Compass swinging

[pessimist]

We've just had some bearings replaced in the steering gearbox which necessitated removal of the pedestal and compass. Obviously received wisdom says we should at least draw up a deviation card when the compass is replaced. As we have switched almost entirely to electronic nav. (with several backups) should we bother or should we regard the compass as largely ornamental? Should mention that we still carry a hand bearing compass, but I can't remember the last time it was used in anger.

 

[lw395]

I would probably check it at a few points to see whether it's miles out.
However much you prefer the GPS etc, it can be re-assuring to know that the compass is 'near enough'.

 

[Deleted member 36384]

I understand where you are coming from. Last year we did have recourse to steer using the compass but only as a reference, the actual number was irrelevant i.e. deviation and variation was not used. I think it is relatively straight forward to do it yourself and produce a deviation card, maybe a bit more difficult to adjust the flinders bar to reduce or eliminate the effects of deviation. As a back up, perhaps just swing it yourself.

A colleague made his own pelorus and mounted it on the central line of the yacht, just on the companionway hatch cover, and swung his compass. As a refinement he mounted it on a camera tripod directly over the compass binnacle and uses that now. He runs a sailing school boat and does his own.

Is it worth it? Probably not for the majority of types of sailing in familiar waters, coastal jaunts in reasonable weather with good electronics. Having said that, after my refit, I am paying a professional adjuster to swing mine. To save on the travel costs he will do it when he has number fo clients in our area, portal;y next month, about £350 quid for me on the West Coast of Scotland.

 

[zoidberg]

.....about £350 quid for me on the West Coast of Scotland.

Strewth!

 

[Biggles Wader]

I reckon lord kelvin has got you by the balls.

 

[lw395]

Find a couple of transits where you can postion the boat. Ideally about 45 degrees different. Use distant land marks a long way apart.
Slowly turn the boat on the transit. One person calls out when e.g sighting across the shrouds lines up, while the other watches the compass.
By sighting both ways fore and aft and amidships, each transit gives you four points.
If everything is within say 5 degrees, be happy.
If things are badly out, re-check the boat for stray bits of iron and consider doing more work.

 

[Skylark]

A colleague made his own pelorus and mounted it on the central line of the yacht, just on the companionway hatch cover, and swung his compass. As a refinement he mounted it on a camera tripod directly over the compass binnacle and uses that now. He runs a sailing school boat and does his own.

Sorry for thread drift but in the interests of us PBOs do you think that you could persuade him to take and post a few pictures (please)

 

[RichardS]

Find a couple of transits where you can postion the boat. Ideally about 45 degrees different. Use distant land marks a long way apart.
Slowly turn the boat on the transit. One person calls out when e.g sighting across the shrouds lines up, while the other watches the compass.
By sighting both ways fore and aft and amidships, each transit gives you four points.
If everything is within say 5 degrees, be happy.
If things are badly out, re-check the boat for stray bits of iron and consider doing more work.

Why not just check it against the gyrocompass?

Richard

 

[Deleted member 36384]

Sorry for thread drift but in the interests of us PBOs do you think that you could persuade him to take and post a few pictures (please)

That was a long time ago. The device was just plywood, a protractor scale marked out and a couple of panel pins in the swivel to line up. Simple stuff.

 

[AndrewB]

Before the days of GPS, for long-distance cruising a really accurate compass was a must. I used to correct mine about once every six months (while in use), and I will describe how it was done. Of course, these days having the compass this accurate is rather OTT.

Does your compass have correcting magnets? All good ones do, usually two pairs located in the base of the compass. These should be set first. For this you need to find somewhere at sea that has both a convenient north-south transit and an east-west transit. These do not need to be spot-on, but should be within about 10 degrees of these directions. You will measure the exact (magnetic) bearings of these transits.

Choose a time when there are not strong currents in the area. Then:

1. Steady the boat on the northwards transit. (Line up the mast, forepeak, and two transit marks). If the compass doesn't read the same as the transit, adjust the athwartship correction magnets until they are the same.
2. Steady the boat on the eastwards transit - if the compass doesn't read the same as the transit, adjust the fore & aft correction magnets until they are the same.
3. Steady the boat on the southwards transit (or if necessary a back transit on the northwards transit) - if the compass doesn't read the same as the transit, adjust out half the difference with the athwartship correction magnets.
4. Steady the boat on the westwards transit (or if necessary a back transit on the eastwards transit) - if the compass doesn't read the same as the transit, adjust out half the difference with fore & aft correction magnets.

The compass is now corrected as far as the correction magnets will allow. Go round the four transits again to check and get measures of deviation. If any are 5 deg or more out, you need to add further correcting needle magnets - at this point I'd recommend professional advice.

Otherwise you now need to draw up a deviation curve. You have four deviations already but you need more. See if you can find further transits that are roughly NW-SE and NE-SW. Sail along these and again note the deviations. (If you don't have all these transits convenient, of course the process can be spread over several weekend sails).

With eight deviations you have enough data to draw the curve. You can just plot them and join the dots, but it will be better to formally fit a sine curve of the type y = a*sin(b.x) + c, where x is the actual bearing, y is the measured deviation, a, b, & c are constants summarising the deviation. This can be done with Excel, and there are several descriptions on the net, perhaps best is https://jkp-ads.com/Articles/leastsquares.asp. (P.S. If the constant c is much different from zero, it may be a warning that the pivot on your compass is shot).

You will probably have done all the testing under power, but it is worth checking under sail (ideally repeating the process under sail) as there can easily be a degree or two difference. There can be far more deviation when sailing on a heel, it is a good idea to measure what this is on each tack.

I'm sure many people will think that using transits is excessive, and that checking the compass against GPS (allowing for variation) is adequate. However, my experience over many years is that using transits does give by far the most accurate results. Only don't use floating buoys for transits, as their position moves.

 

[lw395]

Why not just check it against the gyrocompass?

Richard

That's always an option, if you have a gyro compass.
I don't.
I don't think I've seen one on a leisure boat.

 

[RichardS]

That's always an option, if you have a gyro compass.
I don't.
I don't think I've seen one on a leisure boat.

Ah .... OK. I thought that all boats with electronics nav like the OP's and mine had a gyro or fluxgate compass which could be used to check against the magnetic compass but I stand corrected.

Richard

 

[johnalison]

You can get a pretty fair idea of how the compass is doing by taking bearings with the hand-bearing compass. You can start by doing it from another position such as the foredeck but from somewhere on shore would be better.

 

[lw395]

Ah .... OK. I thought that all boats with electronics nav like the OP's and mine had a gyro or fluxgate compass which could be used to check against the magnetic compass but I stand corrected.

Richard

We have a fluxgate compass.
Depending on what mood it's in when we try to adjust it, it's deviation may be better or worse or just 'different' than the uncorrected Plastimo steering and tactical compasses. It has an adjustment procedure which consists of motoring steadily in circles while pushing sequences of buttons and chanting passages from the manual.
It's the electronic compass which we normally check against transits.
I did a couple of trips on a steel boat, which had an electronic compass with the sensor half way up the mast. Checking that was a good astro-nav exercise.
You can also try taking GPS positions at bow and stern...

 

[zoidberg]

£350 for a simple check swing and correction card! Sheesh!

Try doing a compass calibration swing on a Vulcan, with four engines running and a gale of cold Lincolnshire winter wind blowing up your trousers..... The comparison was made against a Dumpy Level or sometimes a theodolite. That had to be read to within 100th of a degree on 8, sometimes 16, headings. Vibrations from aircraft taking off a mile away made the scale unreadable, until it settled down again. Not a pleasant job.....

 

[Deleted member 36384]

£350 for a simple check swing and correction card! Sheesh!

Try doing a compass calibration swing on a Vulcan, with four engines running and a gale of cold Lincolnshire winter wind blowing up your trousers..... The comparison was made against a Dumpy Level or sometimes a theodolite. That had to be read to within 100th of a degree on 8, sometimes 16, headings. Vibrations from aircraft taking off a mile away made the scale unreadable, until it settled down again. Not a pleasant job.....

I think £350 is a snip. Your fuel bill for 1 seconds worth of engine running time was probably more than that. To be honest it is a rip off as the guy is coming over from the east coast and staying overnight, even though he will be grouping the work with other west coast vessels. He is certified and has all the tools, magnets and skill to do the job. The pedestal, binnacle and compass have all be removed multiple times during the refit, so a one of hit to sort it out is acceptable to me.

I am waiting quotes from another Aberdeen swinger but I suspect it may be the same person who works for a couple of companies.

 

[Resolution]

We have a fluxgate compass.
It has an adjustment procedure which consists of motoring steadily in circles while pushing sequences of buttons and chanting passages from the manual.
.

I recognise that! Often interrupted by a helpful motor boat coming over to ask if you are all right.:encouragement:

 

[NormanS]

Back in the day, I made a simple pelorus. If I remember correctly it was made out of one side of a small plywood cable drum, marked off every 10°, plus some bits and pieces to give sights etc. One of us sat with it on the wheelhouse roof, taking bearings on a distant transit, while the other carefully steered in increments of 10°. Since this was done in Loch Drumbuie, a popular anchorage, and using the distant south shore of Ardnamurchan, seen through the narrow entrance to the loch, as the transit, we did get some funny looks.

Nowadays, though we still use a compass to help with steering at times, it isn't used much for its former navigational purpose. Sadly, I suppose, satellites and fluxgate compasses have largely taken over.

 

[zoidberg]

Your fuel bill for 1 seconds worth of engine running time was probably more than that.

My dear B'O'B, I would not dream of dissuading you. I'm delighted that yet another disciple has the enlightenment..... It does grieve me, though, as an expatriate and impecunious Weegie, to think that a submariner or flyboy navigator from your neck o' the woods would probably do the job in question for, oh, an order of magnitude less mullah and a bottle of Laphroaig.

I'm thinking I might find another late-in-life 'calling'.....

Mind you, should you suffer the misfortune and indignity of stranding your goodself on the likes of the Cleit Rock sometime henceforward, do not underestimate the likely clamour from the nor'nmost BlueMoment mob of reprobates or, indeed, some from around here. You will then have few excuses.

 

[zoidberg]

Back in the day, I made a simple pelorus..... this was done in Loch Drumbuie, a popular anchorage, and using the distant south shore of Ardnamurchan, .

I can't think of a better place to while away an hour or two peacefully 'compass swinging', than Loch na Droma Buidhe. If one can see Ardnamurchan, then it's a fine, fine day.....