swinging a compass

[surekandoo]

What is the correct procedure for swinging a compass?

Whilst sailing last weekend I was able to compare the readings of my main bulkhead mounted compass, my GPS, the fluxgate compass in my autohelm and the compass in my Fuji binoculars. Non of them agreed with each other! There appeared to be +/- 20 degrees difference across them all.

If the boat is, say, motoring at slack water there should be no leeway, so shouldn't the main compass and the GPS agree on the boats heading?

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[Talbot]

Difference between main compass and GPS:

is the GPS set to True or magnetic North. If Mag north, the GPS and Fluxgate should agree.

Main compass and Fluxgate - the difference should be deviation (assuming the fluxgate allows for it, and you have set it up correctly.

Main compass and binos - the difference will be deviation. There will be a different deviation at different places in the boat. Try to establish a neutral spot for taking visual bearings through the binos.

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[Talbot]

There is an exhaustive USCG procedure for swinging the compass at <A target="_blank" HREF=http://www.uscg.mil/hq/g-o/cgaux/Publications/Manuals/boatcrew/chap14-c.pdf> here </A>

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[surekandoo]

Thanks Talbot, that should do the trick.

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[AndrewB]

The USCG paper describes a number of methods, and there are others.

For the main steering compass, I have found the method of multiple ranges (para C16), also known as multiple transits, the most reliable and would recommend it if possible. It is best done in a large harbour such as Plymouth Sound with natural transits in all directions, and requires a good sized chart. Ideally you need about 8 spread around the compass. Avoid 'steering away' transits which are harder to establish in a yacht, also don't use floating buoys for transits, specially those close by. Sail along the transit so the mast and bow are exactly in line with the transit, keep pointing at the distant mark if you slip a little off. Take several readings of each transit, ideally with different helmsmen.

The resulting points rarely form such a nice smooth curve quite as neatly as shown for example in fig 15c. It may be preferable to fit a sine curve to your observations rather than try to connect up the points as they stand - programs for this can be found on the web.

If deviation exceeds 10 deg on any heading, the compass will need adjusting. Para C17 explains the method, assuming you have a compass with easily accessible correcting magnets. Turn them into the neutral position (poles up and down) before starting. Any two established transits at approximately right angles to one another can be used for this process.



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[graham]

When I see a transit come into line I stick the boats head onto it for a moment and record what the compass says.The true bearing can be checked on the chart later.

Once variation is allowed for ,over a time you can get a deviation card made up like this without the need for hours of farting about swinging the compass.

 

[William_H]

It is highly unlikely that the bino compass has errors (as it should have no corrections) in an environment free from other magnetic interference. Try taking it into the dinghy on a long painter out the stern. Check there is nothing in the dinghy to cause errors ie iron work, radios, motors etc.
You could also try just hanging off the sternrail as far as possible. Use this compass to roam around the boat and you may find areas of errors. The main compass may have correctors which are incorrectly adjusted. I was once told that a compass mounted centre in the cockpit over the engine can have errors when heeling on either tack because the engine in the vertical plane is either to port or to starboard of the place where the compass was checked ie middle. I don't know if any one has noticed this effect. regards will

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[pampas]

Simple answer is NO your main compass will have devation and the gps shows variation compensated for the sailing area.
This difference I have used by slowly motoring at 45 degree angles to plot a polar chart, compared to doing it with the hand held, only slight differences show up, by averaging the curves I now have a polar curve the fits my requirements.

I am not an expert by any means, but did it to satisfy my curious nature. Tip. Do not do this if you are near a large iron mass on land, oil refinery or dockyard I give the local dockyard at least 1.5 miles clearance.


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