Sextants.. How precise do you need to be?

[[3889]]

Deep into a YM Ocean course and have broken out my cheapo Ebbco (Special, mind you!!) to give sun sights a go in the real world. I'm taking sights within 4'-10' of predicted altitude at my GPS position. Is this good enough? All the paper exercises have index errors less than 2' so is 4' or more too imprecise?
Do I need to fork out for a metal instrument to get through the YMO? I'm not interested in owning a thing of beauty.

 

[[70521]]

I've chatted this one over with a YM Ocean Examiner he uses a Davis MK15 and gets within 5nm of a GPS position and is happy with that.

How small is the island you are sailing to?

 

[RobbieW]

With YMO I dont think accuracy per se is the point of the exercise. If you know how to use the instrument and are aware of the failings or inaccuracies, and demonstrate an understanding of what you might do to mitigate them, theres no reason why you should fail - either the written or viva part of the exam. As Sandy said, how big is your target.

 

[john_morris_uk]

I'm an Ocean Examiner and when I originally took my ticket it was with sights from an Ebco Special. Every time I took a sight I checked the index error and noted it. The examiner noted that the index error was different for each sight and asked what was going on? Plastic Sextant in the med sun explained everything...

The sights aren't a competition to see who gets nearest. 4 miles off is pretty good. 10 miles off and you're still going to make your landfall.

 

[KellysEye]

>The sights aren't a competition to see who gets nearest. 4 miles off is pretty good. 10 miles off and you're still going to make your landfall.

We had a Freiberger aluminium sextant and only once did we get 5miles in over 100 sights all the rest were about 10 miles

 

[Paddy Layback]

>The sights aren't a competition to see who gets nearest. 4 miles off is pretty good. 10 miles off and you're still going to make your landfall.

We had a Freiberger aluminium sextant and only once did we get 5miles in over 100 sights all the rest were about 10 miles

Interesting thread, would you be checking the accuracy by GPS?

 

[[3889]]

OK. Thanks for replies so far. Maths isn't my forte so can I ask if a 4' error in observed altitude translates into a 4 mile error in position, or is there some exponential buried in the sums which will send things really awry?

 

[sailorman]

Deep into a YM Ocean course and have broken out my cheapo Ebbco (Special, mind you!!) to give sun sights a go in the real world. I'm taking sights within 4'-10' of predicted altitude at my GPS position. Is this good enough? All the paper exercises have index errors less than 2' so is 4' or more too imprecise?
Do I need to fork out for a metal instrument to get through the YMO? I'm not interested in owning a thing of beauty.

Ask Sir Cloudesley Shovell, accuracy does matter

https://en.wikipedia.org/wiki/Cloudesley_Shovell

 

[alant]

OK. Thanks for replies so far. Maths isn't my forte so can I ask if a 4' error in observed altitude translates into a 4 mile error in position, or is there some exponential buried in the sums which will send things really awry?

If your aiming for a landfall at an exact spot, this isn't necessarily the objective.
Mariners pre-GPS, usually offset their landfall deliberately, so when the reached a coast, they knew that 'turn right' or 'turn left', got them to where they wanted to be. Chichester when flying across the Pacific, dare not miss his refuelling island, so he deliberately missed it, then ran down the latitude of the target, until he spotted it.

 

[Mistroma]

If your aiming for a landfall at an exact spot, this isn't necessarily the objective.
Mariners pre-GPS, usually offset their landfall deliberately, so when the reached a coast, they knew that 'turn right' or 'turn left', got them to where they wanted to be. Chichester when flying across the Pacific, dare not miss his refuelling island, so he deliberately missed it, then ran down the latitude of the target, until he spotted it.

I used to use this technique when heading from Scotland to Holland in 60s and 70s. The Gats were a pretty small target, few features on shoreline and often foggy. Much easier to aim more towards Imuiden and turn left when in sight of the coast until I reached Schulpengat.

My sextant was usually accurate enough to confirm I was in the North sea. Mark 1 eyeball used to confirm that I was indeed surrounded by wet stuff. First RDF was a slight improvement, but not enough to ever think of aiming directly at the target.

 

[AndrewB]

Its down to practice, practice, practice. 10nm out when you start is very respectable. You will get that down to 3nm even with an Ebbco cheapie if you use it daily and come to understand its foibles.

 

[Mistroma]

Its down to practice, practice, practice. 10nm out when you start is very respectable. You will get that down to 3nm even with an Ebbco cheapie if you use it daily and come to understand its foibles.

Problem with North sea was that seeing the sun or stars wasn't guaranteed and sometimes even seeing the water wasn't guaranteed. I couldn't assume 3nm, more like 6-8nm and to play safe aim 12-15nm off my final target. First trips were in a slow bilge keel 26' boat, so DR over a couple of hundred miles didn't really guarantee great accuracy either. Tiny 2-stroke engine so spent quite a while just drifting around.

Oil rigs became more useful than a sextant when they became more common.

 

[Lon nan Gruagach]

OK. Thanks for replies so far. Maths isn't my forte so can I ask if a 4' error in observed altitude translates into a 4 mile error in position, or is there some exponential buried in the sums which will send things really awry?

Fair enough that the theoretical maths is a bit OTT, but how about trying the error your self?
Take sighting and pretend it is spot on, then add your own error in steps 1', 2', 4', 10' and do the calculation again.

If you want to delve a bit deeper then you can draw graphs of the Sun's elevation and plot your errors on that. (draw horizontal lines corresponding to 10' offset from any actual value, then, where these cross the graph of actual elevation draw lines down, the distance between these lines is the position error)

To answer directly, the Sun's altitude during a day is close to a sine wave. if you take your measurement at mid day (local time) then the elevation will be changing very slowly and any errors will have a greater effect. if you take your sightings mid morning and mid afternoon, errors have less significance.

 

[johnalison]

To answer directly, the Sun's altitude during a day is close to a sine wave. if you take your measurement at mid day (local time) then the elevation will be changing very slowly and any errors will have a greater effect. if you take your sightings mid morning and mid afternoon, errors have less significance.

That will be true of the sun's altitude, but won't apply to a position line derived from the observation of a star/moon/sun. An error of four seconds equates to one mile, IIRC, but an error of one minute's altitude will, I think, vary according to the altitude of the object. At its extreme, with an object on the horizon, an error of one minute could place you 5000 ft in the air, at least, I think so.

 

[Skylark]

.........................I originally took my ticket it was with sights from an Ebco Special. Every time I took a sight I checked the index error and noted it. The examiner noted that the index error was different for each sight and asked what was going on? Plastic Sextant in the med sun explained everything...

I had a very similar experience during my qualifying passage. It was a southbound Biscay crossing. I also used a Ebco and noted daily increasing index error as the air temperature increased. It was a good point of discussion during the assessment.

Over around 6 days of forenoon sun-MP-afternoon sun running fixes the closest position to the GPS was about 8M. I now have a fairly high quality CP Sailing sextant and I can't get a lot better! The big difference between the two is robustness and the quality of mirrors and optics, in my view. More important for when seeking the little buggars in the twilight sky, not such a big deal for the shooting the sun.

Good luck with the course.

 

[jwilson]

I'm an Ocean Examiner and when I originally took my ticket it was with sights from an Ebco Special. Every time I took a sight I checked the index error and noted it. The examiner noted that the index error was different for each sight and asked what was going on? Plastic Sextant in the med sun explained everything...

The sights aren't a competition to see who gets nearest. 4 miles off is pretty good. 10 miles off and you're still going to make your landfall.

I used an Ebbco Special for some years: including genuinely mid-ocean as primary navigation (no GPS in those days). If you took a single snap sight you could allow 5 miles error if you were confident of the horizon. By taking index error, taking five or more sights, checking index error again, which was often as JM says different, plotting the sights against time on graph paper, and picking an average one, I could usually get within 2 miles, on those occasions when I had a known position to check against if sailing coastal and just testing my nav. Took a while but in real ocean sailing you've usually got time to spare anyway.

Eventually my eyesight got worse and my bank balance improved, and I lashed out on an ex-US Navy sextant that was only slightly older than I was, but with real glass in the telescope instead of Ebbco plastic. By then we had GPS, and it was easy to check errors. Could now get 2 miles average error on a single sight.

 

[KellysEye]

>Interesting thread, would you be checking the accuracy by GPS?

Yes.

 

[alant]

I had a very similar experience during my qualifying passage. It was a southbound Biscay crossing. I also used a Ebco and noted daily increasing index error as the air temperature increased. It was a good point of discussion during the assessment.

Over around 6 days of forenoon sun-MP-afternoon sun running fixes the closest position to the GPS was about 8M. I now have a fairly high quality CP Sailing sextant and I can't get a lot better! The big difference between the two is robustness and the quality of mirrors and optics, in my view. More important for when seeking the little buggars in the twilight sky, not such a big deal for the shooting the sun.

Good luck with the course.

My "qualifying passage", was Cape Town to UK using an Ebco, which was kindly loaned to me (without any cash deposit) by a certain capnsensible of this parish, when he was based with his U-boat colleague in Haslar.

 

[jwilson]

>Interesting thread, would you be checking the accuracy by GPS?

Yes.

Not always, used sextant well before GPS existed. Finding Rockall was one good check - it came up dead on the nose after heading derived from sun-run-sun sights via Ebbco. Early on I practiced anchored in various southish-facing bays here I had a true horizon and sun moon or stars visible.

Ebbco optics are not much good for anything but the brighter stars, though fine for sun and moon.

 

[capnsensible]

My "qualifying passage", was Cape Town to UK using an Ebco, which was kindly loaned to me (without any cash deposit) by a certain capnsensible of this parish, when he was based with his U-boat colleague in Haslar.

Unclean, unwashed, underwater.