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Old 26-02-2011, 01:16   #1
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Sextant Use

We just got a sextant to back up the GPS as we get ready to head out in about 6-8 months.

I'm sure the Astra III B is more accurate than our sightings will be on a 39' sloop, but that's better than the other way around.

While I got a sextant crash course from a sailing professor in the 70s and I have a few of the current sextant books, I'd appreciate some first hand experiences.

First of all, what do you use for a time piece ? A radio signal and stop watch or something else ?

Second, what is your honest assessment of how good your fixes are, lat and long (and what size boat are you on) ? I'm looking for numbers to shoot for with practice, not a competition.

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Old 26-02-2011, 08:49   #2
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Re: Sextant use

I set my Suunto Vector wristwatch to either a radio signal or an Internet source every six months or so. I have to change the battery once a year, so that is also an opportunity to reset it. Frequent checks confirm I may lose one second between resettings...a trivial variance.

I picked the Suunto Vector, which is not a new model, because it was less than $200 in Canada and because it has a decent compass and a barometer in it. Designed for hikers, I have used it extensively on the water. I would not hesitate to get another one. It's waterproof and also has a dim face-illuminating light.

As for fixes, I only know Lake Ontario, although I took a reasonable fix on an Atlantic delivery (about four miles off from GPS reading). I generally get with five miles even in "sloshy" conditions, but if the course is steady, I can get within two miles. I need more practice, however, and I need more star sightings.
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Old 26-02-2011, 10:07   #3
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Re: Sextant use

Any quartz watch will do. Set it from WWV and note the rate of loss checking against WWV. In most instances it will be seconds per month. If you don't reset it regularly, plug in the time correction when you do your shot.

We used a Tamiya without the scope. Kept both eyes open when taking a sight and had no problem getting accurate shots. Read Moitessier to get a more thorough discussion on using the sextant this way. We haven't taken a sight since GPS but always made our landfalls with accuracy I'd estimate within a couple of miles. Need to get the mirrors resilvered and try a few shots and verify against the gps to see just how accurate I am with the sextant.
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Old 26-02-2011, 10:20   #4
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Re: Sextant use

Sven,
I have an old German Freiberger sextant that I practice with and have more out of tradition than backup. One day with little/no wind, we did a series of morning, noon and afternoon sites off the harbor entrance. I was surprised at how close the intercept was to our known position! Granted, we spent the day drifting around, no wind, no swell, but it was good practice!
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Old 26-02-2011, 11:11   #5
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Re: Sextant use

Ditto on a digital watch checked occasionally against the time signal. In my experience, accuracy to within about 5 miles from a small boat. Best results for a small boat were from a sun line an hour or two before noon, a noon sight, and a sun line an hour or two after noon, all advanced to a running fix.

This took several hours' work, and was accurate to within about 5 miles. That's why I haven't done it in the GPS era.
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Old 26-02-2011, 11:31   #6
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Re: Sextant use

There was a recent thread asking about sextant position accuracy, generally the answers fell in the 2-5nm range for consistant real world results.

If I were going offshore right now I would put 2 or 3 Casio G-shocks ($60 or so each) in a metal box with or in the sextant box. Once or twice a week on a regular schedule,I would take them out and check them against an outside time source and keep a record of the errors without resetting them. If I ever lost access to the outside time source, the long term log of time errors would give me a very accurate rate of change for the error of each time piece and an indication of whether the rate was linear.

The metal box is to give the watches the best possible odds of surviving a nearby lightning strike. The box acts as a Faraday cage shunting all stray electrical and magnetic fields around the items within.

At the same time I was checking the reference time pieces I would reset the one I wear and would use it for a sight and LOP at the same time. That way I would stay somewhat in practice in the event I lost GPS reception.
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Old 26-02-2011, 11:41   #7
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Re: Sextant use

Your watch does not need to be exact, you just need to know the error with the actual time. A cheap digital plastic watch on your wrist works good enough.

Three miles with 5 LOP's is a decent fix.
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Old 27-02-2011, 12:30   #8
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Re: Sextant use

All the previous poster's sights have been more accurate than mine. I feel lucky to get within 7 miles on a moderate days practice. The timepiece I use is a cheap Casio that I set to the radio once in awhile. It turns out to be very accurate. I like my Astra IIIB and I also like Davis 25 on a rough day.
I'm certain if I took sights every day I'd do better but I believe I can see land pretty well from 7 miles out so it isn't too bad.
kind regards,
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Old 27-02-2011, 14:56   #9
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Re: Sextant use

FWIW:
When we were relying upon celestial, I used (as others have) an inexpensive quartz digital watch and checked its rate against WWV. Mine had a few seconds a week drift, and could be reset, or just compensated for in the calculations.

Accuracy depended on sea state and just what observations I had made. Running sun fixes depended a lot on how much current was affecting our estimations of distance run between observations. I often used a noon sight coupled with a small program (HP-41 calculator) that did a least squares fit of the observations to a parabola. With that I got fairly accurate fixes for both lat and lon from the one set of observations. But, for accuracy I preferred doing a round of stars, usually at evening twilight. This would usually yield better than 5 mile accuracy.

I enjoyed doing all that, and I'm surely glad that I know how to do it, but I really love my gps!

Cheers,

Jim and Ann s/v Insatiable II lying Morning Cove, NSW, Oz
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Old 27-02-2011, 14:57   #10
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Re: Sextant use

Well, from a practical point, once you see land, pilotage takes over...unless, like Colombus, you are really mistaken in some of your assumptions as to where you really are.

I'll tell you something I can usually get very good results with near land is taking bearings with a compass or estimating heights of shore features and doing the angles.

The use of the sextant for this could be very handy if you are dubious about the compass due to sailing over an ore body or something (hey, it happens).
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Old 27-02-2011, 16:56   #11
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Re: Sextant use

Quote:
Originally Posted by SvenG View Post
First of all, what do you use for a time piece ? A radio signal and stop watch or something else ?

Second, what is your honest assessment of how good your fixes are, lat and long (and what size boat are you on) ? I'm looking for numbers to shoot for with practice, not a competition.
Digital watch. 1-5 miles, depending on sea state and concentration. Land is a lot easier.

Remember also the trick of three points where the sighted objects aren't celestial, but terrestrial. That's a fix of a different sort and when you use it in conjunction with a good map you can really get things down.

IOW. Narrow down your location by celestial. Consult map. Shoot angles to three points (sextant parallel to water - consult Bowditch if this is new). Triangulate.

This is a lot more accurate than just going off celestial.
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Old 27-02-2011, 18:25   #12
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Re: Sextant use shooting stars

Using the sight reduction tables for selected stars, rather than try and bag all 6 listed stars, if conditionss were not perfect I go for the 3 brightest and giving the best intersect angle, and then take each of these stars twice. Other tips, evening twilight, go for the brighter stars in the east first, morning twilight, dimmest stars in the west first.

If you have a bright moon, avoid taking sights of stars on similar bearing to the moon, the brightness of the moon somehow distorts the horizon, so you measure an altitude greater than what it should be. ( I was told this by some real crusty sea dog years ago, I believed him)
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Old 27-02-2011, 18:52   #13
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Re: Sextant use

26' boat here, pretty rolly double-ender:

timepiece : Casio,

time ticks : No, we use GPS time, if the GPS packs up, we know where to find the ticks on SSB, we also know how much the Casio advances,

position accuracy : varies, I bet most of the error comes from inaccurate sights, good day - 5 Nm off, bad day - 20 Nm off,

difficulty level : rookie,

boost to the driver's ego : PRICELESS ;-)

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Old 27-02-2011, 21:00   #14
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Re: Sextant use

Thanks for all the great responses.

I'm heartened by how many still know how to use and occasionally do use a sextant. In the age of the GPS it might seem like an anachronism but one good lightning strike is all that lies between a usable GPS and none.

Now I have to become one of those who know how to use one



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Old 27-02-2011, 21:12   #15
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Re: Sextant use

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I'm heartened by how many still know how to use and occasionally do use a sextant. In the age of the GPS it might seem like an anachronism but one good lightning strike is all that lies between a usable GPS and none.
Well, I've never had a TV on board so there wasn't all that much else to do.
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