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Millard, etal:

Thanks. These are the kind of questions and discussion Silicon Sea was
intended to promote. I also caught an error on my part for my
calculations.

>Nav List,
[Snip]
>        Now to question # 3 Leg 56:
>
>        I may be missing something so feel free to enlighten me.  As I
>look at question # 3  I feel there is not enough data to work the problem.
>Buffeted for two days;  we were buffeted for two days, what was our
>average speed for those two days?  It must have dropped and would have
>been recorded on an hourly basis in keeping  the log as the rules of DR
>require.  I think there are six of the rules for log entries for  DR.
>Entries are made (1) A DR position will be plotted every hour on the hour,
>(2) A DR position well be plotted at the time of every course changes, (3)
> A DR position will be plotted at every speed changes , etc.

The circumnavigation, as played by Silicon Sea, can only be an
approximation. There is just not enough time to lay out a Leg step-by-
step as one would experience it. Stuff is included for color. Use only
the data given in the problem to get the answer. Don't anticipate trick
questions, I make enough mistakes to meliminate that. Other than my
errors, the data required to answer the question is provided with each
Leg.

>Starting at 11:50:00 UTC on the 20/03/1999 at position 42d 39.8'S
>98d 51.5'W, var 30d E dev 3d W and traveling at a speed of 10.3 Kts. we are to
>calculate the DR  for a time of 18:48:23 ZT @ 23/03/1999.   By my
>calculation for the time it is 3 days 13 hrs 58 min 23 sec or 85hr 59min
>23 sec.  When I calculate the DR from these parameters I get a DR of  55d
>38.7'S   88d 04.2' W.
>
> That is the reason I asked in a previous post what did buffet
> mean, our speed must have dropped considerable, and would have given us
>a much lower average speed for the 895.8 nm miles traveled in those three days, the
>distance from our starting point to the DR.  To travel the 895.8 nm, by my
>calculations the speed would still around 10.3 kts, and that gives me the
>DR of 55d 38.7' S   88d 04.2'  W.
>
>I could be way off on my thinking, but I can't get to the DR
>posted by the answers for question # 3 from the data presented.
>What am I missing????

Calculate the elapsed tome from start DR to get the position. Note that
no other information is given, Being buffeted for two days, means, more
than likely ANY DR estimate is going to be a guess. So you use only the
data you can rely on. Time/speed and distance. Which is why I prefer to
DR using a distance log rather than boat speed.

Below are my calculations for Q3 using the DATA of Leg 56 and
Navig94/Navpo. AFTER I corrected my error on calculating ZT at the
departure point:

 DR - SELECTED ZT NAVEX SYSTEMS (c)
-- ZT --  ------- Positions --------    T/i/m   TC  Var  Dev CC   S
04 50 00  L 42d49.9'S  Lo  98d51.5'W   Depart   124 30E  3W  097  10.3
ZT next-
04 00 00    44d40.3'S      95d05.7'W
04 50 00    45d08.1'S      94d07.6'W
24 00 00    46d58.5'S      90d12.5'W
04 50 00    47d26.4'S      89d12.0'W
24 00 00    49d16.8'S      85d06.3'W
18 48 23    51d05.1'S      80d56.2'W

We all are headed for Cape Horn, aren't we?

Dan Hogan WA6PBY
dhhogan@nav.cnchost.com
Navigation-L: http://nav.cnchost.com

   

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