Astro Compass Swinging

[subtuna]

Hi,
I am just entering the new and exciting world of Ocean sailing and using Astro Nav. Which whilst I have GPS etc I am really looking forward to getting out there and playing with a Sextant for real.
After taking my YM Ocean theory Im now in the position of having a little knowledge and no experience. Still we all have to start somewhere.
Im writting to ask if any of you seasoned sea dogs out there could explain the Compass Swinging procedure using an Azimuth. We covered it very quickly on the theory course and I don't think I truly understood.
The rest of it I cant wait to put into practise.
Many Thanks in advance for your help.


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[snowleopard]

it's not actually swinging the compass but checking the accuracy on a single heading. you use the 'Amplitude' or 'sun's true bearing at sun rise/set' table to tell you what the bearing of the sun is at sunrise or set. when the sun is apparently half its diameter above the horizon it's centre is in fact on the horizon (allowance for refraction).

take a compass bearing of the sun, apply variation and the remaining difference between compass and calcualtion is your deviation. eeasier to do than to describe.

there are several ways of taking the bearing: if you have a compass with a shadow pin in the centre of the card it is easy. on ships they have an azimuth ring that lets you take a bearing off the main compass. a few people have a pelorus.

a less satisfactory method is to point the boat at the sun but this doesn't give a deviation for your current heading. it will however let you know if there has been a change in deviation.

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[John_Lana]

You can swing the compass this way as well - all you have to do is to sail on various headings while taking a bearing of the sun on each heading. Normally though, on a passage, all you are interested in is any compass error on your particular heading. Back in the "good old days", it was part of the morning routine to take an azimuth bearing of the sun and check the compass.

John

<hr width=100% size=1><A target="_blank" HREF=http://www.john-lana.com>http://www.john-lana.com</A> Mainly about boats, but also a bit about my other toy, my Caterham 7

 

[Robin]

We did a complete swing of our compass on our last boat using a Davies plastic Pelorus (from West Marine in the USA) and a shadow pin. Suns bearing was easily obtained using the sun sight program published years ago in PBO which we had loaded into a Casio mini computer, time obtained from the GPS set. We built up the deviation card over a period, taking advantage of any berths or fore/aft moorings that gave a good steady heading at various bearings, rather than motoring round in circles.

On our current boat I haven't done this yet, but we did an initial swing (that has proved pretty good) at slack water neaps in a fairly tideless bay using a dGPS. Some headings have been subsequently confirmed by transits since we found as much as 9 degs deviation on an easterly heading. One day I will do the sun sight swing again when I get that Round Tuit that SWMBO promised for Chistmas. By the by, previous owners (YM Ocean Instructors) said there was no noticeable deviation hence there was no deviation card, the log was also under reading by around 1 knot when we calibrated it and this despite the boat having completed the AZAB and a tour of the Med (which must have had some trips with an East heading!). Just shows the power of modern electronic position finding devices....

<hr width=100% size=1><font size=1>Sermons from my pulpit are with tongue firmly in cheek and come with no warranty!</font size=1>

 

[Mirelle]

To SWING the compass using an azimuth of the sun you will almost certainly need a pelorus, because it is most unlikely that the sun will be visible from the boat's compass on all headings, even if the compass has a shadow pin (many, including the Sestrel Major and Moore, do) or (very much less likely, indeed I have never seen one on a yacht) an azimuth ring.

Professional compass adjusters, a body of men who should be encouraged, but whom we yotties seem to try to avoid, often do use an azimuth and a pelorus to correct a yacht's compass. The drawback to the method is the usual one with all astro-navigation i.e. those big fluffy things up in the sky, indeed even more so because unlike a sun snap you need to see the sun on each heading, all the way round.

To CHECK the compass, on a given heading, is much easier; you need to know when and where you are, of course, but these days the GPS will tell you that!

<hr width=100% size=1>Que scais-je?<P ID="edit"><FONT SIZE=-1>Edited by Mirelle on 16/07/2004 11:57 (server time).</FONT></P>

 

[Robin]

That was why I bought the Pelorus, I had tried to make one in true PBO Blue Peter fashion (compass rose from an old chart, bit of plywood, some perspex strip to make the sight arm), then saw the Davies version in the West catalogue and brought one back next trip. It sits up nice and high on 3 mounts, in our case screwed on the mainhatch, and is detachable, it is even gimballed with water used as a counterweight. We had previously had the services of a professional compass adjuster, his first effort was way off as we confirmed the next weekend by missing Cherbourg and checked on a Cherbourg transit. He returned and repeated the swing, having accused us of 'moving something' which we had not, this time it was better but still out so I decided to do it myself since I really didn't trust it. In this case the irony was the original un-swung compass was probably OK, the adjuster man used the correctors and when I re-swung it I didn't zero or remove the correctors, so my (correct) card had deviation of +/- 3 degs or so whereas without the compensator magnets who knows it might have been very near zero.



<hr width=100% size=1><font size=1>Sermons from my pulpit are with tongue firmly in cheek and come with no warranty!</font size=1>

 

[Mirelle]

I've had the opposite experience with our local adjuster, whose task was not made easier by my installing a vertical steel rod (gear change linkage - ancient engine!) adjacent to the compass!

Anyway, do you reckon we've answered the question? Here's another attempt:

1. Find out when and where you are.

2. Armed with this information, the Almanac will tell you where the Sun is.

In practice, the Sun is infinitely distant, and magnetic compasses are not interested in minutes and seconds of arc, so;

3. Do a sight reduction using your actual position as the AP and any simple round number for the Altitude and out comes the Azimuth.

<hr width=100% size=1>Que scais-je?

 

[bluejuice]

many of the replys to this post have missed the point a bit i feel as surely what we all use the compass swing by azmuth techinque merly to check weekly daily etc our extng deviation card on a long passage. to ensure espacally when really the sextant is out we end up with acrate ep's. and for this the specailst equipment is not nesscary in particular.???

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