Sextant.. Navigation tool or ornament?

[Seajet]

GPS availability has always exceeded clear skies in my experience.

Well that will come as a surprise to the sailors just before Gulf War 1 who according to their GPS were doing 500 knots at 35'000 feet !

The U.S.have said they won't do it again, but though I am a supporter of the U.S, if I was in a serious war that's one of the first things I'd turn off, or better still spoof...

 

[Nostrodamus]

So, in honesty, when you take a sight do you plot your position on the chart from this or do you check it against the GPS, think I am 3 miles off which is pretty near and then plot your position on the chart from the GPS?

Most people also carry a back up GPS. Do you carry backup sextants, watches and tables or is the sextant a fail safe in case the GPS goes down or just because it is an historic way of finding ones position.

My view is that I will always use the GPS when I can and although it can be used in cases of emergency the sextant is there to prove to yourself that you can fnd your position if you need to and usually you will try to find how near to the GPS position you can get from a sight rather than use it.

 

[VO5]

So, in honesty, when you take a sight do you plot your position on the chart from this or do you check it against the GPS, think I am 3 miles off which is pretty near and then plot your position on the chart from the GPS?

Most people also carry a back up GPS. Do you carry backup sextants, watches and tables or is the sextant a fail safe in case the GPS goes down or just because it is an historic way of finding ones position.

My view is that I will always use the GPS when I can and although it can be used in cases of emergency the sextant is there to prove to yourself that you can fnd your position if you need to and usually you will try to find how near to the GPS position you can get from a sight rather than use it.

I don't know about others, but as I explained in my previous post, my priority is to maintain my navigation skills (traditional skills) in use so as not to allow them to slide away by not using them.

Then I try (except in really bad weather or reduced viz) to fix by observation and calculation. I plot the result on my chart.

Only then do I consult the GPS. I usually get results of less tha 5 miles, often 2 or 3 and occasionally a few cables, which is very satisfactory.

If I am sailing with crew, then I try to do my DR and EP as accurately as possible, using traverse tables, just for the excercise.

I have a method of accurately calculating leeway also, which is a help.

In bad weather or reduced viz, eg., muzzy horizon or cloud cover, I still calculate a DR or EP and check it out with the GPS.

 

[alant]

I don't know about others, but as I explained in my previous post, my priority is to maintain my navigation skills (traditional skills) in use so as not to allow them to slide away by not using them.

Then I try (except in really bad weather or reduced viz) to fix by observation and calculation. I plot the result on my chart.

Only then do I consult the GPS. I usually get results of less tha 5 miles, often 2 or 3 and occasionally a few cables, which is very satisfactory.

If I am sailing with crew, then I try to do my DR and EP as accurately as possible, using traverse tables, just for the excercise.

I have a method of accurately calculating leeway also, which is a help.

In bad weather or reduced viz, eg., muzzy horizon or cloud cover, I still calculate a DR or EP and check it out with the GPS.

Whats your "method of accurately calculating leeway"?

 

[scotty123]

I didn't need to work out my position. Note that it was an afternoon sight so it ran NW-SE. I simply had to sail East or South until the transferred position line passed through the landfall point (an offshore buoy) then sail down the bearing of the position line. It's just a modification of 'running the Easting down'.

For the less knowlegeable, could you expand explanation - perhaps with diagrams pls.

 

[Jamesuk]

Do you still think there is a use for the Sextant or should fix?

Do all of your navigation using a sextant and you will not need to ask the question, IF you never use it - whats the point, because when you "do" need to use it you would have probably forgotten to buy the update Nautical Almanac.

I am just in the process of buying a sextant and will be navigating by the heavenly bodies soon.

 

[Nostrodamus]

Do all of your navigation using a sextant and you will not need to ask the question, IF you never use it - whats the point, because when you "do" need to use it you would have probably forgotten to buy the update Nautical Almanac.

I am just in the process of buying a sextant and will be navigating by the heavenly bodies soon.

So if you haven't bought a sextant as yet how do all your navigation?

 

[VO5]

Whats your "method of accurately calculating leeway"?

Verri thimple...

Rig a fine line with a fishing weight at one end. Attach the other end to your stern, to which is fitted a protractor. Then leeway is easy to read off.

By the way, don't forget....leeway is caused by the effect of wind.
Set is caused by the effect of tide and / or current.
Drift is the combination of leeway and set, or leeway against set.

Thimple, U C ?

 

[snowleopard]

For the less knowlegeable, could you expand explanation - perhaps with diagrams pls.

OK, here we go...

You take a sun sight that gives you a sun position line. You know that you are somewhere on that position line. Draw a parallel line through the intended landfall, i.e. the transferred position line.

Choose an arbitrary point A on the sun position line. Sail South (green track) until your plot shows you have reached the transferred position line (point B). You now know you are somewhere on the TPL. Turn and sail along the bearing of the TPL and you will hit the landfall.

Note that I have omitted the calculation of tidal set & drift (and leeway) for clarity.

 

[Lady Campanula]

'The Uses of a Single Position Line.....'

Once upon a time, every navigator had to learn these by rote - then go and practice using them.

Nowadays, most wouldn't recognise a LOP if it bit 'em!

Brave new world....

 

[maxi77]

GPS availability has always exceeded clear skies in my experience.

Well that will come as a surprise to the sailors just before Gulf War 1 who according to their GPS were doing 500 knots at 35'000 feet !

The U.S.have said they won't do it again, but though I am a supporter of the U.S, if I was in a serious war that's one of the first things I'd turn off, or better still spoof...

I would suggest that Gulf Wars are somewhat more predictable than the weather. In reality since GW1 the dependance of particularly air traffic on non P code GPS means that it will take something really bad for the septics to do more than well advertised local tampering with GPS. For a start far to many of their own grunts us comercial gps sets to try to avoid their traditional freindly fire incidents to let too much tampering go on. During GW1 the availability of civilian handheld GPS sets was almost zero because US soldiers families were buying them to send to their lads so they didn't get lost in the desert, the same was happening to a lesser extent in the UK.

I suspect we are not that far now from multi constellation sets as the Gallileo system becomes available, and some may even give a Glonass capability too.

In my time at sea I spent more time fiddling with radio aids to get a fix than I ever did taking stars, because when you needed the bl**dy things you could't see them.

 

[KellysEye]

I think it's must for ocean sailing and in a liferaft. If you tire of doing the calculations use a programme called Astrocalc.

 

[timbartlett]

In my time at sea I spent more time fiddling with radio aids to get a fix than I ever did taking stars, because when you needed the bl**dy things you could't see them.

Ah but the endless fascination of fiddling with RDF -- the headset that looked (and smelt) as though it had been salvaged from a Lancaster bomber-- the bizarre antenna contraption with a tuner that covered the entire MF band in a quarter of a turn of a knob about the size of a modern 5p piece. Stations so weak that each null spanned about 175degrees, crossed with others that were so strong that you didn't really need to tune the thing in to pick them up -- and whose null was no more than a barely-perceptible reduction in volume.

To this day, I can still hear dit-dah-dit dah-dit-dit ringing in my ears from Roches Douuvres!

And then, at last the deep joy and satisfaction of drawing it all onto a chart to find that you'd got .... a cocked hat the shape of Guernsey but four times the size!

And that was only thirty years ago!

 

[alant]

OK, here we go...

You take a sun sight that gives you a sun position line. You know that you are somewhere on that position line. Draw a parallel line through the intended landfall, i.e. the transferred position line.

Choose an arbitrary point A on the sun position line. Sail South (green track) until your plot shows you have reached the transferred position line (point B). You now know you are somewhere on the TPL. Turn and sail along the bearing of the TPL and you will hit the landfall.

Note that I have omitted the calculation of tidal set & drift (and leeway) for clarity.

Excellent explanation.

Nothing like that on a Day Skipper course!

 

[Seajet]

RDF

TimBartlett,

while I feel some here are missing the point about sextants and backups, I agree 100 % with your description of basic RDF, including the controls as I had several of these devices which I can only assume must have been made & sold for a bet.

I found, fortunately just before I spent my money, that the 'digital' system I fancied was in fact worked by a rheostat turning the display up & down, not particularly related let alone calibrated to the frequency 'selected'

The only good signal I got mid-Channel was when I didn't need it, or from aero beacons which were supposed to be unreliable through 'land effect bending the beams', though Guernsey's seemed to work, possibly assisted by a thing Nevil Shute mentioned: watching the route of the aeroplanes ! ( 'Trustee From The Toolroom' ).

 

[maxi77]

Ah but the endless fascination of fiddling with RDF -- the headset that looked (and smelt) as though it had been salvaged from a Lancaster bomber-- the bizarre antenna contraption with a tuner that covered the entire MF band in a quarter of a turn of a knob about the size of a modern 5p piece. Stations so weak that each null spanned about 175degrees, crossed with others that were so strong that you didn't really need to tune the thing in to pick them up -- and whose null was no more than a barely-perceptible reduction in volume.

To this day, I can still hear dit-dah-dit dah-dit-dit ringing in my ears from Roches Douuvres!

And then, at last the deep joy and satisfaction of drawing it all onto a chart to find that you'd got .... a cocked hat the shape of Guernsey but four times the size!

And that was only thirty years ago!

I was lucky enough to be using a pucka HFDF set in the warmth of the Third Wireless Office. Mind you it was difficult to get the consol bearing to match the HFDF bearing of the same station, even after applying half convergency corrections. It was however much better than the little handheld sets used in yachts. Later I had access to Loran A where all you had to do was move one a scan spike over another and read of the time difference, then find that line on the latice chart a bit like 'real' Decca.

 

[Jamesuk]

So if you haven't bought a sextant as yet how do all your navigation?

On line of sight

 

[snowleopard]

In the days before Decca & GPS it was the norm to grab a position line whenever one was available. The neat navigation exercises of fix by cross bearings were all very well but it was just as valid to cross a sun position line with a compass bearing or even a depth contour.

Then there were night-time fixes by dipping lights. A departure from Start Point and, not long afterwards, a landfall on the Casquets.

OTOH entering the channel in thick weather the gap between the Cotentin peninsula and the IoW could seem narrow!

 

[alan_d]

I was lucky enough to be using a pucka HFDF set in the warmth of the Third Wireless Office.

I am struggling to understand the meaning or significance of Third Wireless Office. What happened in the other two? Or was it a class thing and the First Office had antimacassars and reading lights? Can't have been anything to do with the Third Programme, they wouldn't have needed DF to tell them where they were...

 

[ytd]

I have no problem with people using astro nav because they enjoy the problem solving aspect. It's a bit like sudoku. But it's a bit rich claiming astro is a necessary safety precaution for any ocean crossing. Some backup. You need the tables, separate book for each 10 deg lat, almanac, or laptop, accurate time and sextant. Each of these (except the watch) is less robust than a hand held GPS. Drop the sextant and the mirror will likely break. Let the tables get wet and the pages you need will be stuck together.

I used astro in pre GPS days and it was useless in the tropics for 8 months of the year because of the heavy overcast. The idea that you could use an astro to help navigate between 2 coral reefs is ridiculous. You can't rely on a chart plotter for reef navigation unless you have been to the same reef previously and verified its position on the plotter.

By all means enjoy astro as a hobby but don't claim it is a necessary back up. I enjoy the engineering in steam trains but I would expect a present day train driver to pass a test to operate a steam train to get a train drivers licence.