Making a sextant: Help appreciated.

[richardandtracy]

Thanks for your comments. Being mildly skint I'm making it myself, the cost of new & second hand good ones have frightened me off!

Regards

Richard



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[HaraldS]

Sounds like a fun project! I would be too scared to try producing such precision stuff myself, but you sound like you know what you are up to.

Just looked at the certificate of my Plath, it says vernier check to 0.1 minutes OK, reproducability across arc 9 arc seconds.

Nothing about temperature range, but I know that this is a big part of the design.

The so called "Bowditch" the Ameriacn Practical Navigator has a lot of good information about sextants, but the section got thinner in the recent releases, so try to get an older 'green' copy (when it wa stsil two volumes).

Semi transparent mirrors are quite common now, it's easier to catch the sun with them, but the loss of light less good with star sights. Same is true for polaroid filters, as opposed to a bunch of single ones.

I have an artificial horizon, got it from my father who bought it in the sixtees, still fits the new plath. It works with an air bubble whos size you can adjust. You have to make bubble and sun concentric and calculate for center. But this isn't possible in any seaway as the bubble dances like crazy. The thing goes in place of the telescope and includes a 2x or 3x scope.

As to buying a sextant instead? My answer is> nobody really needs a sextant in GPS days and using or building and using it is strickly fun, go for it!

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[richardandtracy]

GPS vs Sextant

I'm a self sufficiency nut. I always believe everything should be repairable/replaceable from materials carried on board when going on a longish voyage. With this in mind I prefer the idea of a sextant - but will have a single handheld GPS when I go too.

As to overall self sufficiency - you'll probably think I'm nuts (no, I'm sure you will), but my internal layout has been built around a 17' long timber. This timber can be stripped out as a [short] spare mast to be lashed/ scarphed to whatever stub remains after a pitchpole or capsize. I've got 5off 10' long timbers along the cabin sides for spare battens for the Junk sail, and I chose a Junk design so that in extremis I can use the sheets off the bed as sails. The fwd cabin is laid out as a machine shop, and will be populated with the tools just before I go. That way I'll be able to re-make any part of the vessel in the middle of any ocean just so long as it stays above water! The long keel is filled with steel scrap ballast - and is accessable if I need to plunder it for raw materials when my normal stock runs out.

And where might I need all this emergency stuff? I hope (one day) to go to the cold bit at the bottom of the world, and want to be adequately prepared so there is no need to put anybody else's life in danger rescuing me. I admit I'm probably suffering from congenital insanity (inherited from the children, I'm sure), but everyone's got to have a dream.

Regards

Richard.


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[HaraldS]

Re: and my wife calls me nuts

and I'm by far not as crazy as you. You should have seen the looks when I said I was going to take a lathe on board...
There has been a lot of discussion about sextant vs. GPS. We take out the sextant on passages just for fun and I think other than determining current and tide for tacking the high accuracy and instant availability of GPS fixes isn't so great, just showing slow progress. Where I think the GPS is fine is close to land at night, the porblem is you start to use up the extra safety margin you get from it, and it is near land where it is more likely to fail. We found GPS jammed near Faro for some 20 minutes this year. So to me it's more a question of GPS vs. classic piloting, and not sextant.
Anyway, I hope I'll be near your boat when I need a real odd part in a real odd place.

Regards,
Harald

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