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View Poll Results: Blue Water- is a Sextant Necessary?
Absolutely essential 24 18.90%
Desirable, but not essential 52 40.94%
Good fun, but little practical use these days 39 30.71%
Don't waste your money and time on this 11 8.66%
Sextants make excellent dingy anchors. 3 2.36%
Multiple Choice Poll. Voters: 127. You may not vote on this poll

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Old 02-11-2012, 14:53   #106
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Re: Poll-Blue water. Is a Sextant Necessary?

[QUOTE=Vasco;1074565]Well, at this stage of the discussion, it's time to bring on Marvin Creamer.
Marvin Creamer - Wikipedia, the free encyclopedia

Didn't know about Marvin. Just goes to show mod-cons like sextants and charts are only necessary for those who can't read waves.

I love sextants, we would have one guy on the lead-line, two on sextants as we puddled around in our dinghy plotting updates for our port's navigation charts. A great day out and we got paid for it!
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Old 03-11-2012, 01:17   #107
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Re: Poll-Blue water. Is a Sextant Necessary?

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Originally Posted by DumnMad View Post
I love sextants, we would have one guy on the lead-line, two on sextants as we puddled around in our dinghy plotting updates for our port's navigation charts. A great day out and we got paid for it!
People that have not used celestial navigation very much often have a very false idea of the accuracy and frequency of position fixing using this method.
Quotes like this can perpetuate this myth to those that have not used the technique.
Perhaps explaining how you were using the two sextants ( for celestial or measuring angles and heights) together with other tools like a HB compass would help clarify things.
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Old 03-11-2012, 02:10   #108
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Re: Poll-Blue water. Is a Sextant Necessary?

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Originally Posted by noelex 77 View Post
People that have not used celestial navigation very much often have a very false idea of the accuracy and frequency of position fixing using this method.
Quotes like this can perpetuate this myth to those that have not used the technique.
Perhaps explaining how you were using the two sextants ( for celestial or measuring angles and heights) together with other tools like a HB compass would help clarify things.
I must be reading a different post from you, noelex 77

I don't see DumnMad saying anything whatsoever about celestial. Or anything in his/her post to suggest lack of talent for position fixing (specifically, surveying)

Theodolites can be used for celestial navigation (at night, or with suitable shades, and much caution), and conversely sextants have always been (and still are) used for surveying.

And pilotage.

... no heavenly bodies required (luckily for those of us not in possession of one)
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Old 03-11-2012, 02:21   #109
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Re: Poll-Blue water. Is a Sextant Necessary?

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Originally Posted by Ex-Calif View Post

The sextant in the lifeboat scenario is a red herring because no experienced skipper has ever said getting a shot from a pitcjing dinghy is a likely event.......
I was going to be a smartarse and mention Frank Worsley (Shackleton's Captain) - but what he did was pretty unlikely.

However it has to be said that dinghies don't pitch excessively in a long swell, more of a problem in waves. Prolonged calms will result in disappearance of waves.

An experienced user with unlimited time on their hands away from equatorial waters can establish a reasonable latitude on good days. No tables required.
Once you reach the latitude of a known island, turn east or west and sail that compass course.

Latitude sailing is a lot better than nothing, if no-one's coming to get you, at least it fills in time !
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Old 03-11-2012, 02:32   #110
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Re: Poll-Blue water. Is a Sextant Necessary?

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I must be reading a different post from you, noelex 77

I don't see DumnMad saying anything whatsoever about celestial. Or anything in his/her post to suggest lack of talent for position fixing (specifically, surveying)

Theodolites can be used for celestial navigation (at night, or with suitable shades, and much caution), and sextants have always been (and still are) used for surveying.

And pilotage.

... no heavenly bodies required (luckily for those of us not in possession of one)
Yes I think the sextants were being used to measure angles and with known heights distances can be calculated etc rather than for celestial navigation. That is why I asked for the elaboration.
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Originally Posted by noelex 77 View Post
Perhaps explaining how you were using the two sextants ( for celestial or measuring angles and heights) .........
A lot of sailors have the mistaken belief that celestial navigation will provide an instant fix with much greater accuracy than possible. In short they see it as a complete substitute for, and to used like, GPS navigation. It's important to dispel these myths.
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Old 03-11-2012, 02:36   #111
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Re: Poll-Blue water. Is a Sextant Necessary?

Sorry, Noelex, I misread the last sentence in your post. Point taken.

Especially given that the topic title is "Blue Water" - surveying and pilotage could arguably be seen as a cryptic segue ...
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Old 03-11-2012, 03:43   #112
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Re: Poll-Blue water. Is a Sextant Necessary?

Quote:
Originally Posted by noelex 77 View Post
People that have not used celestial navigation very much often have a very false idea of the accuracy and frequency of position fixing using this method.
Quotes like this can perpetuate this myth to those that have not used the technique.
Perhaps explaining how you were using the two sextants ( for celestial or measuring angles and heights) together with other tools like a HB compass would help clarify things.

Last 2 weeks have been teaching our cadet the black art of using a sextant.
Couple of days ago she was thrilled to bits to get a fix using horizontal angles whilst at anchor off Banff. Within a cable of the GPS position, not bad for the first go at HSA's.
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Old 03-11-2012, 04:08   #113
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Re: Poll-Blue water. Is a Sextant Necessary?

Its simple Noelex but takes a while to explain. Very similar process to celestial navigation but a small scale. We had to have two sextants and be quick because unlike celestial surveying we had tidal currents and were moving significantly relative to our fixed survey points. We would simultaneously take a sounding & each sextant measure the angle to known survey points on the harbor perimeter, write them down and later plot them onto a harbor chart. With two adjacent angles measured we had a fix. I've always assumed that Cook used a method similar to this for his chart making. In reality using the dinghy was rare as we normally used the pilot boat. We would choose a nice sunny day and enjoyed the day on the water. Plotting the soundings was a little tedious. As I write this it makes me feel like I'm two hundred years old!! It was less then 40 years ago!
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Old 03-11-2012, 06:19   #114
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Re: Poll-Blue water. Is a Sextant Necessary?

Yep.

I too think the accuracy thing is hardly ever considered. Much of confusion comes from the fact that claimed accy often comes from deck officers of big ships. In a small sailing craft, in rough water (most any water is rough to most sailing craft) accy suffers.

The good news is that if you know where you are to within 5 or 10 miles, you can then make a safe approach by the eye.

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Old 03-11-2012, 09:44   #115
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Re: Poll-Blue water. Is a Sextant Necessary?

Or, if you just happen to have 4000 meters of chain in your raft, then you can anchor almost anywhere. Big raft though
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Old 03-11-2012, 10:33   #116
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Re: Poll-Blue water. Is a Sextant Necessary?

Quote:
Originally Posted by noelex 77 View Post
A lot of sailors have the mistaken belief that celestial navigation will provide an instant fix with much greater accuracy than possible. In short they see it as a complete substitute for, and to used like, GPS navigation. It's important to dispel these myths.
Do people really believe that?

I see them as complementary and mutually reinforcing methods, myself, but I realize that if I'm getting within a mile or two of a known position (say a charted nav aid) with the sextant in a plot, I'm doing very well, indeed.

On passage, however, "sextant-close" is good enough. You need to see how far you've come in a day and in what direction. That can be 135-140 NM and NE, not 137.9 NM and 36 degrees T. when averaged over 24 hours.

Sometimes I think it is the decimal point precision of GPS displays that convey a sort of infalliability and over-reliance on how their information should be integrated into an entire navigational scenario. By this I mean that it is not merely about where the boat is, it is also what is below it, what is nearby and what is directly in front of it. Charts, both paper and electronic, are subject to error. A sextant enforces situation awareness of this because it, too, is a relatively blunt instrument with a relatively wide margin of error. It's a hammer, not a brad nailer. You need to know where you've left your thumb.

So it would be foolish to claim that sextants and GPS are equivalently accurate. It would be equally foolish to state that GPS accuracy is the defining attribute of successful navigation. If you set up a waypoint and motor under AP to it in full confidence that you've input the precise lat/lon., the island chunk in your forepeak you thought was a mile west might wish to debate you.
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Old 03-11-2012, 16:17   #117
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Re: Poll-Blue water. Is a Sextant Necessary?

Decimal point precision and blind reliance on GPS and chart plotter leads to things like the Costa Concordia.
There's the story of the brand new Third mate out of the Merchant Marine Academy when asked to plot his position sharpens the pencil, and puts a small dot on the chart.
The First mate having been at sea for a few years takes the same pencil, draws a cocked hat based on the same round of sights and says "We're in here".
The Captain with 20 years at sea takes his grizzled finger, pokes the chart and says "Somewhere around here is about right".
Unless you're piloting from buoy to buoy, thinking "Somewhere around here" keeps you more alert than "knowing" with in a few meters. Many boats run into trouble cutting hazards too close because they are so certain of their exact position they forget to look out the window and reality reaches up and bites them.
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Old 03-11-2012, 16:20   #118
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Re: Poll-Blue water. Is a Sextant Necessary?

S/V Alchemy makes some good points.
Have attached a picture showing how accurrate GPS is and using AP to steer to a waypoint, especially when the way point happens to be an oil platform
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Old 03-11-2012, 19:34   #119
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Re: Poll-Blue water. Is a Sextant Necessary?

I think Alchemy's point about the problem of GPS positions sometimes being falsely suggestive of pinpoint accuracy is a valid one.

As an analogy: It's hard to keep it in the forefront of your mind that the answer to "what diameter is the inlet hole" originated from someone who reckoned it looked to be about 5/8", when it comes back as "15.875mm"

The same happens when doing "broad brush" design using a computer. Most CAD packages do not allow you to quickly flash up a rough sketch: the result generally looks 'ready to build', and is geometrically perfect.... unlike a hand sketch with wiggly lines done with a nice blunt, soft pencil. This can be a problem EVEN to the person who drew it, say if they come back to it at some later time, and mistake it for a portrayal of reality.

Navigation is all about trapping errors, and in difficult circumstances, counsels of perfection, on ready offer on forums (along with from-the-hip comments along the lines of "you'd have to be an idiot to fall into that trap"), are actually not helpful.

I reckon sailronin also makes an excellent point. I used to draw circles on the chart with the tip of my finger when asked where we were. It didn't make me popular, especially if I was paid navigator, but the job description did not mention popularity.

I figured it was better to remind my self that being approximately right beats being exactly wrong, anyday.
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Old 03-11-2012, 20:04   #120
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Re: Poll-Blue water. Is a Sextant Necessary?

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(...) Unless you're piloting from buoy to buoy, thinking "Somewhere around here" keeps you more alert than "knowing" with in a few meters. Many boats run into trouble cutting hazards too close because they are so certain of their exact position they forget to look out the window and reality reaches up and bites them.
+1!

I will just add that buoys may be off their charted position too.

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