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    Re: True vs Magnetic
    From: Lu Abel
    Date: 1999 Jul 18, 11:19 AM

    This sounds like it could be the beginning of an interesting thread.  Never
    being afraid to express an opinon, I'll start it.
    As mentioned in a previous post, I've taught USPS coastal piloting courses
    for 15 years.  The Power Squadron is purely "always plot and calculate in
    true, then go through TVMDC"
    While I absolutely agree with the need to do this offshore where one might
    be going through changes in variation, everything I do for coastal piloting
    is directly in Magnetic.
    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
    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.
    Even the Power Squadron sort of acknowledges the advantages of marking
    courses in Magnetic.  After teaching everything in True, they note in an
    appendix to their text that sometimes when navigating in a small coastal
    area that marking courses in Magnetic may be more convenient.
    As a side note, my favorite course-plotting tool is the Pocket Instant
    Navigator.  The PIN consists of two plastic disks and a pointer all riveted
    together so they can independently rotate.  The base disk has a 1/4" square
    grid of N/S and E/W lines imprinted on it as well as a 0-360 degrees marked
    around the periphery.  One drops the PIN over the starting point for a
    course (the axle rivet is even hollow so one can see the point through the
    PIN), aligns the base disk N/S by aligning the grid with the meridians and
    parallels on the chart, spins the pointer to the destination (or along the
    course line) and reads the course.
    But wait, there's more!  In normal practice, one takes the second disk
    (also imprinted 0-360 degrees, but with no grid), rotates it to the value
    of variation, and then when one reads a course as described above one also
    gets the magnetic course.  (Since I do most of my sailing in an area of
    constant variation, I lock the two disks in position with a bit of Scotch
    Tape).  In fact this "magnetic" disk has its degree scale outside the base
    or "true" disk's, so the most obvious number under the pointer is the
    Magnetic course.
    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."
    At 07:39 AM 7/18/99 -0800, you wrote:
    >I have seen a few small boat sailors and fishermen plot using Magnetic.
    >But since day one I was taught to plot on charts or plotting sheets
    >only true course. Then to do TVMDC for magnetic/CC. The reason being
    >that variation will vary with your position and deviation with your
    >>He was working with True Course and I
    >>had taken the variation from the charts and converted my course  to
    >>magnetic.  When I asked him did he consider the variation, he realized we
    >>had the same course only one was true and the other magnetic.  We set the
    >>auto helm to the course  corresponding to the compass course, and to my
    >>amazement Block Island, RI showed up a day and three quarter later.  We
    >>never touched the helm until the entry into the Great Salt Pond at New
    >>Harbor, Block Island, RI.
    >Dan Hogan WA6PBY
    >Catalina 27 "GACHA"

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