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Old 10-06-2010, 15:52   #31
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I use pen, paper and slide rule or printed logarithm tables and trig functions when working a sextant to find my position. You can even use a sextant to find distance if you have a landmark of known height in sight.
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"slide rules...printed logarithim tables and trig functions". My brain hurts thinking about this stuff. I quess I'll have to re-learn all this but in the meantime, I'll get by on one of my three GPS systems.
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Old 10-06-2010, 16:00   #32
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Now there is an idea. At noon you use the spinnaker pole to measure the shadow of the mast (a known height) across the water which gives you the angle of the sun.

Only problem I see is the mast might move a bit at sea. So how about a sun dial mounted on top of one of those self leveling radar scanners instead

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Old 10-06-2010, 18:36   #33
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OK...Hypothetical question.

We are all aboard a boat headed for Hawaii....we left San diego on a heading to intercept our fist way point somewhere 100 miles at sea to or first turn picking up the trade winds ( Or what ever winds they are)

After the turn onto your rhumb line we loose GPS for 7 days...We know the basic current pattern...we know our course heading, we know the deviation, the wind direction and speed, we know our speed through the water.

How far off in 7 days can we really get?

If we never get GPS back are we at a huge risk of missing the islands completely? Next stop main land China?
I sailed for years like this. Every day I would think: 5 kn most of the afternoon, then it died for an hour or two, a burst into the 7's during that squall then 5 kn again so let's say 110nm. I was seldom out by more than 5 miles. So after 7 days 35 miles. Round it up to 50 for safety and no problem finding the Big Island (it's very high). Even if you can't see it you should notice the change in the shape of the waves, maybe even the boat will pitch a little as it feels reflections of the swell, the breeze will probably ease and shift a bit, there will be a stationary cloud at the island, the colour of the watrer may change, you will notice more weed and flotsam, and easiest of all at around 50 miles you start to see land based birds whicj=h will fly directly towards land around sunset.
Easy peasy.
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Old 10-06-2010, 18:39   #34
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And if you know your stars you can find one that is at the same latitude at HI and sail under it till you get there -latitude sailing.
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Old 10-06-2010, 19:34   #35
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The art of running a good DR may be dying with electronics that provide an instant fix.

With a good DR and an occasional Noon sight, you cannot miss Hawaii.
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Old 10-06-2010, 20:04   #36
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So a sextant is not a necessity? Stars and DR will get you there?
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Old 10-06-2010, 20:28   #37
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So how about a sun dial mounted on top of one of those self leveling radar scanners instead
Pete
Have you tried a sun dial in the tropics?
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Old 10-06-2010, 22:07   #38
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Knowing how to DR should be absolutely necessary in my opinion. If you can't take a noon sight for days, and that's a real possibility at sea in bad weather or fog, then one has to compensate for drift due to wind and current. One may or may not be right on the money, but with a little care careful compass use and plotting, one can stay in the ball park at least until you can get a decent noon sight and establish your position. Before GPS and RDF, that is simply how it was done, and men like Cooke and Bligh could fetch up almost exactly where they wanted to be. Now, that said, I will admit that I do rely a lot on GPS if sailing close to land, but at the same time, it does not hurt a bit to have old-fashioned, but tried and true skills to fall back on in case one's electronic marvels suddenly goes out on you.
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Old 10-06-2010, 22:11   #39
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So a sextant is not a necessity? Stars and DR will get you there?

Go to your library and look for "We, The Navigators" by David Lewis, he tells you how it is done.
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Old 10-06-2010, 22:14   #40
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Will do thanks...
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Old 10-06-2010, 22:26   #41
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Can you see the horizon in a thunderstorm?
It's easy to see the horizon in many thunderstorms. Thunderstorms create very localized disturbances in the afternoons in the tropics. You see the big cumulonimbus clouds going up to 18,000 feet, but they are only a couple of miles across. We dodged thunderstorms all the time in the Bahamas and in the tropics/Caribbean.

When I am flying up to the Apache Indian reservation, I sometimes have a thunderstorm both on the right and left sides of the aircraft. We fly around these localized disturbances, because if you get in the downdrafts, the 70 mph winds might rip your wings off. Very violent and scary stuff. From 10,000 feet, I watch lightning strikes on the ground. It's interesting to see lightning at eye level and below you. Those thunder storms are easy to fly around. You just fly a few miles out of the way to avoid them.

When we sailed, we deviated our course 90 degrees in the presence of a thunderstorm, and that kept us out of the worst part of the danger zone.

Tropical thunderstorms that are localized disturbances won't prevent you from taking a sun sight unless the thunderstorm is between you and the sun.
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Old 16-06-2010, 10:18   #42
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Have you seen the cost of a decent sextant!!!! I was rather amazed. Glad someone gave me one.
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Old 16-06-2010, 11:29   #43
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I use a plain (alloy?) sextant made in former East Germany and plain calculation formulas. Calculator does help. I want to go for logarithms now because they will let me drop the calculator. A second sextant could be a good thing too, just in case the real thing suffers a bad accident.

The NASA must justify their budget somehow. They will keep on acting just like the WHO acted during the fake pig flue pandemic.

In any case, I will not be surprised when, not if, one day something wipes out part of our oh-so-very-reliable technology.

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Old 16-06-2010, 11:44   #44
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This is a great thread. I always learn from people that have been there and done that. I will look forward to Hawaii on my DR/sextant ways. I had almost given up on DR but you guys have lifted this wacky old man's spirits...
I wonder if there should be a different circumnav category for those that do it without electronics....
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Old 16-06-2010, 12:12   #45
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Go to your library and look for "We, The Navigators" by David Lewis, he tells you how it is done.
I second that... We, the Navigators, The Ancient Art of Landfinding in the Pacific - David Lewis - ISBN 9780824815820

Lewis was an amazing guy and a true sailor who understood that navigation in the old days without GPS was also an art borne of generations of observations

David Henry Lewis - Wikipedia, the free encyclopedia

Today's quick fix society may need to re-learn some old tricks if what they say could happen...does
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