3-Armed protractor thing for plotting horizonal sextant angles..........

wooslehunter

wooslehunter

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If you take horizontal sextant angles between three objects, you can then set a three armed protractor to the angles & then by laying the arms on the three points, the centre of the protractor is at your position.

What's the three armed instrument called?
 
GeorgeLlewellin said:
Three Arm Protractor

http://www.bookharbour.com/other-navigation-tools/3-arm-protractor/

This one is by Weems & Plath, good quality and seems reasonable at £19.95
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That's very interesting, I presume it's designed to use with a sextant, not a compass bearing?
I love my hand bearing compass and feel naked without it, but no success on steel vessels due to mega deviation obviously. That thing combined with a plastic sextant looks like a quite quick way to a 3 point fix if you can't use a compass. Food for thought, I'm just going to the "wanted" section...
 
I have one aboard my boat. An Ebbco 'combined station pointer and distance off calculator'. Its my second favourite piece of old school nav kit. three plastic arms pivoted from a douglass style protractor. It has a hole in the middle of the aluminium knob that you put a pencil through to mark your position.

my no 1 item is like a hand bearing compass only a million times better. Its a vintage monocular (looks like half a 7x50 binocular) with a compass prism'd to appear at the bottom of a crisp lubber line that appears through the lens. its heavy, about 40 years old and beautiful. makes a plastimo iris look like something free with a box of cornflakes. best bit was finding it on ebay for a fiver 'cause some goon listed it under antiques not boating gear :)
 
Nope, haven't used Snellius but it looks interesting.

The reason I like horizontal sextant angles as opposed to compass sights is that even with the cheapest nastiest plastic sextant,you can easily get way better results. If you're doing this the objects you're sighting a generally only a few miles off at most. 1 degree is easy with a sextant but impossible with a hand bearing compass unless it's absolutely flat. Even then you have to be sure the deviation is accounted for. Variation doesn't come into it since you need the difference between bearings.

Having said all of this it's all just a bit of fun these days unless GPS fails..........
 
Of course, you can also use a 10" square plastic Douglas protractor - which also has several other uses, usually printed on the protective cardboard sleeve.

Simply draw in the angular bearings of the 3 charted 'objects of interest' to Left and Right of the Centre one ( make that one 360° ) as 3 pencil lines on the protractor radiating from the hole in the centre - the 'origin' - and shuffle the protractor around the chart until each of your pencil lines passes through one of the charted 'objects of interest' ( headland, church tower.... )

The 'origin' is your fix. Place a pencil dot there, with the time, and any other nav info you wish to record on the chart.

Simples.... :cool:


....And if you're intent on showing orf, a vertical sextant angle on the most suitable of the 'objects of interest' - or any other big sticky-up thing that's charted - and you have a 'distance off', too.
 
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wooslehunter said:
Nope, haven't used Snellius but it looks interesting.

The reason I like horizontal sextant angles as opposed to compass sights is that even with the cheapest nastiest plastic sextant,you can easily get way better results. If you're doing this the objects you're sighting a generally only a few miles off at most. 1 degree is easy with a sextant but impossible with a hand bearing compass unless it's absolutely flat. Even then you have to be sure the deviation is accounted for. Variation doesn't come into it since you need the difference between bearings.

Having said all of this it's all just a bit of fun these days unless GPS fails..........
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May I politely ask why deviation would matter, assuming you do three bearings standing in the same place, with the boat maintaining a compass heading? Am I wrong or just being a big fat pedant,the compass card doesn't move in relation to the boat.
I use my hand bearer a lot more than ' a bit if fun' , I use it constantly for pilotage,and I agree about the general lack of precision. Cheers Jerry
 
As long as you don't move about compass error is irrelevant as you are only using the relative difference in bearings.
Only trap that I know off : If you and the objects used are on the periphery of the same circle --- you can be anywhere on that periphery.
 
jerrytug said:
That's very interesting, I presume it's designed to use with a sextant, not a compass bearing?
I love my hand bearing compass and feel naked without it, but no success on steel vessels due to mega deviation obviously. That thing combined with a plastic sextant looks like a quite quick way to a 3 point fix if you can't use a compass. Food for thought, I'm just going to the "wanted" section...
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No not nessesseraly.

While the tecnique is called a horizontal sextant angle a sextant is not actualy required.
A sextant is very precise instrument which can mesure angles to decimil points of a minute of arc. you cannot plot a position to the sextans acuracy on a chart unless it a very large scale chart.
you can mesure the same horizontal angles by taking the difference in the three compas bearings, the actual bearings don't matter is the angles which matter so varriation and deviation don't matter
you can use a hand compass,

once you plot the position you can determin your compass error by comparing the true bearing from your position with the compas bearings.
 
Right about deviation - it doesn't matter either assuming the boat remains on the same course & you remain in the same place while you take the sights.
 
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