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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@nav.cnchost.com
http://nav.cnchost.com
Catalina 27 "GACHA"

   

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