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Re: Silicon Sea: Leg 71
From: Dan Hogan
Date: 2001 May 15, 09:59 EDT
From: Dan Hogan
Date: 2001 May 15, 09:59 EDT
On 14 May 2001, at 21:52, Paul Hirose wrote: > 1. DR position at log reading 89.6 is 13 17 N 63 02 W. OK > The problem doesn't say when the log was zeroed. I assume that happened > when departure was taken. > > There's enough info to compute the estimated position (allowing for > current) too: 13 16 N 63 09 W. I don't make trick questions(intentionally), so no need to answer more than what the question asks for. ) > 2. Lacking an almanac, I don't know what celestial phenomenon is > occurring at the DR time. Could probably figure it out from the USNO Web > site, but that's too easy. Well an Almanac or Almanac program would give you the answers for Sunrise, Sunset, twilight and time of LAN problems. > 3. Fix May 30 0852 Z is 13 02 N 61 29 W. This is 24 miles south of the > departure position, yet we were supposed to have sailed 90 miles west! > I'm puzzled that two other people got results substantially identical to > mine but didn't comment on this gross discrepancy in the DR. Your latitude is off by a degree. I checked my data and it looks ok. > The sights are so close in time I decided to ignore vessel's movement > and just use the mid time of the sights as the fix time. LOP > intersections were a bit sloppy, probably due to the long intercepts. > Given the realities of small craft celestial, I didn't think it > worthwhile to re-reduce from a better assumed position. > > > NOTE: Sextant Index correction is on the arc 1.5d. Right, it should read on the arc 1.5'. > > I got confused over the above. Of course common sense tells me it's got > to be 1.5 minutes, not degrees. But it's not clear what sign to use for > the correction when it's described that way. A menomic to use is "On the arc off(-), off the arc on(+)." > For a more realistic problem, don't tell us the IC. See if we know how > to get it. E.g., "Just before the round of sights, the images of the > horizon are brought to coincidence to determine index correction. The > sextant reads 1.5'." > > > 4. Aruba Gap is 502 miles and 275 true from the fix. OK > 5. Compass course to Aruba Gap is 286. OK > 6. Time enroute will be 49.5 hours. OK > 7. Speed made good to Aruba Gap should be 10.1 knots. OK > > Used the USNO celestial nav Web page to get celestial body coordinates, > but performed sight reductions with HO 229. Did all plotting on pieces > of typing paper. Time/speed/distance computations performed on an E-6B > aviation "whiz wheel". Explains your plotting. I still have my original plastic Weems E-6B and a metal "Aerodyne" issued by the company. In practical use I only interpolated to the nearest 0.5 Degree. Dan Hogan WA6PBY Catalina 27 "GACHA" dhhogan@XXX.XXX Navigation-L List Owner http://www.wa6pby.com