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Hi Renee

I remember you popping up some months ago after Silicon Sea Leg 88 was
posted, and am glad you've worked this problem through.
I am not an expert, but happy to try to help.

> + For a leg of over 1600 miles across open ocean, does a skipper
>     prefer a rhumb line course?  Why?  because it is easier and less
>     prone to error?

You have posted both rhumb line and great circle solutions, both are
accurate, so that's fine.
You're right that a rhumb line course is easier to follow. The difference in
distance here is fairly small. Generally, courses to the north or south show
little difference (since lines of longitiude are great circles) and courses
to the east or west, particularly at high latitudes (the equator is also a
great circle) show the most difference. But shortening the course then
involves even higher latitudes which may not be practical. A small sailing
boat can seldom enough follow its desired course for an extended period
which often becomes the bottom line.

 In the northern summer I flew to Paris from Hong Kong. First we followed
the flight path to Beijing (but didn't land). A great circle route from Hong
Kong, while shorter, would have taken us over Lop Nor, which is where the
Chinese test their nuclear devices (and whatever else they get up to there)
so it seems even planes have considerations other than navigational to take
into account when planning routes. Fom above Beijing we took the great
circle route to Paris, over the Altai mountains and further north into
Siberia before heading south again, over the Baltic states. Dawn began with
a rosy light on the tips of the Altai Mountains and it was still early
morning many hours later as we approached Paris, as we were flying with the
apparent  movement of the sun.

> + Why does the navigator prefer to work with DR rather than EP?
>     (easier?  less error? not important unless current gets stronger?)

This is really a question for the author to answer. All I can say is that
the Silicon Sea series uses DR positions established by courses and
distances only, except for when calculating compass course to steer. I also
found this confusing to begin with. In the real world I can't imagine a
navigator having information on the current and leeway and not taking them
into account to give a more accurate Estimated Position. The basis of
traditional navigation is this process of running forward a DR or EP,
calculating postion according to all the factors affecting it. Position
lines, or other methods when available, are ways of checking the calculated
position.

> + I figured the Rhumb Line course from DR #1 towards MOP2 should
>     be a course made good of 78.5 T.  If I attempt to compensate for
>     current, that gives me a course to sail of 79.3 T, which agrees well
>     enough with the 79d40.3'T sailings implied by the points given
>     (DR #1 and DR #2) (bearing in mind that the navigator and I likely
>     used different methods for DR work, as he likely had an
>     appropriate chart, and I felt to lazy to construct a sheet of graph
>     paper for the appropriate latitude).

Don't be lazy! Plotting helps to visualize what is happening, is much more
informative than just the numbers. DR movements can be plotted accurately
enough for practical purposes (calculations can check this) and the same
plotting sheet (you don't need graph paper) can probably be used for
plotting the position lines and fix.

> 4) Compass Course is calculated in each case.
>     I presume it is also a good idea to correct for current.
>     Rhumb Line: CC 69.5d (ignoring current) or 68d (correcting for
current)
>     Great Circle: CC 57d (ignoring current) or 55d (correcting for
current)
>
>        Which did the navigator choose?

What you need is a simple vector diagram to plot the difference the current
makes, then convert true to compass course.

>  Peter -- I disagree on the solution to part 5
>   You can't set
> the Intercept to 0.

My calculated intercept via one method, capable of resolving this to the
nearest minute of arc, was zero.
I've now checked this with another method (don't be lazy myself!) and agree
with your answer. Its so easy to make mistakes within the process and so
difficult, sometimes, to find them yourself.

> Peter Fogg seemed to have completed the exercise without
> any difficulties, but I found myself continuously mystified,
> I suppose that, by the time I work all 87 of the 88 legs through,
> I'll understand more about the habits of (at least) one navigator,
> and won't be quite so mystified at every turn.

Practice certainly helps, and the Silicon Sea series is a great resource. A
magazine called 'Ocean Navigator' also includes a nav problem in each issue.
If you can get hold of old copies (going back about 25 years) the problems
were much more varied and more interesting. These days each problem seems to
be yet another noon sun sight, yet the problems often contain errors, which
really make you triple check the process and question yourself, so are
possibly an unexpected bonus. Recent problems are available online at
http://www.oceannavigator.com/site/csrv/bType.asp?v=7

If you have access to a sextant, but not a boat, you can always take sights
and reduce them from a convenient coastal headland. With a shallow pan of
some reflective fluid you can make observations almost anywhere, at any
time, remembering to halve the sextant altitude (and any error of
observation it includes - an advantage of this method) before applying the
index error.

regards
Peter Fogg


----- Original Message -----
From: "Renee Mattie" 
To: 
Sent: Friday, January 23, 2004 8:11 AM
Subject: Silicon Sea Leg 88 questions


> After some passage of time and much perusing of my copy of
> Bowditch (1966), the NAV-L archives, several websites,
> and the solutions offered by Peter Fogg and Joe
> (for which, thank you both -- they helped me spot my errors)
> I have once again attempted Silicon Sea Leg 88,
> with some success, I think.  Peter -- I disagree on the solution to part 5
> (http://www.i-DEADLINK-com/lists/navigation/0308/0022.html).  You can't set
> the Intercept to 0.
>
> I am a self-taught beginner, and full of questions.
> If you have any expert opinions on how these things are done in practice,
> please post.  Much appreciated, as I am trying to become a navigator,
> which will be a few steps beyond learning how to work these problems.
>
> Though I worked it through to the end, I am left with some questions
> + For a leg of over 1600 miles across open ocean, does a skipper
>     prefer a rhumb line course?  Why?  because it is easier and less
>     prone to error?
> + Why does the navigator prefer to work with DR rather than EP?
>     (easier?  less error? not important unless current gets stronger?)
> + I figured the Rhumb Line course from DR #1 towards MOP2 should
>     be a course made good of 78.5 T.  If I attempt to compensate for
>     current, that gives me a course to sail of 79.3 T, which agrees well
>     enough with the 79d40.3'T sailings implied by the points given
>     (DR #1 and DR #2) (bearing in mind that the navigator and I likely
>     used different methods for DR work, as he likely had an
>     appropriate chart, and I felt to lazy to construct a sheet of graph
>     paper for the appropriate latitude).
> + Did the navigator use the current data during this leg?
>    Or was it used to determine the course to steer between DR #1 and DR
#2?
>    What course did helm actually steer between DR #1 and DR #2, and why?
>
> Thanks to everyone just for being there on the list.
> It's been a fascinating (selective) read so far.
>
> Renee
>
> My answers:
>
> 1) DR #1 39d39.4' N 154d 42.4'W
>
> 2) ZT is 08:30:20
>
> 3)  I begin to get confused. "TC and Dist to MOP2"
>     Since this is a distance of more than 1600 nmi,
>     I presume that the navigator would choose a great
>     circle course, broken into several legs.  Perhaps
>     4 or 5 legs of ~400 nmi, or 7 legs (one per day?)
>     In this case, there is not a single TC to MOP 2.
>     Which implies, perhaps, Rhumb Line sailing.
>
>   78.5d, 1652 nmi via the Rhumb Line
>   67d, 1639 nmi via the Great Circle. (67d for first leg.)
>
> 4) Compass Course is calculated in each case.
>     I presume it is also a good idea to correct for current.
>     Rhumb Line: CC 69.5d (ignoring current) or 68d (correcting for
current)
>     Great Circle: CC 57d (ignoring current) or 55d (correcting for
current)
>
>        Which did the navigator choose?
>
> 5) arrive at DR #2 39d46.1'N 153d54.8'W, at 22:12:34 UT,
>    and take a sun sight
>     For this noon sight, Azimuth is assumed to be 180, dec = 5d06.8'
>
>     Lat = 90-(Ho+Dec) = 40d08.3'.
>     Intercept is 22.8 nmi Away
>
> 6) The unknown body must be Arcturus.  This would be more difficult
>     if I had a "real" almanac to use.
>
> 7) The running fix:
>     The course from DR #1 to DR #2 is 79d40.3'T
>     So the navigator must have chosen rhumb line sailing!
>     Using this course, DR #3 is 39d58.6'N 152d25.9'W
>
>     The sight of Arcturus reduces to Z = 267 Intercept 20.8 Away
>
>     From my plot, RFIX is 40d21.6'N 152d00.4'W
>
>     I also did a quick check by approximating the Arcturus Z as 270,
>     skipping the plotting to come up with an approximate fix of
>     40d22'N 152d05'W, which is approximately 20 nmi from my "best"
>     attempt at a fix.
>
> Peter Fogg seemed to have completed the exercise without
> any difficulties, but I found myself continuously mystified,
> I suppose that, by the time I work all 87 of the 88 legs through,
> I'll understand more about the habits of (at least) one navigator,
> and won't be quite so mystified at every turn.
>
> I have been using Navigate, v 1.5 by Rick Chapman
> (using the default WGS 1984 ellipsoid) on my Palm V
> for DR (though I have also attempted mid-latitutude graphing)
> I get my celestial data for 2002 from the US Naval Observatory
> http://aa.usno.navy.mil/data/docs/celnavtable.html
> which also works Hc and Zn from an assumed position, which
> is wonderful if you don't later decide (when the internet is out of
> reach) that you made a mistake figuring the DR
>

   

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