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Old 09-06-2010, 18:06   #1
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Sextant: Okay, I'm in - What Next ?

I heard a story over the weekend first hand from a cruiser who was completely rolled in the Roaring Forties when crossing from the Caribbean to Africa last year. It was a harrowing story, but the part that stuck with me was that he was broached and completely inverted at one point. In and of itself, a terrifying prospect, but it was his description of the "aftermath" that got me thinking. Naturally, when he finally righted, hove-to and bailed out, there were no nav electronics functioning onboard. He's not sure, but the handheld back-up VHF/GPS disappeared in the chaos as well - presumably floated away.

Ultimately, they were fine, and made it safely to the west coast of Africa because they had and knew how to use a sextant. I couldn't help but ask myself - what the heck would I have done? I suppose it would have had to reach for the EPIRB.

I'm not a conspiracy theorist and I don't believe the US government is going to one day discontinue a service upon which militaries and entire shipping and transportation industries have come to rely. Solar flares? Well... maybe. But the point is not so much a reliance on the GPS as a system, but on electronics generally.

I'm not a Luddite, and I embrace technology that makes my life easier, more enjoyable, or safer.

So... sextant? For sextant neophytes, there's an intimidating mystique to the devilish contraptions - a complex engineering origami of mirrors and glass. Are they as complex and difficult to use as they at first appear? Would anyone recommend a book or particular course?

I'd really like to teach myself the basics, perhaps "double-checking" the GPS each day when cruising just to get in the habit and to develop the skills.
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Old 09-06-2010, 18:22   #2
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Learning to use the sextant would be fun, put you closer in touch with the universe, and have some marginal value as a backup. In practice, for the cost of one decent sextant, you can buy 3 or 4 handheld gps units and lots of AA batteries. Properly stowed, they'll survive any roll over at least as well as a sextant, and they'll provide accurate position updates night or day, in any weather.
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Old 09-06-2010, 19:53   #3
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I have an abandon ship bag with a waterproof VHF and a water resistant GPS, as well as a hand held compass, and a ERIB. I keep a sextant under my bed, and I am learning to use it. What does that make me? Either very paranoid or very careful, haven't figured out which...
Next project? Make sure I have up to date flares, including white ones...
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Old 09-06-2010, 20:04   #4
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Multiple GPS backups may not survive a lightning strike...EMF will take them out.

You don't have to be a practiced all-round sextant navigator to use a sextant for simple sun sights. A single sight of the sun's lower limb at local apparent noon (LAN) and some simple math will give you your latitude. Only reference required is a Nautical Almanac showing the sun's declination and corrections thereto.

Additionally, you can take simple sights a few minutes before and a few minutes after LAN and, armed with just a Nautical Almanac, you can compute your longitude. Close enough for government work, anyway.

So with a few sextant sights around noon, a Nautical Almanac, an accurate timepiece, and a pencil and paper you can work out your noon position.

Some references:

Celestial Navigation Net

Mary Blewitt's "Celestial Navigation for Yachtsmen"

Jeff Toghill's "Celestial Navigation"

THE COMPLETE ON-BOARD CELESTIAL NAVIGATOR
Everything but the Sextant

By: George Bennett

Bill
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Old 09-06-2010, 20:05   #5
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[QUOTE=RSMacG;466447] Naturally, when he finally righted, hove-to and bailed out, there were no nav electronics functioning onboard. He's not sure, but the handheld back-up VHF/GPS disappeared in the chaos as well - presumably floated away.

Floated away? I'm not buying it.

I carry two hand-held backups, both of which are submersible and will float. I feel confident that they're no more likely to float away than any sextant in a wooden case.
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Old 09-06-2010, 20:30   #6
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Multiple GPS backups may not survive a lightning strike...EMF will take them out.
Simple solution: make a box with a tighly fitted cover out of some sheet copper and put the GPS inside. This will isolate your spare GPS from the lightning induced EMF. An aluminum box might work almost as well.
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Old 09-06-2010, 21:01   #7
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When you use a sextant to take a sun sight, make sure that you are using plenty of filters when looking at the sun. If you experience an after image in your vision that lasts more than a couple of minutes, that means you have burned your retina.

I put all the filters in place, and then remove them one at a time until I have a dull circular image of the sun. Doing things that way, I don't get a burn on my retina.

I am a retina physician, and I have seen lots of burned retinas over the years. Most commonly it happens when people use dark sun glasses or dark exposed x-ray film for protection when watching a solar eclipse. The people that try that out end up with a creamy white burn in the center of their retina, and frequently there is a permanent reduction in central vision, depending on how persistent they are in looking at the solar disc.

Occasionally we see someone on drugs who stared at the sun and sustained permanent damage to their central vision.

The sun filters are there for a purpose, and use as many of them as you can while still seeing the sun when taking a sight.
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Old 09-06-2010, 21:09   #8
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Originally Posted by RSMacG View Post
I heard a story over the weekend first hand from a cruiser who was completely rolled in the Roaring Forties when crossing from the Caribbean to Africa last year. It was a harrowing story, but the part that stuck with me was that he was broached and completely inverted at one point. In and of itself, a terrifying prospect, but it was his description of the "aftermath" that got me thinking. Naturally, when he finally righted, hove-to and bailed out, there were no nav electronics functioning onboard. He's not sure, but the handheld back-up VHF/GPS disappeared in the chaos as well - presumably floated away.

Ultimately, they were fine, and made it safely to the west coast of Africa because they had and knew how to use a sextant. I couldn't help but ask myself - what the heck would I have done? I suppose it would have had to reach for the EPIRB.

I'm not a conspiracy theorist and I don't believe the US government is going to one day discontinue a service upon which militaries and entire shipping and transportation industries have come to rely. Solar flares? Well... maybe. But the point is not so much a reliance on the GPS as a system, but on electronics generally.

I'm not a Luddite, and I embrace technology that makes my life easier, more enjoyable, or safer.

So... sextant? For sextant neophytes, there's an intimidating mystique to the devilish contraptions - a complex engineering origami of mirrors and glass. Are they as complex and difficult to use as they at first appear? Would anyone recommend a book or particular course?

I'd really like to teach myself the basics, perhaps "double-checking" the GPS each day when cruising just to get in the habit and to develop the skills.
I'm trying to figure out why he needed a sextant to find something the size of Africa. A compass and a last fix would do.
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Old 09-06-2010, 21:36   #9
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Amelia and Noonan missed Howland island but it is pretty darn small. They did find Africa...

I know this is slightly off topic but when in site of land it's amazing how people navigate. Predominantly they look at the chart and try to locate the harbor. This is what some on my crew were doing going to Indonesia a couple of weeks ago. I finally pointed out a big mountain behind the marina on the chart and said why don't you all see if you can spot that mountain?

Navigation - Big picture to small.

i.e. Find Africa first then get specific.
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Old 09-06-2010, 22:56   #10
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I'm trying to figure out why he needed a sextant to find something the size of Africa. A compass and a last fix would do.
The trick is not in finding Africa but in arranging the near miss that is most desirable, i.e. 6 fathoms with a sand bottom.
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Old 09-06-2010, 23:19   #11
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Do both.

Buy one or two water resistant battery powered backup GPS's and put them in two different locations in zip lock bags. The chances of lightning or a capsize destroying both would be highly unlikely.

Buy the sextant and learn how navigation was done not all that long ago. I really enjoyed learning this science and art and how it relates to the sun, stars, this planet and the universe. Celestial navigation is one of those things that expands the mind and makes going to sea more interesting. Because something may seem useless to some does not mean it is not worth learning.

Different forms of navigation are a supplement to each other, not a replacement of the other. I think its a potentially dangerous mistake to think GPS has replaced all other forms of navigation. As I see it, the idea of a backup is not an obsolete concept, especially when going to sea when a West Marine is nowhere near.
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Old 10-06-2010, 00:26   #12
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Buy one or two water resistant battery powered backup GPS's and put them in two different locations in zip lock bags. The chances of lightning or a capsize destroying both would be highly unlikely.
There is almost no place on a boat where electronics are safe from lightning damage if the boat is struck. Even if there is no current passing thru the equipement, the electric field gradient around the conduction path is so high that induced currents in the electronics will fry a myriad components. This is out to at least 10's of feet from the conduction path.

The one place that might be protected is inside the oven if it doesn't have a glass front. The oven acts as a faraday cage, the metal shell shunts electrical fields around the interior, to a greater or lesser extent depending on openings in the shell and a bunch of other stuff I didn't pay enough attention to in physics.

Best bet, is store emergency electronics (gps, vhf, shortwave, 2 or 3 watches, whatever) in ziplocs in metal boxs (i.e. business cash box), put that in the oven whenever lightning threatens. No garuntees even then, lightning may still fry everything or maybe just 1 item in the box.

Don't forget to take out of the oven afterwards, oven will bake electronics if lightning didn't fry them.
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Old 10-06-2010, 01:01   #13
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It never hurts to have the skills necessary to navigate by sextant. Once you get the hang of it, and it really isn't all that hard, it actually becomes fun and satisfying to use a sextant. I use my 70 year old naval Plath all the time, at least when the sun is visible.
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Old 10-06-2010, 01:33   #14
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A sextant is a wonderful instrument, and I do not go to sea without one - my wife calls it my security blanket... It is easy to master, essentially one quarter science and three quarters art, and you look just soooo cool and intelligent perched up on deck using one. Apart from the obvious deep sea easy position fixing, a sextant can be used (horizontally) for coastal position fixing when two or more charted land marks are visible, can be used (vertically) for giving distance off when a landmark of known hight is charted... and most importantly, it is a stand-alone instrument, not subject to the vagaries of satellites, the boat's electrical system, dry cell batteries, moisture in the works etc. Shooting stars is difficult from a small and probably heaving deck and the small boat sailor is probably best advised to limit him/herself to shooting the moon or sun. In this respect, you can draw some comfort from the fact that the sun and moon have continued to rise for millennia and will probably continue to do so for some time yet, so its reliability is exceptional.

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Old 10-06-2010, 01:58   #15
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A sextant, almanac, and a watch can get you around the world....

Quote:
Originally Posted by RSMacG View Post
So... sextant? For sextant neophytes, there's an intimidating mystique to the devilish contraptions - a complex engineering origami of mirrors and glass. Are they as complex and difficult to use as they at first appear? Would anyone recommend a book or particular course?
RSMacG,
1) Fisrt off, NO, in actuality, a sextant is NOT complex at all.....
Breaking it down to the basics, a sextant just measures angles very precisely.....in most ways, you can think of it as just a very precise (and accurate) protactor....
That's pretty much it.....

2) Bill's answer is by far the best you're going to get.....commit this to memory....and you'll be the envy of your marina/yacht club/etc...
Quote:
Originally Posted by btrayfors View Post
You don't have to be a practiced all-round sextant navigator to use a sextant for simple sun sights. A single sight of the sun's lower limb at local apparent noon (LAN) and some simple math will give you your latitude. Only reference required is a Nautical Almanac showing the sun's declination and corrections thereto.

Additionally, you can take simple sights a few minutes before and a few minutes after LAN and, armed with just a Nautical Almanac, you can compute your longitude. Close enough for government work, anyway.

So with a few sextant sights around noon, a Nautical Almanac, an accurate timepiece, and a pencil and paper you can work out your noon position.

3) I learned to use a sextant 35+ years ago......and while I only do it for fun nowadays, I still use it for fun (and practice) when offshore.....but's it been almost 35 years, since I've done anything but a Sun Sight.....

{As simple as shooting the sun is, I cannot fathom why use of a sextant seems to be thought of in such mystery.....but, oh well, that's life...}

As Bill wrote, shooting the Sun at local noon will get your lat.....and doing a few sights before and after, and noting the exact time of the "highest" postion, will give you your long......
Not within feet....but within miles at least....


4) As for where to get more info, books, etc......the answer is Celestaire....
Can't think of anyone better.....
Welcome to Celestaire


5) Not trying to confuse the issue, but what you, the navigator, are trying to do is simply measure an angle of a specific object (like the Sun) above your horizon, and if you do that and know the exact time you made that measurement, everything else you're going to is MATH.....and that will give you a Line of Position (LOP)......

If you have a "idea" of where you might be (you are keeping a log, right?), you can use that position (or your current estimated position derived from a DR plot), and plot this against your celestail LOP.....(or even better LOP's from a morning, noon, and afternoon sights...)

And for basic celestial navigation, that's it......
I know the "purists" will say that you should learn to do star sights, use the tables, etc....and while I DO agree with them if you're interested in this, but for "back-up naviagtion", probably not necessary....


6) Now, just by happenstance, this past weekend a new owner of a sistership to mine asked me about "navigation plans", which sort of confused me, until he explained that he was looking for info/data on how others navigated......
And, I thought you may find my answers to him useful....
Quote:
As for "plan A", "plan B", "plan C", etc.....some of this changes, depending
on WHERE you are sailing.....
Here's what I do.......(not sure if this will be of help or not, but here it
is anyway...)

1) My primary navigation / charting ("plan A") is:
Using paper charts, main steering compasses, and depth sounder (when depths / depth contours are a factor)......with GPS position plotting.....(usually running 2 separate/independent GPS units at all times)
When "depth plots" do not agree with "GPS plots", I'll usually plot both and
note the difference on the charts....
Keeping position and time plots on the chart, and updating course as
needed....allowing use of DR should electrical/electronics/GPS failure
occur.....
(When well offshore, I also try to do a daily noonsite on my
sextant.....just in case...)


2) Secondary navigation / charting ("plan B") is:
Eyeball Navigation......reading the water depths by eye......
This is used in the Bahamas, etc. where charts (paper or electronic) are not always accurate.....(Explorer Charts are good, and I do use them......but in the Bahamas there are many places that you must read the water by eye....)


3) My "plan C" is:
Navionics Platinum Charts, in Raymarine E-120 and E-80......(with GPS input from the Raystar 125, and other data from my on board Sea Talk data bus...)
I'd also include radar in this category......since Florida, Bahamas,
Caribbean, and North Atlantic sailing doesn't require much (if any) use of
radar......but if sailing New England or Pac NW, radar would probably in
"plan A".....


4) My "plan D" is a two prong plan:
Basic celestial (sextant, almanac, tables, accurate watch) with a morning,
noon, and late afternoon shots if possible.....and, Basic DR using paper
charts and my main steering compasses.....


I do carry back-up GPS units, including a spare Raymarine GPS 125, and three Garmin GPS 76's.....as well as sextant, almanac, tables, multiple timepieces (and a "celestial calculator"), as well as spare radios and receivers, spares for autopilot, instruments, wiring/cables, etc....
But, hope to never need them......
(And, yes, if I believe thunderstorms are likely, I do stash a Garmin GPS
76, spare Icom handheld VHF, and some batteries in the microwave....just in case......and I also have a decent lightning ground for the mast....)

I do hope this helps...


7) Lastly, as for the specifics of the boat you mentioned "finding Africa"......that's really not a big deal......
Finding land is pretty easy, even without anything more than a compass....head in the "approx direction" you think land is, and evenutally you'll find it......
Finding your way to a specific island in the middle of the ocean, that's another story.....

{I'd not recommend it, but, you actually can navigate the "Captain Ron" way......and you'll eventually find land.....("....besides, if you get lost, you just pull in somewhere and ask directions...") }



RSMacG, I realize this was a long answer....but I thought you might find this useful....

Fair winds.

John
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