Times, they are a changin' (or are they?)

[Little_Russel]

Times, they are a changin\' (or are they?)

I am off to get the new compass swung tomorrow as it is the seamanlike thing to do. However, is it strictly essential in this era of GPS chart-plotters et al. One of which I happen to have by the way. How many of you have your compass deviation cards blu-tacked up over the chart-table? Even if you have, how often do you refer to it?

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[Ohdrat]

Re: Changing Times ...so are the Poles!

<A target="_blank" HREF=http://www.ybw.com/cgi-bin/forums/showflat.pl?Cat=&Board=ym&Number=348774&page=0&view=collapsed&sb=5&o=31&part=>Magnetic Flips</A>

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[Twister_Ken]

Re: Times, they are a changin\' (or are they?)

Must have sailed and navigated on 20 or more yots, and I ain't never seen a deviation card except in books about navigation.

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[BrianJ]

Re: Changing Times ...so are the Poles!

Not to mention the Russians
BrianJ

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[Peppermint]

True

Charter & Sea School boats always had a card but not any more.

I check mine, each year, on transits to see if any new equipment has screwed it up.

I would only get the swingers in if it needed major adjustment.

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[ccscott49]

Re: Times, they are a changin\' (or are they?)

I have one, but my compass is never more than 1 degree out, even the autopilot can't steer to that accuracy and I wouldn't want it to, it would use too much power, I prefer to allow a little wandering allowing for the sea state.

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[AndrewB]

1 degree deviation? What about 35 degrees!

Checked mine a couple of weeks ago, 35 degrees deviation. That's not uncommon after a winter ashore working on my steel yacht. After a lightning strike a few years ago it was even higher.

'Swung' her last weekend by adjusting the correction magnets (new ones, thanks to Mirelle) and taking bearings on transits. 6 degrees deviation - not perfect but good enough. Last season it was 12 degrees. Deviation curve calculated yesterday evening will be pinned above nav station in place of the old one.

In practice we steer to the GPS. As I think a conventional compass is easier for the helmsman to steer by, we work by the navigator calling course corrections (e.g: turn 10 degrees starboard) rather than giving a compass course to steer. That way deviation isn't really an issue -- unless of course the GPS packs up.

GRP yachts don't have such a problem but no magnetic compass I've ever personally checked on board a yacht has been as low as 1 degree on all headings.

 

[Twister_Ken]

yes but

>GRP yachts don't have such a problem but no magnetic compass I've ever personally checked on board a yacht has been as low as 1 degree on all headings.<

As the average yotdriver has difficulty holding a course to more than about 10 degs accuracy, deviation - unless very major - is a minor problem

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[ccscott49]

Re: 1 degree deviation? What about 35 degrees!

Seriously, I had mine swung and proffesionally adjusted before I left uk, no more than 1 degree, always around the east-west axis, as magnetic interference is always strongest the further you go away from north-south. Next time I'm on the boat I'll scan the deviation card and post it. Wooden boat and well positioned. I do subsurface surveying, so know a thing or two about compasses and magnetic anomalies, not to mention gyro stabilized accelerometer platforms.

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[ccscott49]

Re: yes but

Just as I said, didn't I?

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[AndrewB]

No but

Watching a helmsman in a bit of a sea, its true the compass swings all over the place, 10 maybe 20 degrees either side. But a reasonable helmsman will do some kind of job of averaging out. And even if it's not accurate (many tend to err on the up-wind side as a result of weather helm), a good navigator can check the bias and allow for it. Between them, I expect accuracy to within 3 degrees on a steady course. After all, a 10 degree steering error implies 10 miles out in 60 travelled. Surely we can do better than that .... can't we?


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[ccscott49]

Re: No but

It's true a good course can be held. allowing for sea etc, I totally agree

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[AndrewB]

Re: 1 degree deviation? What about 35 degrees!

Yours may well have been that accurate when it was professionally swung. That's not unusual for a good compass. But what was it like 12 months later? And just how many people have had their compass swung professionally in the last 12 months?

Even then, unless the compass was swung in position on the boat, and checked against transits while under way as part of the process, the compass may be perfectly accurate "on the bench", but it won't be capable of delivering that accuracy in use.

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[tome]

Devious Variation

I did a seismic survey off Gabon some years ago where the magnetic variation (declination in survey speak) changed by almost 5 degrees over a 40km distance! Played havoc with our streamer positioning - each streamer has a calibrated fluxgate compass every 300m, and the 4 streamers were each 4.5km long with a GPS tailbuoy on the end.

In the end we had to measure it and then apply a dynamic correction, which gave our software boys ashore a bit of a challenge.

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[craigbalsillie]

Another Newby Question..

What does it mean "to have your compass swung.."?

sounds painfull..

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[ccscott49]

Re: 1 degree deviation? What about 35 degrees!

Mine appears, from checks I've made to be pretty accurate, but as you say things change, usually rapidly! We check the declination here every 24 hours!! I don't want to drill into another well producing 5000 bbls a day, it wouldn't be conjusive to longevity of employment, not to mention health!!

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[Twister_Ken]

Physchology of yottigation.

>Surely we can do better than that .... can't we?<

That's the secret of good yottigation (or it was before GPS). Allow for the helmsmen's errors, then come up with a line that goes something like "Mmmm, The cross tide seems a bit stronger than the tables. I reckon you can come off another 5 degress to port" (coz you know he's been steering high). Then when Barfleur light pops up just where you said it would be eveyone thinks that Vasco's done a good job tonight, while the helmsman (helmsmen) think - "Gee, I can really steer a good course". But you've got to make a correction without blaming them, coz they go all prima donna-ish.

And you can't just allow for the fact they're going to aim high in the first place, and say nowt, because then they think "I knew the course was higher than the one Vasco gave. Useless sod."

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[bigmart]

Re: Another Newby Question..

The metal parts (engine, Keels & other bits) of a boat affect the accuracy of a compass depending on which way the boat is pointing. The effects can be counteracted to some degree by placing small magnets around the compass.

The compass is swung by placing the boat in an area where the position of the boat can be checked, usually with some transits so the boat can be positioned accurately. The boat is turned through all the points of the compass & the inaccuracy(Deviation) is checked. The magnets are placed to make the deviation as small as is practical & a chart showing the deviation in each direction is drawn up. This chart is known as a deviation card & usually looks something like a vertical Sine Wave. By applying the corrections from the deviation card to the compass readings a more accurate course can be plotted.

Obviously a steel boat suffers more from this problem. As you will see from other posts here, on glass fibre boats the deviation is more than likely to be less than the helmsman can practically steer.

Sorry for the long reply but I hope that helps.

Martin

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[Mirelle]

Just how many people have had their compass swung?

Well, I have. Wooden boat, with old fashioned compass in a brass binnacle; the compass is on the bridge deck over the engine and adjacent to a vertical mild steel rod which operates the gearbox (see MD2 thread for more gory details...)

Deviation was 25 degrees, is now 2 degrees until the next time when I do something absent minded with a tin can....

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[craigbalsillie]

Re: Another Newby Question..

Thanks for that. I don't care how long the replies are to my Newby Question posts.. It's all about learning for me at the moment.. Thanks again..



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