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» Online Classroom   »   » Public Discussion of Cel Nav   » Hawaii by Sextant: logbook questions

Author Topic: Hawaii by Sextant: logbook questions
David Burch

 - posted April 01, 2016 12:15 PM      Profile for David Burch           Edit/Delete Post 
This post is a transfer of a couple valuable notes from Rafal Orlik about HBS. As we proceed we have asked all readers to use this forum for discussion. Answers marked ====

(1) “I have started my trip to Hawaii. As at the beginning the structure of log book is explained I tried to calculate ship's speed again. I think I found some errors. These are marked in the attached PDF file. I think that log in the book was refined and maybe some points that were in the original one are missing.”

=== I took a look at that, and there are indeed some 5 or 6 average speeds listed that differ by a tenth or so from what one would get from speed time distance computations between log entries.

It is easy to see how this could happen. The “logbook” entries in the book are not from the actual deck log filled out every day. At the time the book was printed, we did not have that document. As it turns out, we have since found it, and indeed the logbook was entered about every hour, every day. So our goal is to eventually type up that log and we should then get better DR to fix data for the final analysis.

However, this does not affect the main results of the book. The logbook values shown were taken from another notebook that was used for the individual sight session analyses. On each of these pages there was also recorded a DR position, time, and average speed. How these averages were determined back in 82 I do not recall. They could have been log to log computations or maybe some attempt to improve on that knowing we need this only during a running fix. Also most, not all, were marked average speed (i.e. an S with a bar on top) but some were just S. So it is not surprising that they do not all agree with the log to log computations. In fact, it is rather more surprising that so many do agree. We

(2) “There is also a gap in the numbering, see page #3 of the PDF. In the book the sequence is '61, 63, 64, ...' whereas it should be '61, 62, 63, ...'. That is why I suspect that some points were intentionally removed.”

=== That column was just for a way to refer to an entry it is not used, and specifically says not related to navigation. That one is missing was a surprise to all of us here. That must have been a blunder in copying from excel i would guess. It does not mean anything was intentionally removed.

(3) “And the last thing. On page #4 of the PDF it is explained how to read log book. But the numbers given in this table do not correspond to the whole log book. Specifically, there is not entry for July, 8th, 08:00 PDT.”

==== Thanks for pointing this out. We need to apologize for this. It is intended to explain how the log book works, but unfortunately it does not use examples from the actual log book. It really should say “a logbook” not “the logbook.” We just missed this in the proofing. I am not sure when that logbook explanation was made. It looks like it was a section of the real book, then edited to be more general for purposes of explaining how it works The main point here is that was not a section of the actual logbook. Maybe we can find a more general section of the actual log to put there.

(4) “If these speed's errors are in fact true, then one can get incorrect results when transferring LOP to one chosen moment of time.” ==== these are not real values, so they do not affect the navigation.

=== thanks again for bringing up these points. We will address the earlier posts very shortly.

From: Starpath, Seattle, WA

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