Should You Get a Plastic or Metal Sextant?
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- Опубликовано: 30 сен 2024
- A common question is whether a plastic sextant can do the job at sea. In short, yes it can. In this video I compare a Davis plastic sextant to a Celestaire metal sextant.
Although the metal sextant is definitely sturdier and a little more accurate, the plastic sextant is also well built and a great starting point for either emergency navigation or just getting started in celestial navigation.
amzn.to/2P7KvYb Davis Mark 15 Sextant, about $175
amzn.to/35U7trD Celestaire Astra IIIB, about $800
I have a Davis Mark 15. I'm just a hobbyist, and wanted to have the means to actually put the theory to practice without breaking the bank. Instability in the index correction is really noticeable; you can pretty much monitor the sextant heating up or cooling down by taking a time-series of index error measurements on the horizon. When taking sights for practice, I always inject a horizon measurement for index error before and after each celestial sight. In a series of consecutive sights, this leads to pretty good repeatability, although sometimes all of my LOPs from a given session are systematically several miles off from my actual position, while all remaining consistent with one another. (Of course, user error remains a distinct possibility.). Sometimes I nail my position within 1 nm, although this may just be luck. I do notice potential issues with backlash, and/or a little bit of skittishness with how the micrometer engages with the notches on the arc. I like the way the different sunshade colors work, and how these can be further modulated with neutral density shades... I don't know how this might be different in the various metal sextants. I'm toying with buying something nicer our of curiosity to see how much of my current errors are just me, vs the tool (I'd like to think I would get the same outcome Nautical Man14 reported). Even though the Davis sextant is not super high precision, I am absolutely confident it would perform sufficiently well to navigate anywhere within a reasonable margin of error that should be applied to celestial navigation.
When I switched from plastic to metal (exact same sextants you have shown) - there was a huge leap in obtained accuracy with the metal.
... That you're somehow not able to disclose...?
I learned on a plastic Sextan and as a graduation present I got a metal Astra that I love.
On a Mk 15, I am seeing about 5-7 arc minutes of backlash in the rack gear, so it is important for me to always turn the micrometer in the same direction when taking measurements. I also check index error before & after readings. I sometimes see 1-2 arc minutes of difference there, which I then average & use for correction. I am curious how much better the Astra is.
Night and day difference.
With a plastic sextant, you’ll constantly have to recalibrate it. A real pain. It’s hard to gain confidence as your not sure if your technique is off or if it went out of calibration again. After buying a Mark 25 plastic sextant, I bought a $300 eBay Astra IIIb clone. Works great. Calibrate it once per year, but really doesn’t need it.
I think the question here is are you married or not. $800 vs. $175? Somebody's gotta get her hair done every 5 minutes. Good luck with that.
Bought a beautiful metal yacht (slightly smaller form factor) sextant made in East Germany (with certification papers) on Ebay for $180 Aud.
Don't think it was ever used.
Love the weight of it compared to my plastic Davis.
My Davis "plastic" Sextant was out performed by a HOME MADE Sextant put to gather from bits and pieces left in the junk box at the engineer department at school. Was highly accurate, even when tested at sea on a 48 foot sailboat traveling from San Diego to Hawaii !
So, no. I'm not going to spend any more cash on these plastic toys.
Give me metal every time.
The thing to add is that plastic is a bit more unstable and needs to have the mirrors checked more often for alignment, metal less so. Metal is heavier which you have to get used to especially when taking a lot of sights for the Noon Sun. Thanks for this!
I had a bad experience with DAVIS. I bought a new M15 sextant (on eBay) and the lens came loose from the 3X telescope. There was no way to fix it. I sent an email to Davis, including photos, and got no response. I sent it again and was told that customer service would respond. The answer came after I insisted, once again, and was "we don't have stock of this part. I asked for more information and no answer. They totally ignored me. So, a serious company is one that cares about the customer and this was not the Davis case with me.
For actual use at Sea?
I NEVER use or recommend the plastic "Toys".
Get metal. Even the one from China is several orders of magnitude better. On every level!
I've had my astra sextant for over 20 years as well, and love it.
Ha the early plastic ones had you 60 miles out if they were in the sun for more then a minute , Thank god for eyeball navigation & DR.
You bet! I would average a ton of sights with a plastic one to do the best you can, but you are definitely right there.
Plastic is less stable as in temperature and humidity impacts.
Plastic is alright but I want the traditional metal one
Awesome tips! I can’t wait to get out there!
Metal 🔧🪛⛏️🧲🪚⚙️
I would like to see more of that tall ship. Did you take the stickers off your Celestaire?
Enjoy the south pacific!!
Haha good question, the sextant in the video is one of the ship's stock, mine still has the "custom flame decals" of course!
You mentioned around the 2:08 mark that the metal sextant requires some maintenance. What maintenance does it need?
Usually on our ships every 3 months we clean with fresh water, lube the gear mechanism, change the battery if so equipped, and adjust the mirrors. We have like 6 sextants on each tall ship, so it is part of a routine maintenance schedule. For the plastic ones, needs about the same treatment but a bit more forgiving if you forget since they don't corrode (in my opinion anyway)!
With Mark 25 Sextant I get good 4 Nm accuracy (4 minutes) using, an TI 84 calc with programs to record and even plot the observations...BUT when u make a measurement u can only Scroll DOWN in one direction, if u start scrolling UP and DOWN when trying to catch the horizon, it becomes worse like 6 minutes.
4 minutes of arc? 4 degrees is 240 nm....
@@karhukivi pardon Minutes).
@@karhukivi A minute of arc is different from a minute of time. A minute of arc is a fraction of a degree. 60 minutes = 1 degree. In Latitude, 1 minute of angle = 1 nautical mile.
@@jima4656 That is correct, his original comment has been edited.
Professional navigators use only a metal sextan...
Totally, but many beginners will start with the plastic one, which is totally fine.
@@NavigationTraining Hi! Yes I know! But it,s like going hunting with a child,s plastic toy gun.
Living seaman use metal. Adrift and forlorn seaman use plastic. 😕😩
@@MartinSageNot really. I have several sextants. Metal and Davis MK15s. I see little difference in accuracy between any of my Cassens and Plaths and my Davis models.
Question to your Sextant experts. I am in need of an instrument that would allow me to determine the angle between two objects at a distance of 2 to 3 miles. If I were to turn a sextant horizontally? A cheap $50 sextant should be able to achieve this, otherwise it is going to cost thousands for a theodolite that I can't carry up into alpine environments anyway. Any thoughts that come to mind on what I should buy, and if this is possible in your opinion? I plan to transfer these angles to Google earth, and to then determine the GPS locations for objects at a distance that I can't get to because of terrain, ice, rock cliffs, and craigs. I know there are instruments that can do this, rangefinders and such, but I'm not going to spend 5k on a range finder, and then another 3k on a surveyors theadolite. I could hang plumb bobs at a distance in front of me, and from tripods to transfer the inclination between objects to two parallel plumb bob strings, and use the sextant to sight the two plumb bob strings in order to get the angle between the two strings. The plumb bob strings would be superimposed over the two objects at a distance. One object possibly the summit of a mountain, and the second object, located 1,500 in elevation below the summit. Again, the plumb bob strings would convert differences in elevation to two parallel lines that can then be read by the sextant as an angle. This would then be transferred to Google Earth, and the GPS location would be extracted.
How close are the two objects to one another?
I have several sextants. Two Davis MK15s and several metal ones. Mostly Cassens and Plaths.
A sextant will do the job but calibrating one for such close distances could be an issue. Parallax at short distances can be an issue.
Your idea should work but a bit of work is required on the position calculations if they are at different elevations. I have used my Davis 15 to measure the horizontal angles between three lighthouses in the bay near where I live. What I did was to get their location coordinates and use maths to calculate my position, a process known as resection. That was done in the old days by a three-armed device known as a "station plotter" probably used in the 8th and 19th century by sailors making charts of the coastlines. All good fun!
@@karhukivi Sory about my crappy initial post (no editing). Continuing to work on this project. Will be using some device thus summer when the snow melts. I'm currently looking at a surveyors compas. I may do testing across a river with GPS knowns and see how accurate I can get. If my accuracy is less than acceptable, I will move to some other device. I continue to believe the sextant would do the job well. 10 minutes of accuracy. A sextant with a calibrated compas may be how I will proceed.
@@TimKaseyMythHealer Interesting project - let us know if it works, good luck!
I'd imagine the choice would also depend on the size/kind of your boat. A small boat or a racing yacht where you've got salt water breaking over the decks and soaking you while you're trying to get a shot suggests a plastic sextant might survive better? On the bridge wing of a freighter the metal one would seem to be safe enough :-)
Definitely a good point!
What's the sizes and what does the size does it refers to?
Sextant itself refers to the amount of degrees you can travel with the index arm. There are old fashioned "octants" and "quadrants" as well. Thanks!
Come join Practical Celestial Navigation on FB for more...
Thanks Nautical Man, I'm on there!
@@NavigationTraining Yes, and to others as well!
Get a C. Plath. Germans make good stuff. A plastic will be off 5mi on Ave but she’ll get you to your destination if it’s not Too far away. My life is worth the extra cost of metal.