Cheapest workable sextant

[roblpm]

Having questioned the need for astro in the other thread I thought I should at least be able to see if I can do it myself. Maths isn't a particular problem for me. Spending £500 on a decent sextant is.

Anyone had any success with a plastic thing. Sort of google cardboard equivalent?

 

[Paddy Layback]

Make your own from a couple of Lego bricks and a Barry Manilow CD..

 

[Budgieboy]

The Astro I thought was far too much for me ..... I picked up a Davis 25 for about £100 on eBay , had to wait a couple of weeks for it from USA , all good and teacher gonna help take sun sights off the wall in Brighton soon , guy had it up as nearest offer so had a deal with him ....

 

[Jeannius]

I used, just for fun, a Davis Mk3 sextant while doing a rtw. I found I could get within 10-15 nm on a noon sight without to much difficulty. I never expected to need to use it in anger, I just wanted to know if I could be accurate enough to know when I was getting close to land. I could.

 

[laika]

I have a Davis Mk 15 which I've used for star sights (even if some people say you can't use plastic sextants for star sights...) but doubling of the USD price since I bought mine compared with a rubbish exchange rate don't make them look quite the bargain they were unless you can pick one up cheaply.

 

[[3889]]

Well, I going for my YM Ocean with sights from a £25 EBBCO off Ebay. On land I get within 4-10' of predicted altitudes. How this degrades when at sea will be revealed in a fortnight. I've found that one advantage of wobbly mirrors is that they can be persuaded to dewobble without faffing with tiny screws. Unseamanlike, but convenient.

 

[JumbleDuck]

Ebbco used to do two, the standard and the special. The standard was pretty rubbishy, but the special was OK, certainly for practice.

 

[TQA]

The Ebbco is OK yes you need to check the index error often and avoid leaving it out in the sun.

I used mine for 3 years and found my way across the Atlantic and round the Carib.

 

[Skylark]

I bought an Ebbco in the early 1990s and used it regularly on a small dive rib to measure distance off (quite successfully). I used it in 2007 for a YMO qualifying passage. I sold in on eBay and treated myself to a higher quality metal one, single mirror and good optics. I also bought a Davis Mk15, brand new in box for £30 from a boat jumble, couldn't resist a bargain!

 

[bdh198]

I recently bought a Davis Mk25 for a transatlantic passage and was impressed with the accuracy I could get. However, accuracy improves significantly with practice, and by practicing I learnt the quirks of my sextant and thereby how to get a reliable reading.

One particular quirk with my sextant, which I suspect is more common with a plastic sextant, is that the index error was different depending on which way I twisted the micrometer before taking a reading. This could be as significant as twisting one way the index error would be +2 but the other way it could be -5! However, once I understood this I just learnt to take a sight by only turning the micrometer in the direction that produced the lower error. It meant it took a few seconds longer to get a sight, but it was quite manageable. I was also careful to check the index error every time I used the sextant as it did vary throughout the passage.

I suspect these kind of 'quirks' are less common with an expensive brass sextant; however, by getting to knowing my sextant I was able to ensure its quirks did not interfere with my sights and was able get some very accurate sights - often with between 3-5 nautical miles of accuracy, and my best was only 2.2Nm off the GPS reading!

 

[Iliade]

That is backlash. One should always approach a measurement from the same direction to avoid this error, regardless of the quality of the (mechanical) instrument used.

 

[duncan99210]

I picked up a 1970s Simex off eBay for about £170. It's a rebadged Tamaya, proper metal framed job, so it is possible to get a decent sextant for not a lot of money if you're prepared to watch for a bargain on eBay.

 

[jwilson]

The Ebbco is OK yes you need to check the index error often and avoid leaving it out in the sun.

I used mine for 3 years and found my way across the Atlantic and round the Carib.

Agreed, also the way you squeeze the handle can flex it and alter index error: nevertheless perfectly adequate for sun/moon/bright stars genuine navigation.

 

[jwilson]

The trouble with Ebay is that sextants are easy to damage, and potentially expensive/impossible to fix if damaged. I would not like to pay much for a sextant I'd not been able to handle and do a few basic tests, If you can correct out side and perpendicularity errors and there is no backlash, all testable almost anywhere with a distant view, it is probably fine. When I gave up on Ebbco as my eyesight was getting worse I bought a S/H metal sextant from a chart agent to get a real glass lensed device.

 

[john_morris_uk]

You don't have to pay £500 for a sextant.

My original sextant was an Ebco special (£5-00 from a jumble sale). Index error was variable....

Having passed my YM Ocean with it I treated myself to one off eBay. It cost £150 and there is a slight risk in buying unseen because (as others have noted) they're easily damaged. However mine is fine and with a bit of cleaning and fiddling the light to illuminates the scale now also works.

Shop around and be patient.

 

[bdh198]

That is backlash. One should always approach a measurement from the same direction to avoid this error, regardless of the quality of the (mechanical) instrument used.

That explains it, thank you! On another thread that would perhaps be evidence of the value of doing an astro navigation course as part of Ocean Theory as I suspect understanding backlash would be one of the first things the instructor would tell you when showing you how to use a sextant. Backlash isn't mentioned in either the RYA or Tom Cunliffe books, and was not mentioned in the guide that came with the sextant.

 

[jwilson]

That explains it, thank you! On another thread that would perhaps be evidence of the value of doing an astro navigation course as part of Ocean Theory as I suspect understanding backlash would be one of the first things the instructor would tell you when showing you how to use a sextant. Backlash isn't mentioned in either the RYA or Tom Cunliffe books, and was not mentioned in the guide that came with the sextant.

Backlash probably not mentioned because good sextants in good condition should have virtually nil......

 

[Martin_J]

I've always finished by turning the micrometer drum in the same direction before reading... didn't think to do otherwise

Interesting little article here showing how Plath and then Tamaya and Astra designed things mechanically to minimise backlash..

Sextantbook description of backlash

To the original poster.. Did you manage to find a cheap enough sextant?

I might have a spare ebbco plastic one on board that you could have for the cost of postage..

 

[JumbleDuck]

I've always finished by turning the micrometer drum in the same direction before reading... didn't think to do otherwise

Anyone used to working with a lathe or milling machine would do that without thinking.

 

[Martin_J]

Anyone used to working with a lathe or milling machine would do that without thinking.

..and tuning a guitar!