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Old 29-02-2016, 22:33   #31
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Re: How to determine distance from an object of know height using a sextant?

Hamburking hails from a port near me. He seems a reasonable and well experienced sailor.

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Old 29-02-2016, 22:57   #32
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Re: How to determine distance from an object of know height using a sextant?

This is a great discussion about some fundamental piloting skills. Almost makes me want to resurrect my distance log and knotmeter which I abandoned years ago during an electronics "upgrade".
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Old 29-02-2016, 23:22   #33
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Re: How to determine distance from an object of know height using a sextant?

In my way of thinking, all of the electronic nav aids, are just that. Aids to one's basic navigational skills.
Maintaining a DR plot should never go out of style, nor out of practice.
Ditto on reading all of the pertinent navigation texts, Notices to Mariner's, etc... even local Cruising Guides, etc. for the area in which you'll be operating.

For as I see it, you have 2 choices. You can let someone else decide what information you're fed on any particular area. Or You can decide what's important enough to dig into in depth, on an area, or not.
Whom would you prefer to be doing the screening?

And bottom line: Who's responsible for the safe operation of your vessel, & the safety & comfort of it's crew?

PS: Anyone care to field an (honest) poll on how many folks out there can't properly read a chart or map, & navigate sans electronics; due to GPS & ChartPlotter dependency?
I'd venture to say that the numbers are higher than in the single digits, percentage wise. Sadly.
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Old 01-03-2016, 00:31   #34
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Re: How to determine distance from an object of know height using a sextant?

The old Stadia computer (circular sliderule) was used in navigation and other endeavours but has become a rare thing these days.

A cheap sextant is a useful tool for both horizontal and vertical angle finding.

The approximation of the methods shown above is also still used in the compass and watch basics taught for visual navigation of aircraft. In that scenario the small angle " One in Sixty" rule is using the approach of the sine function to 1 to give a new heading to correct drift or offset from desired track. Two choices result, correct and parallel the desired track or double the change in heading to close to the original track in the same distance already flown.
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Old 01-03-2016, 07:18   #35
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Re: How to determine distance from an object of know height using a sextant?

Quote:
Originally Posted by UNCIVILIZED View Post
Maintaining a DR plot should never go out of style, nor out of practice.
There was one of the Delos videos where they were threading their way through some shoals along a tidal river when their GPS/Nav went out.

It's nice to be think you can rely on paper charts and sextants as a backup, but if you've been relying on the electronics, and they suddenly go out, while you're in a tight spot where you don't have sea-room, how long will it take you to dig out the paper charts and identify your position?
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Old 01-03-2016, 08:29   #36
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Re: How to determine distance from an object of know height using a sextant?

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Originally Posted by Ungvar View Post
I'm thinking height of the object over Tan Angle would give you a basic distance, but I don't think many people navigate like that. I don't any way.

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I think the OPs question was more academic than anything. I can't see any other use for it. I don't even think the OP did either?
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Old 01-03-2016, 08:56   #37
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Re: How to determine distance from an object of know height using a sextant?

Easy.

distance = hight . cot(angle)
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Old 01-03-2016, 11:02   #38
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Re: How to determine distance from an object of know height using a sextant?

Thanks for all the help guys. I was teaching my wife some basic chart plotting and getting fixes. She is a former Army officer and familiar with using topo maps and ground nav, but not on the principles of running fix, single point and multi point fixes, DR plotting, and calculating set n drift. We always plot a fix every 4 hrs and try to maintain hourly DR plots and running fixes whenever running a marked channel or coastal navigation. Yes, we have a chartplotter, actually two plus a backup handheld GPS, but you shouldn't rely on electronics if you can't do it by hand. Anyway, Thanks again for all your help and input!
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Old 01-03-2016, 11:15   #39
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Re: How to determine distance from an object of know height using a sextant?

If you are dusting off the sextant, then try some horizontal angles, should be able to get some good fixes.

As hamburking showed in an earlier post, the position can be plotted using a station pointer, but it's just as quick to use a protractor and lay off the lines.
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Old 01-03-2016, 12:44   #40
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Re: How to determine distance from an object of know height using a sextant?

Quote:
Originally Posted by Jdege View Post
There was one of the Delos videos where they were threading their way through some shoals along a tidal river when their GPS/Nav went out.
This is Exactly why one keeps a running DR, on Paper Charts, all of the time. Otherwise they're simply excess baggage.

It's nice to be think you can rely on paper charts and sextants as a backup, but if you've been relying on the electronics, and they suddenly go out, while you're in a tight spot where you don't have sea-room, how long will it take you to dig out the paper charts and identify your position?
Assuming that for whatever (foolish) reason you haven't been keeping a proper DR plot, & this happens. Then the proper course of action (if possible), is to hold your position. Via either anchoring, & or by shooting bearings to prominent topographical features, using your echo sounder, etc.
Until you pull out your paper charts, get your head extracted from your backsided, & fix your position. So that you can then carry on with making way... While maintaining a Proper DR plot, whilst doing so.
To maintain a proper DR, you plot your position (with course & speed info) every;
- Hour on the Hour
- Every Course Change
- Every Speed Change
- And of course, for every; Fix, Running Fix, Sun Line, Line of Bearing, etc., etc.
Plus, basically for any & every time that you acquire a piece of navigational data which helps you to establish where you are, or can be used in conjunction with another piece of nav data, in order to do so.

BTW: Most, if not all, of the above is stuff which gets entered into the ship's log by the designated watch keeper anyway. Along with a number of other things. At regular intervals; usually the ones delineated above.

Though here's a question, which is Key to all of this/answering your question.
--> Why rely entirely on electronics, & electronic charts, completely, to begin with? <--
Is it not simply prudent to use a variety of types of navigational tools, & formats, in order to keep track of your position?

To me, it makes sense to make up a "cheat sheet" of all of the key points, nav wise, for a new area. Both; on a sheet of paper, & on the chart(s). As compiled from the pertinent nav references.
So that what I have is a "briefing sheet" for the new area, which covers all of the key points, in an easy to read at a glance format. Thus, if/when the fecal material hits the fan, I'm already prepared nav wise.

(PS: Said briefing sheet can be pasted into either; the Ship's Log Book, or a Nav Reference Book. So that if you're operating in that area a little while down the road, then you have the basics to work from/with already - though in need of updating).

Making up such an info/data sheet, is (a longer, nav type version of) kind of like plotting escape bearings, as well as danger bearings, once I'm bedded down in a new anchorage. So that if I have to leave in a hurry, & visibility is poor to non-existant, I simply follow my pre-planned route.

All of the info's right there. So that I can concentrate on the 101 other crazy things which will undoubtedly pop up in said scenarios.
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