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Subject: Re: Leg 83
From: Arthur Pearson (arthurpearson@XXX.XXX)
Date: Sun Sep 22 2002 - 22:33:13 EDT
Peter et. al.
Many thanks for these exercises, they are a great project now that the
boat is put away for the winter and we have to find another way to keep
the skills fresh.
I am consistently getting 290d True for Q2 below. I am using 1958
Bowditch Traverse tables and I have worked the problem as a Mercator
Sailing (looking up the MP's of the two latitudes to get m, dividing
that into DLo, using that ratio to get a NW bearing) and as a Mid
Latitude Sailing (using DLo at 19d and 20d to interpolate for departure
(p) and dividing by change in latitude (l) and using that ratio to get a
NW bearing. Both methods bring me to 290d as does my PalmPilot running
CelestNav for a Mercator Sailing. In no case can I find my way to 285d.
I concur that the adjustment for current is about -1d and the combined
variation and deviation is -11d, bringing me to a course to steer of
278d.
Where have I strayed?
Arthur
-----Original Message-----
From: Navigation Mailing List
[mailto:NAVIGATION-L@XXX.XXX] On Behalf Of Peter Fogg
Sent: Saturday, September 21, 2002 11:52 PM
To: NAVIGATION-L@XXX.XXX
Subject: Leg 83
Q1) navigator (9ft) to horizon 3.51nm
Mauna Loa (4169m) to horizon +133.66 nm
navigator to top ML on horizon 137.2 nm
Q2) First calculate TC along rhumb line from (N18d 37'.2 W152d 13'.6)
to (N19d 30'.0 W155d 36'.0)
by MP&MD 285.3d
MP 285.3
PS (ML) 285.4
by plot 285d
Then, by plotting, with fast speed and weak contrary current, I get CMG
286d and SMG 9.3k,
thus TC to steer 284d
combined error E -11d
CC to steer 273d
Q3) Time Zone at (W150d +/- 7.5d) is West 10 hours, 10 hours behind UT.
Q4) traverse from N18d 37'.2 W152d 13'.6 at 290d for 140 nm.
MP&MD N19d 25'.3 W154d 32'.5
ML N19d 25'.1 W154d 32'.0
plot N19d 24' W154d 31'.5
elapsed time is 140 nm / SMG 9.3k = 15h 03m 14s
Q5) Am a little unsure what is meant to be done here, assume what is
needed is the CC to steer from
calculated DR (N19d 25'.3 W154d 32'.5) to DO point (N19d 30'.0
W154d 41'.6).
First calculate TC along rhumb line (distance
MP&MD 298.6d 9.8 nm
MP 298.5 9.8
PS(ML) 298.7 9.8
plot 299.0d 12.0 nm - from
plotted DR)
Then another vector diagram, this time with current setting to the
west. TC to steer 300d
combined error E -11d
CC to steer 289d
elapsed time is 9.8 nm / SMG 10.5k = 00h 56m 00s
Q6) rhumbline calculation for distance from (N19d 30'.0 W154d 41'.6)
to (N19d44'.0 W155d 04'.0)
by MP&MD 25.3 nm
MP 25.4
PS (ML) 25.3
plot 25.8
There is no information on current for this leg so shall assume speed is
10k.
elapsed time is 25.3nm / 10k = 02h 31m 48s
Q7) For the ETA at Hilo add the time for each leg to 01:44:36 UT
on 16/07/2001
+ 15:03:14
+ 00:56:00
+ 02:31:48
= 20:15:38 UT on 16/07/2001
= 10:15:38 ZT on 16/07/2001
Notes:
MP&MD Meridional Parts and Meridional Distances
MP Meridional Parts
PS (ML) Plane Sailing (Middle Latitude)
As all the distances involved in these rhumb line and traverse
calculations are fairly short ones in low latitudes there is little
difference between the different calculations.
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