Here’s my idea:
Davis Mark3
3 cheap casio watches ($15 each) plus a Molinja pocket watch ($35 or so Ebay)
G.Kolbe Long Term Almanac.
Moon almanac printed on waterproof paper for several years to come including laminated correction tables.
Rite in the Rain notebook
Plotting sheet printed both sides and laminated
Two sun, 1 moon and 3 star worksheets printed together front and back on a
single sheet and laminated.
Grease pencils and/or erasable markers in fine point.
A
steel cash box to hold the watches, a spare GPS, a small
solar battery charger, some NiMH
batteries to fit the GPS and some alkaline
batteries.
Total cost about $200, excluding the spare GPS
For a week prior to voyage the pocket watch is wound daily at the same time and rate of error are established. The quartz watches are also checked These errors are tracked in the Rite in the Rain. The Casios are NEVER reset, except just after the finish of a voyage where there will be time to recheck the error rate before another voyage starts.
Underway everything is pulled out once a week and a
single sight is shot (not a noon sight) and the GPS
MOB button is pushed at the same time. The sight is worked and plotted. After the fact the GPS coordinates are retrieved and plotted to give feedback on how close the LOP is to the
MOB. Error for each of the timepieces is checked and logged in the notebook.
So then the unpleasantness occurs.
In the worst case the Casios are fried and the Molija was let to run down and time is
lost.
Wind them and set them to the same time making a best guess as to what the time might be. To regain at least semi-accurate time shoot the moon and a few other bodies. The moon needs to be significantly east or west of your position for this to
work well (let’s say 30degrees east or west of true north or true south). You should get some vague agreement among the misc. bodies with the moon line even further out. Add or subtract a set amount of time to all of the sights and recalculate. The
rule of thumb is 2 minutes of time for each minute of longitude disagreement between the moon and the ‘fix’ for the other bodies. After the first 2 cycles you should be able to interpolate or extrapolate. If the moon line is to the west add your correction, if east subtract the correction. When the moon agrees with the rest of the fix you have figured out how big the error was for your best guess of time. You have re-established time. Would probably be good to immediately or the next day reshoot a round including the moon and double check the time correction. Thereafter recheck weekly. In a
lightning strike I would worry that the high magnetic fields might affect the watches rate of error so that should be tracked over time.
This method of regaining time is not of my own devising, it is from John Letcher’s book on navigating with HO-208, a good read all around though I did not come to adore 208 the way he did.
Here's a free book you can download:
http://www.teacupnavigation.net/Cele...a_Teacup_u.pdf
which was found at
Celestial Navigation
This uses a calculator for the sight reduction, not appropriate if you want to learn to use the tables for sight reduction.
Has a good chapter at the end about how to use a sextant to determine your circle of position by measuring the horizontal angle between two landmarks.
The Mark 3 is not Davis's least accurate sextant. I read a reasoned argument at
Strait of Magellan: Celestial Navigation 101 indicating that the Mark 3 is more accurate than the 15 or 25 even new out of the box.
Why go for the cheaper resource for the ditch kit? Several reasons I can see.
A) You don't need great accuracy in a lifeboat except in a very few locations such as the
south pacific where the only islands you can hit for thousands of miles are very small. Most places you will have a fairly large target to hit so 15nm either way would be acceptable. Greater accuracy would require greater size and weight in the ditch kit.
B) Minimizing the cost of this item and its ancillary
equipment allows you to spend more
money on other
safety gear to stow in the lifeboat or ditch bag. All told you would need $150 or so for the whole setup including sextant, long term almanac with reduction tables, 3 watches and a good case. That's about the cost of a cheap GPS.
C) The Mark 3 being dirt simple also means there is a lot less to go wrong.
D) The vernier reading helps keep you from developing a false sense of precision about your results. A pity more people can't keep this idea in mind when using the results of their GPS.