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Subject: Re: Leg 56
From: Dan Hogan (dhhogan@XXX.XXX)
Date: Thu Jul 15 1999 - 20:24:24 EDT
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@XXX.XXX
http://nav.cnchost.com
Catalina 27 "GACHA"
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