by much more than a degree.  Net
    >result is that the number I read off my preplotted course line is the
    >number I steer.  No math needed along the way.
    On the boat I use mercator sailing and TVMDC in a pocket sized
    notebook. Simply because I have gotten used to it.
    [Snip USPS, agreed with and PIM description)
    The PIM sounds interesting. I have never seen one, but would love to, I
    enjoy gadgets. At one time I used an E6-B for DR.
    >Takes more time to explain it than to use it.  Bought mine many, many
    >years ago, don't know if it's even available any more, but it's absolutely
    >my favorite navigational tool.  Beats hell out of walking courses across
    >to a compass rose on a chart or using a traditional course plotter which
    >gives only true courses.
    >Lu Abel
    >PS - Does anyone know how to calculate leeway (for either power or sail)?
    >The Power Squadron asks one to use a leeway allowance in some its course
    >calculation problems, but it's always stated as "based on your experience
    >with your boat, you decide to allow 5 degrees port leeway."
    What I was taught to use for leeway is to look back at the wake and
    estimate the angle the wake makes with the heading. But on long
    passages It's just another complication. Since the errors seem to
    balance out, usually.
    Dan Hogan WA6PBY
    Navigation-L: http://nav.cnchost.com

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