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Old 17-01-2020, 09:06   #31
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Re: Sextant - Aluminum or Brass

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Originally Posted by Santiano View Post
Press release:

I've made a deal with mnh for his Astra. Thanks to everyone else, especially rbrazil, for their offers and input. Sorry as well, rbrazil, I was too impulsive to wait and see what you found in Petaluma.
Cheers!
Hey Santiano I hope your deal went thru. Astra are great for the purpose. I had a Frieberger aluminum, light and very good, but it had a split mirror.
One thing to note is the index mirror type. A full horizon is much easier to use on a sailboat, and they say that eventually you may move to a split horizon, but we don't purchase sextants too often, so you may use this as a criteria.
More details here:

https://www.celestaire.com/selecting-a-marine-sextant/
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Old 17-01-2020, 15:31   #32
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Re: Sextant - Aluminum or Brass

Free Tip: If your Astra has a light, DO NOT LEAVE THE BATTERIES IN if your not going to use it for a while, don't ask how i know this
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Old 24-01-2020, 22:46   #33
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Re: Sextant - Aluminum or Brass

TIP: If the seller says "brass sextant", there is an excellent chance that it is only a display reproduction of a real sextant. Most are bronze. Or aluminum. I think a lot of posters here who mention brass sextant, actually mean bronze. Big difference. Most brass alloys are too soft for use in sextants.



Salvaged sextants from the shipbreakers are often an excellent buy. As another poster said, they are mostly very lightly used.



Half mile accuracy is pretty doable on a ship, but actually rather difficult on a small sailboat except in very calm conditions. Happily, a triangle that is 2 or 3 miles across is perfectly adequate for midocean navigation.


Good idea to keep a notebook in a ziplock. Write your navigation stuff in pencil. If it gets wet, it won't run the way ink does. Ask me how I know. If you always record your hourly speed and course, and all your fixes, you can always reconstruct your plot, and you should know how to construct a plot sheet on blank paper.


I like the Astra sextants. Pretty decent instruments. The one and only one I have now is a vintage Plath scored on fleabay. Seems okay enough though honestly I have not had much opportunity to mess with it since purchase. Davis plastic sextants are popular and we could argue all day about their suitability but while I respect the decision by many to go cheap, I am personally firmly in the metal camp.



Sight reduction apps are nice but remember, you are dealing with computer or tablet or celphone or celestial calculator, and if you want an alternative with zero dependance on electricity or electronics, you should know how to add and subtract manually, and use sight reduction tables. And keep them aboard, at least the relevant volume or volumes for your voyage, along with an almanac. My suggestion is work it out longhand and check it with a sight reduction app, and then compare your final fix with the GPS fix for the same time. You can also just always record the GPS position and just see if lies on your LOP or not. Use all your resources and be a true navigator. Use only your sextant and you are merely a seagoing Luddite. Use only electronics and well, I will stop right there. LOL!



Have pencils. Lots of them. Erasers too. Triangles. When you get used to using them, much faster and more accurate than parallel rules or fancy pants rolling rulers. Two pairs of dividers, not just one. You want to keep one with a pencil mounted, for use as a drawing compass.


One of the most neglected skills of the navigator in small boats is compass checking. This is one celestial task that at least on U.S. ships, is likely to be done daily. The quick and dirty way is to take an amplitude. Easy as pie. Even if you have NO sextant, you can do that, or do an azimuth. Creating a compass deviation card is satisfying and useful.



Another useful tool for the primitive navigator is a chip log. Much more accurate than just guessing your speed. Make it yourself!
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Old 25-01-2020, 06:46   #34
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Re: Sextant - Aluminum or Brass

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Originally Posted by IslandHopper View Post
Free Tip: If your Astra has a light, DO NOT LEAVE THE BATTERIES IN if your not going to use it for a while, don't ask how i know this
Actually that goes for any sextant with a battery. I spent a long time cleaning up the battery connections on the squadron's sextants when I took on being education officer.
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Old 25-01-2020, 08:28   #35
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Re: Sextant - Aluminum or Brass

Hey, Mr. Monster, that's a great post, but I have two questions.

"The quick and dirty way is to take an amplitude." How do you do that?

Another useful tool for the primitive navigator is a chip log. Care to give basic directions for a DIY freak?
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Old 25-01-2020, 08:31   #36
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Re: Sextant - Aluminum or Brass

"I spent a long time cleaning up the battery connections on the squadron's sextants when I took on being education officer."

This is a good reason to carry an old fashioned #2 pencil in your nav kit - cleaning AA battery prongs in the bottom of a plastic hole with the eraser.
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Old 25-01-2020, 09:22   #37
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Re: Sextant - Aluminum or Brass

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Originally Posted by tkeithlu View Post
Hey, Mr. Monster, that's a great post, but I have two questions.

"The quick and dirty way is to take an amplitude." How do you do that?

Another useful tool for the primitive navigator is a chip log. Care to give basic directions for a DIY freak?

Compass Error by Amplitude
It looks involved but take it a step at a time and you will see that there is only one time where you need to break out the calculator.



This vid is sort of interesting. It never occurred to me to use the moon. I have only ever seen the sun used.


You might also refer to Bowditch.



A chip log is just a triangular or pie slice shaped piece of wood with a bridle and a long lanyard for retrieval. The lanyard is measured and knotted. To simplify, the chip log is dropped in the water and allowed to fall behind the boat as it sails onward. The amount of line paid out and the elapsed time are used to calculate the speed. For a one minute logline you would have knots ever 1/60 of a Nautical Mile. Not a very handy distance, a lot of line to pull back in. So you could use the classic 50'8" and let it run out for a half minute. If two knots pass over the transom in a half minute, you are making 2 Knots. You can make your marks every 25'4" and go 15 seconds, much handier on a fast boat. Still, at 10Kts you are looking at 254' of line to retrieve so you could maybe use an even shorter timeframe at some slight cost to accuracy. You can just cut a pie wedge out of plywood or whatever, drill holes near the points for the bridle, and weigh the bottom so it floats vertically and catches the water. The pointed end should only barely break the surface of the water. Don't make it too big, or you won't be able to retrieve it. It only needs to be a few inches across. You can also use a breakable thread for the two bottom bridle points. A sharp jerk will break them and make log retrieval easier. Up to you.




In this vid they seem to be using 10 seconds so their line is knotted accordingly.


And the wiki
https://en.wikipedia.org/wiki/Chip_log


Keep in mind that a log does not measure speed over ground. It measures speed through the water. Just like your compass does not measure course made good. It only tells you what direction the boat was pointing. Set and drift are found when you take a fix and compare your fix position with the DR position from the same time.
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Old 26-01-2020, 21:51   #38
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Re: Sextant - Aluminum or Brass

I have an excellent brass/bronze sextant that has been retired and is available. I do not plan to cross oceans or need it again. In its wooden case. But I will not be home to ship it until the end of April. Take care of it and it will take care of you.
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Old 27-01-2020, 07:06   #39
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Re: Sextant - Aluminum or Brass

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Originally Posted by tkeithlu View Post
Another useful tool for the primitive navigator is a chip log. Care to give basic directions for a DIY freak?
Without a chip log, you can measure your speed with a watch and two marks on the deck say 30 feet apart. Drop a wad of paper ahead of the first mark and measure the number of seconds for the paper to pass between the marks. If it takes say 5 seconds...

30 feet /5 seconds = 6 feet per second
6 feet/second * 3600 seconds/hr = 21600 feet/hour
21600 feet/hr / 6000 feet/mile = 3.6 knots

or 6 feet/second * 0.6 = 3.6 knots

You can actually do pretty good just sitting in the cockpit on a pretty day gazing over the side and counting as the foam bubbles go past. Work on your mental math skills and try to outguess your knotmeter.
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