NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Leg 83
From: Arthur Pearson
Date: 2002 Sep 22, 22:33 -0400
From: Arthur Pearson
Date: 2002 Sep 22, 22:33 -0400
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@LISTSERV.WEBKAHUNA.COM] On Behalf Of Peter Fogg Sent: Saturday, September 21, 2002 11:52 PM To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM 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.