Compass swinging...best method

[starboard]

Looking to produce a Compass deviation card ( DTI requirement) I have my own thoughts i.e. using a good quality hand bearing compass away from all magnetic influence to compare error on ships compass, or using GPS/Track plotter to track accurate magnetic couses to compare error every 30 degree's.......I would be keen to listen to any good solid idea's and views on subject.

Paul.

[fluffc]

I used a crude (but accurate) pelorous to take a relative angle of conspiuous church tower, and known (gps) positon to determine precise boat heading. My boat being ferro had quite a lot of deviation - up to 20 degrees. In this instance I seem to remember doing it in 10 degree increments.

[Phoenix of Hamble]

the traditional method of crossing a transit at various angles is possible with a 'dome' style compass, and probably easiest... basically cross the transit at 0deg, 30 deg, 60 deg etc etc, and read off the bearing towards the transit as you cross it... then compare with what it should be, and there is your deviation. You can also do this at anchor by pulling the boat round in a circle using the tender... but if your compass is bulkhead mounted then you are stuck with either using the handheld (best in a dinghy behind the boat to get an accurate reading), or finding several transits....

I wouldn't use the GPS.... bearings are never that accurate in my experience when taken from a GPS... you could easily change by 5 deg, and it wouldn't catch up quickly enough to avoid a significant error being made in your measurements...

One other thing.. don't know how old your boat is but when I swung my compass, I was suprised to find upto 2 deg difference between deviation with the egine running and the engine off.... worth perhaps checking, as I suspect it was due to an aging lump of an engine, and not great electrical systems

[tazzle]

Remember doing this donkey's years ago at a fave spot on the Orwell where you could moor up to a buoy and swing her round with the tender lining up church towers, electric pylons etc and comparing compass with chart (after var) - seem to remember getting about a dozen bearings to make a dev card from.

[landaftaf]

one thing to remember is to do the compass swing in your normal sea going state - or do several to suit your sailing mode (i.e. with or without local magnetic influences)

eg - if you have a tender with a big chumk of metal on it near the compass when under way, and you use it to swing the boat during the swing, then you compass correction readings will be a nonsense. [img]/forums/images/graemlins/cool.gif[/img]

[Phoenix of Hamble]

Yes, good point.... in my case, it would be the outboard on its bracket being absent if I used it to tow the boat about....

As an aside, I prefer the anchor/mooring approach, using the tender to turn the boat, as you have much more time to let the compass settle, and get an accurate bearing.....

[BradC]

I'm doing mine progressively, first I checked that there wasn't much effect on the hand bearing compass standing at the stern of the cockpit by steering a circle whilst checking bearing on a distant fixed point. At the back of my daily log jotter I've made 4 columns headed hand, port, starboard and deg. of heal(2 x bulkhead compasses) I check heading at every opportunity whilst I'm out sailing. It doesn't matter that they are not consecutive bearings as i can collate them later. It's tricky though, needs two people, one on hand and one at bulkhead. I'm also using known transits when they crop up.

[lenseman]

[QUOTE FROM MAGNA CARTA]
I was suprised to find upto 2 deg difference between deviation with the egine running and the engine off.... worth perhaps checking, as I suspect it was due to an aging lump of an engine, and not great electrical systems
[END QUOTE]

Very valid point, and also Swing with you anchor in its stowed position as if on passage for exactly the same reason.

And finally, do check what happens to your compass when you turn on any major electrical supply in the cabin below. Sometimes the wiring runs very close to the binnicle and can effect it greatly. This can be tested alongside a pontoon even before you find a good location to swing.

Try turning on your radar nav lights, engine, generator and various other loads whilst carefully watching the compass.

[bilbobaggins]

Have just looked at your 'bio' and, if this is not a troll, then I'm a monkey's uncle!

FWIW, do please remember that Compass Correction was and still is a marine profession, and there's a fair bit more to it than you pick up on your average RYA nightschool course.

However, you could A) buy the book 'Practical Guide To Compass Correction' or similar B) find a crusty old yacht club member in the bar who used to do compass swings as part of his living and slip him a few G&Ts to do yours C) find a crusty old forum member who used to do compass swings as part of his living - such as moi! - and drag him off for a weekend on your boat to do yours.

I'm betting that you know several qualifiers from group B.....

As for -
[ QUOTE ]
using a good quality hand bearing compass away from all magnetic influence

[/ QUOTE ] - that simply won't do. Just how do you determine with any confidence that your HBC is 'away from all magnetic influences'? You cannot unless and until you do a check swing.

The question really becomes - do you want the confidence of knowing your compass heading, corrected to a degree, for use when it's clamped-in; or do you just want bragging rights that you did it yourself and 'didn't need all that palaver'?

- And be wary of doing a 'swing for deviation' on a laid mooring. The riser and ground chains are certain to be deviating influences. Perhaps your best bet is borrowing an anchor, short chain and long warp. Using that keeps your own anchor and chain in its normal sailing position.

- Oh, and do check the wiring to your steering compass light. If it is other than 'twisted pair', you are likely to find that THAT is a deviating influence when powered.

"And another thing...." mostly for t'others on 'ere.....

- if you're chartering, or doing a course, on a Coded Boat, then check the validity of the legal-requirement compass deviation card, which should be fitted adjacent to the chart table. If it's out-of-date or missing, the boat is technically illegal. That alone might get you a severe reduction in charter fee but ( much more important ) go then and check the date-validity of the supplied lifejackets, fire extinguishers and liferaft. Any charterer/operator who is so cavalier about the requirements of the Code as to have a visible and easily-checked thing like an invalid compass card is likely to be just as careless about the other, more crucial - but hidden - stuff.

Er, that's not in the Syllabus either......

[img]/forums/images/graemlins/wink.gif[/img] [img]/forums/images/graemlins/wink.gif[/img]

[Steve Clayton]

[ QUOTE ]
- if you're chartering,......on a Coded Boat, then check the validity of the legal-requirement compass deviation card,

[/ QUOTE ]
Clarification: in Spain if <=20 miles from a safe haven then a deviation card is not a requirement. >20 miles and it is.

Under "Portsmouth" coding (assuming this is still a valid code) is a deviation card a legal requirement for charter boat?

Steve
http://www.seraph-sailing.com