Horizontal sextant angles with beam converter?

[Jdege]

I found a cheap Davis 25 sextant on eBay, so I'm playing around with it.

This is the one with a half-mirrored beam converter, instead of split mirrors.

It's easy enough to see how that works with sun sights, but I've not been able to get a clear image when measuring horizontal angles. Laying sun over horizon works, but lighthouse over hilltop doesn't.

Any suggestions? Or do I need a 15?

[GordMay]

Mark 15 & 25 Users’ Guide ➥ https://www.davisinstruments.com/pro...0_IM_00025.pdf

[Jdege]

My problem is getting both images to be visible in the beam converger at the same time, when they're both just landscape, and don't have the contrast of Sun vs. horizon.

Are there any tricks to it?

[Adelie]

You don’t need to get a new sextant, just replace the whole horizon partial mirror with a traditional half mirror.

[Jdege]

Searching the net, it looks like I can pick up a split mirror intended to fit either the 15 or the 25 for $20.

I'm going to give it a try.

Thanks.

[S/V Alchemy]

Originally Posted by Jdege

Searching the net, it looks like I can pick up a split mirror intended to fit either the 15 or the 25 for $20.

I'm going to give it a try.

Thanks.

If you like coastal pilotage (and I assume you are taking bearings to do distance off), you can also try this with a handheld compass and put tape on your lifelines indicating 45 degrees (or 30 or 10) forward of your helm and the same aft (works better with a tiller!). Same with running fixes. I'm so glad I took a pilotage course in 1999 just before everything went GPS.

[Jdege]

Seems to me if you have two landmarks on the coast, you can get a quick fix using compass bearings, faster and easier than using sextant angles. But I want to know how to do it.

[NevisDog]

Originally Posted by Jdege

... if you have two landmarks on the coast, you can get a quick fix using compass bearings, faster and easier than using sextant angles.

Yes but the quickest use of sextant is to measure the height of a known landmark above HW, a lighthouse or small hill (to calculate distance off). Have only ever used a split mirror for this but I expect the contrast may be quite good with half-silvered whole mirror. We used the height thing combined with multiple transits to locate fishing floats well offshore.

[El Pinguino]

Once you have your sextant sorted you will need a Station Pointer... https://www.celestaire.com/product/p...ation-pointer/

That one is a bit fancy but there are plastic ones out there.... plenty good enough for yacht navigation...

And you can even dispense with the sextant... just use your HBC to take three bearings... subtract one from t'other to get your 'sextant angles'.

Beauty of the method is that variation and deviation play no part....

[Badger007]

Originally Posted by El Pinguino

Once you have your sextant sorted you will need a Station Pointer... https://www.celestaire.com/product/p...ation-pointer/

That one is a bit fancy but there are plastic ones out there.... plenty good enough for yacht navigation...

A sheet of clear plastic and a texta would be a very economical alternative. Compass bearings would be OK but sextant much more accurate.

[KiwiKen]

Originally Posted by NevisDog

Yes but the quickest use of sextant is to measure the height of a known landmark above HW, a lighthouse or small hill (to calculate distance off). Have only ever used a split mirror for this but I expect the contrast may be quite good with half-silvered whole mirror. We used the height thing combined with multiple transits to locate fishing floats well offshore.

Geometrical plotting of a pair of horizontal Sextant angles will give you the most accurate manual fix. ( other than a transit which is ususally a single position line)
A sextant vertical angle range, as long as the height of the object is adjsuted for height of tide is equally accurate, but there is still an error in the taking of a bearing



Traditional hand bearing compass fixes will have a 1-2 Degree error in the bearing accuracy, which at 15nm of the coast will give a circle of error of 1/2nm radius round your fix.


The shame is that GPS is seeing the fading of the traditional fixing skills . The problem is what happens when the GPS fails.

[Jdege]

Originally Posted by KiwiKen

Traditional hand bearing compass fixes will have a 1-2 Degree error in the bearing accuracy, which at 15nm of the coast will give a circle of error of 1/2nm radius round your fix.

But what are you going to be taking bearings off of, at 15nm distance? All I can think of would be mountain peaks. And I'm not sure I'd be able to tell one mountain from another, at 15nm.

[NevisDog]

Most will likely know already but anyway, the great thing about those vertical angle measurements of charted objects (before GPS made all this redundant) is you can easily calculate and set a "safe angle" on the sextant and, so long as that angle is not exceeded when you squint through it at the landmark, then you know you are outside a safe distance from that landmark. When the tide falls, the preset angle ensures you remain at an even safer distance, which is kinda neat

[NevisDog]

Originally Posted by Jdege

But what are you going to be taking bearings off of, at 15nm distance? All I can think of would be mountain peaks. And I'm not sure I'd be able to tell one mountain from another, at 15nm.

All this may be of historical interest only but I found it intriguing how those old salts found their lobster pots at 10 or 15 miles offshore, out in the open Atlantic. Of course they knew names of all the local mountains like the back of their hand but that was not essential to finding their fishing gear. They drew detailed line drawings of the distant high peaks intersecting the nearer low peaks soon as they dropped their gear and before steaming for home. I call these transits, as each detail - where each low ridge in foreground intersects a more distant shoulder of mountain on the skyline, etc - each carefully recorded sketch provided a multitude of transits from which to find the gear again next morning. The recorded heights from sextant angle simply speeded up the process of getting there next day. Incredibly accurate.

I remember an abandoned red van that had to be precisely in line with a particular black rock when entering (or leaving) the tiny natural rock-strewn harbour they used. If anyone ever moved that old wrecked van, no more lobster fleet! (No, I'm sure they had some back-up plan, someone would have placed a marker there.)

[CaptainJohn49]

I have a Davis 25 and you are right. You can buy the split mirrors from Davis for a few bucks and convert it. Later if you want you can convert it back. The process is very easy.

CaptainJohn49