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I have been assuming that, if I calculate all courses and DRs, I can get the 
"right" answers to the Silicon Sea problems.  I assume that Lat/Long or 
Course should be correct to within about a minute, and times to within a 
couple of minutes.  I assume this because I assume the working group 
calculated their answers, rather than doing the problem graphically.

Are these realistic assumptions?

When I attempt Silicon Sea, Leg 11, I find my ETA at the waypoint is off over 
an hour, and my course to sail is off by a couple of degrees (which is the 
same size as the compass correction).  In real life, I ought to be happy to 
have come so close to my planned course/ETA.  But, since I am preparing a 
book, I want to be sure I understand it properly, so I don't muck anything up 
during production.

I was reviewing the answers to Silicon Sea, Leg 11.  I am stuck on 11-7 and 
11-8. If anyone can enlighten me, I would appreciate it.  I have uploaded a 
worksheet to http://www.offsoundings.info/download/attachments.htm (scroll 
down to today's items)

I figured that, overnight, I should set a course from the evening fix to the 
waypoint, start a new DR plot from the FIX position, and attempt to sail that 
course until the morning fix.  The problems do not list compass courses 
sailed, so I have to assume the vessel's sailings were as planned.

By my calculations, the course from FIX (38d 58.5'N 04d 22.1'E) to WP (38d 
30.3'N 10d 42'E) should be TC=95.4, CC is 93.2.  Answer to 11-7 states it is 
91.2.  Problem 11-3 says that, at 0244, the DR position is 38d 56.2'N 05d 
34.1'E.   I would have calculated this to have been 38d 53.2'N 5d 34.1'E, 
since the problem does not state that the sailings were any different from 
the planned sailings.  This also means that my MNT DR (11-4) is
38d 52.7'N 5d 40.2'E (instead of 38d 56.0'N 5d 40.1'E as in answer to 11-4).

Using the 11-4 answer value for the DR at MNT, the heading to WP becomes 96.2 TC or 94.2 CC

Problem 11-8 asks for the time, from the MNT DR position, to the Waypoint.
Using the 11-4 answer position, I get a distance of 236.95 nmi, for a time of 
31.59 hours.  If I use my own MNT DR, I get 31.55 hours.  These both differ 
by more than an hour from the 11-8 answer of 33h 9m.

Am I missing something?

If you have your notes from working this leg, I would appreciate hearing from 
you.  Should anyone reading the book be able to work the 'exact' answers if 
they use the same calculator?  Or should we leave them guessing if they've 
applied the compass correction correctly, used the wrong starting point, etc.

Thanks

Renee Mattie

I have ignored set and drift information throughout, because they do not 
affect the DR.  The problems do not ask for the EP, which would use any 
available current and/or leeway information.

I have figured course-to-sail and DR locations using a mid-latitude 
calculation in EXCEL (formulas from Andres Ruiz's Marine Navigation page 
(http://www.geocities.com/CapeCanaveral/Runway/3568/marinenavigation.html ) 
and the NavTrig page (http://www.angelfire.com/nt/navtrig/index.html ))

My course-to-sail and DR calculations agree very well with those given in the 
SSEA archive (http://www.offsoundings.info/navfiles/ssea.zip ), until I get 
to 11-7 and 11-8.

I have not done the FIX calculations.  Right now, I am just reviewing the DR.  
My calculation of TC and distance from FIX to waypoint agrees with answer 
11-2.


----------------------------------------------------------------------------
The relevant parts of leg 11 are:
1) [material deleted] What is our Fix?
With our Fix at hand, we plot our new course to our point at the Strait of Sicily(38 30.3N 10 42.0'E)
2)  What is the TC and Dist to the Strait Of Sicily?
The wind has picked up and is now blowing at 15 Knots from 285d. We are on a 
downwind run. Our speed has increased to 7.5 Kts. At 02-44-30 we are at 
DR(38d 56.2'N  5d 34.1'E)
3)  Using the current DR at what ZT does MNT occur ?

4)  How many more miles will we have traveled at this MNT? What is this DR?
5)  What is the brightest body available at MNT? What is its Hs and Zn?
6)  What is the angle of one Point on the compass?
The local variation is 2dW. Our compass Deviation is 4dE.
Current Drift is 0.5Kts, set 225.0d.
7) What is the procedure for going from a TC to Compass Course(CC) called?
What is the acronym for this procedure?
What is the Compass Course(CC) to the Strait of Sicily?
8) How many hours will it be until our arrival at the Strait of Sicily?
---------------------
Answers:
1) Fix= 38d 58.5'N 04d 22.1'E  D Off = 4.6 @ 157.4
2) TC= 95.4ø  Dist= 298.9 mi
3) MNT= 03-22-00
4) Dist= 4.7 DR= 38d 56.0'N  5d 40.1'E
5) Moon   Hs= 24d 53.9'  Zn= 124.7d
6) 1 Point= 11.25d
7) CC= 91.2   Called= "uncorrecting"  Acronym= TVMDC
8) Time= 33h 09.1m

   

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