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Renee

Can't find upload at 'offsoundings' site.

> 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.

Be aware that for calculations of rhumb line courses the answers will be 
dependent on the method used, of which there are a few: plane sailing, 
mid-latitude, Mercator (a graphical method) and so on. Here is the link to 
the relevant chapter in Bowditch:
http://www.i-DEADLINK-com/bowditch/pdf/chapt22.pdf
There are also lots of other useful formulae there.

The method I favour uses tables of Meridional Parts and Meridional Distances 
and is supposed to be the most accurate. Note that great accuracy is a purely 
academic exercise as in real life there would be too many imponderables to 
cloud the issue.

> I assume the working group
> calculated their answers, rather than doing the problem graphically.

To the best of my recollection Dan Hogan calculated all his data, but he 
should be the best authority on that.

> 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.

11-7.
Q. What is Compass Course? My answer: 096.1 True; 094.1C
Official answer: 91.2

Q What is procedure?
My answer: Working from True to Compass; add West corrections and subtract 
East corrections. The opposite when going from Compass to True.
Official answer: Uncorrecting

Acronym: True Virgins Make Dull Company
Official answer: TVMDC

11.8
From a calculated distance of 237.6nm:
Q. How many hours?
My answer: 31.68 hours, or 32 hours (to the nearest hour)
Official answer: 33h 09.1m

In other words, close to your calcs.

This comes from notes from a while ago, when these legs were worked out. Later 
this month I may be able to find the time to recalculate legs.

Hope this helps


> -----Original Message-----
> From: Navigation Mailing List [mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]
> On Behalf Of Renee Mattie
> Sent: Thursday, 15 June 2006 4:27 AM
> To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
> Subject: Silicon Sea, Leg 11 question
>
> 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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