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    Re: Leg 56
    From: Dan Hogan
    Date: 1999 Jul 15, 6:24 PM

    Ed, etal:
    
    I use Navig94 to verify and set up the problems. I also double check
    using a programmable calculator with Mercator Sailing for the DR when
    checking and doing what-if's.
    
    When using plotting sheets and manual calculation for DR your answers
    can vary up to 30 minutes of arc. But practice and care will get it
    down to less than 4 minutes, but you need to interpolate carefully. The
    intent of Plotting Sheets is to provide plotting for an area of 2
    degrees. Trying to use it for more REALLY increases the error factor.
    
    Don't forget DR is time and distance applied to a Sailings calculation.
    Each method has it's limitations due to the construction of the
    formulae used with each method. Each method has its limitations in
    accuracy:
    
            Plane Sailing about 50 miles.
    
            Mid-Latitude Sailing about 600 miles. Probable to 1200 miles but
        errors creep in. Need to calculate to more than 4 places in 90d and
        270d direction.
    
            Mercator Sailing about 2000+ miles. But within the navigation
        quadrant you are sailing in. If you cross a quadrant it takes the
        LONG way 'round.
    
            Rhumbline Sailing the greatest distance, most accurate for DR. But
        a constant rhumbline will take you to the pole.
    
    
    Estimated position (EP) is the DR calculation with the current affect
    accounted for.
    
    
    >> My problem with these multi-day legs is that I've always worked out
    >> shorter leg DR problems graphically on a chart.  I can now easily
    >> generate charts for anywhere in the world thanks to the chart plotting
    >> software this group has led me to, but the scale isn't large enough to
    >> work out the solution graphically.  Are you using some sort of distance
    >> formula or a table somewhere (or a Nav. calculator) to arrive at the DR
    >> position.  I've only done day-long coastal cruises but I always figured
    >> you would still work out DR positions over a shorter elapsed time, if
    >> for no other reason then that the wind conditions would vary enough to
    >> give different cruising speeds and even headings.
    
    In practice you break down your distance's to what ever method you are
    using. And start a new leg as you near you maximum distance. However,
    if I did that with every Silicon Sea Leg, my 3 yearold grandson would
    finish this trip. If you have the Silicon Sea archive, take a look at
    the first 20 legs out of Mallorca to see what I mean.
    
    >> I'm not complaining, mind you, just that I could use some help in
    >> working the long-leg DR problems.  What's the trick?
    
    You need to calculate using one of the Sailings. Either longhand, or
    with trig tables(UGH!), or use a calculator or computer program.
    
    >I'd like to know that too.  I sometimes do it graphically
    >on a makeshift chart, and sometimes mathematically using
    >crude trig, but afterwards, I have to skip ahead to the answers
    >and use the official DR solution to finish the problems.
    
    As the old story goes. After plotting a FIX the Midshipman placed a
    cross with a sharp N0.3 pencil on the chart, and said, "We are here."
    The first mate placed a finger over the cross and said, "We are about
    here." The Captain came in, slapped a big beefy hand down on the chart
    and said, "We are somewhere around here."
    
    A WWII merchant marine Captain that tutored me used Mid-Latitude
    Sailing for his dead reckoning, H.O. 211, Ageton for sight reductions.
    All pertinent data was kept in a notebook and handmade plotting sheets.
    He had to pay for his own charts and navigation equipment so he seldom
    marked up the charts.
    
    Dan Hogan WA6PBY
    dhhogan{at}nav.cnchost.com
    http://nav.cnchost.com
    Catalina 27 "GACHA"
    

       
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