sextants

[wishbone]

Just started to get into astro nav; for a possible trip from USA to north east UK; have seen a pucker second hand but twenty years old or do I go for a new plastic one, trying to work out best split screen or all view

Any thoughts???

Wishbone
Rolling, rolling, rolling keep them doggies moving!
Where’s me chuck wagon gone?


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[Rowana]

20 year old one will probably be all brass, and subsequently very heavy. Plastic one may possibly not be so accurate, and more easily damaged.

If the purpose is just to get up to speed with astro nav, and you are flying to the USA, go for the plastic one, as the other will use up a lot of baggage allowance! You will be using GPS for navigating, I presume, and I would expect you to get to somewhere around +/- 5 miles by astro after a bit of practise.

When you think about what's +/- 5 miles in the middle of the ocean? Even when you approach land, it just means you sight the hard bits an hour or so earlier or later.

When I did my ocean yachtmaster course, we went out into the Sound of Jura one weekend for a bit of practise. It was quite a nice day with not much swell, and after taking quite a few sights using 2 different sextants, we then reduced these to get a position line. We were about 0.5 miles out from the GPS position, but as I said, the conditions were ideal, and we were all taking extreme care. It would be a different proposition in the middle of the ocean, with a bit of swell running.

Personally, I would love to do such a trip, and I would do the whole thing by astro, but have the others on the crew use the GPS. It would be an interesting exercise, I think.

Good luck, and do let us know how you get on.

Jim

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[trevor_loveday]

Like any new hobby, I would suggest it largely depends on how serious you are and how much you wish to invest. I am doing my YM Ocean at the moment and, after a short trial, felt that the Davis plastic type are not very appealing to use at all. They need frequent re-calibration and JMIrvine's suggested accuracy appears accurate. However, this may suit your purpose and they are the lowest cost.

If you have the choice of a good metal sextant for a similar price to a new plastic one, there's no comparison, IMHO. It will also always have a residual value if you decide you no longer wish to keep it.

In answer to your question about full or split view, the full view uses a semi-transparent mirror. This cuts light transmission by 50% so makes it difficult to use feint stars. This may or may not be a problem to you. The consensus opinion appears to be that full view is easier to get started with but split view is preferred by experienced users. I went for split view from the start and haven't had any problems with it.

For more information, try this site: <A target="_blank" HREF=http://www.celestialnavigation.net/index.html>http://www.celestialnavigation.net/index.html</A>. The "Resources" link on the home page takes you to some really useful links, including tutorials.

Good luck.

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[trev]

Go for an all metal split screen job. My sextant is a forty year old 'three circle' and still does the job - but only used once or twice a year for the pure hell of it. Make sure the mirrors and index glass are clear and bright and the arc glides smoothly. Also check that the telescope ring is not distorted and sits at right angles to the plane of the instrument.

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[wishbone]

It's hard to tell the condition as it is on sale on ebay, but the seller has said that it is good working order

maker is Thomas Foulke last tested 1974
Wishbone

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[AndrewB]

Sounds like it was last certificated 30 years ago, and is presumably older.

You will be taking a big chance that this one is still suitable for serious navigation. As is well known, one hard knock damaging the frame, and a sextant may still 'work' but is pretty well useless. Can the owner guarantee accuracy?

A new plastic one will be quite serviceable.

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[Guest]

20 yrs ....

Blimey thats young and only just getting into order !

It is reckoned that a good sextant will last all your nav working life ....... can you say that about a plastic one ?

For Yacht use a basic Davis or Kelvin Hughes Mate sextant should be fine ..... or if you can get a second-hand Plath / Tamaya etc. ...... then that would be excellent.

MAKE SURE that a) it's still with original box and not some bodge job, b) it has its correction certificate from makers inside lid, c) it still has its tools and lube bottle !!

The sextant relies on its being properly cradled when not in use and the original box is made for that particular sextant ....

Get hoold of a good sexatnt book and BEFORE buying check it for its 3 forms of error ..... a) straight undamaged frame by eyeing up reflected with unreflected arc of the sextant, b) eying the horizon at zero setting and seeing that the horizon line is good across reflected and unreflected in eyepiece, c) that index error is within tolerance of original makers error certificate by sighting a star and adjuting images well off each other and back again .... reading the resulting settings .... repeat ! OK - this means that seller must accept you have the sextant for a trial ..... if he says no, then c can be done later.... if a + b are in good nick.

........


<hr width=100% size=1>Nigel ...
Bilge Keelers get up further ! I came - cos they said was FREE Guinness !
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[Guest]

Accuracy ?

Noon sight with running morning fix ...... expect average +/- 10 miles,

Stars ....... good to +/- 5 miles

But that is a nav who has been at it for a while .... OK so there are many occasions where fix is near spot to the GPS .... but you have to remember that this is false - as the reference DR used is probably from the GPS .... increasing the accuracy from the start ! It used to be a run up EP from previous astro fix to derive sight parameters .....


<hr width=100% size=1>Nigel ...
Bilge Keelers get up further ! I came - cos they said was FREE Guinness !
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[wishbone]

Well I did'nt win the one for sale on ebay; but after reading all your posts perhaps it is just as well! could'nt check it! and it went for nearly 190 quid.

I can see astro nav will be a challenge, and something to while away the dark evenings and that long trip from across the pond.

Thanks everybody for your input, still think i'ts a great forum..........


Wishbone
Rolling, rolling, rolling keep them doggies moving!
Where’s me chuck wagon gone?



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[richardandtracy]

You've got another option.. Build your own.

But how long to your trip?

I've just completed (last 4 days) the design work behind a brass sextant with a micrometer style vernier. PM me with your e-mail address if you want the design in AutoCAD, DXF or GIF format. I'm hoping the design will get posted up with the Octant and CD Sextant on http://www.tecepe.com.br/nav at some point in the fairly near future. Omar Reis, the site manager, has agreed to knock up a page with it on, but I think he was slightly surprised at the number of drawings. Anyway, publishing the page is beyond my remit.

I've started making the simpler bits, and [so far] don't have any problems. Looks like the cost will be approx £50 for the raw materials.

Regards

Richard.


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[wishbone]

Hi
thanks for the offer; I just have a problem with the time; I have so many things on the go getting ready for my retirement, I do have facilities to make most things have been in maufacturing for over 30 years. The date is'nt set for the states yet Im'e going over to buy a boat to bring back, fed up with the inflated prices over here.
Iv'e read best time for the trip back is around end april.

All the`best
Wishbone
Rolling, rolling, rolling keep them doggies moving!
Where’s me chuck wagon gone?


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[KipH]

Well if you're going to the States, you could consider an aluminium Astra 3b. 400 plus squids over here, 400 odd $ over there. Celestaire is a place you might consider.
http;//celestaire.com

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[wishbone]

Thanks very much for the info, very interesting and informative site

Kind regards

Wishbone

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