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. /forums/images/graemlins/cool.gif

 

[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.....

 

[KRG]

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......

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[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
www.seraph-sailing.com

 

[Andrew_Fanner]

Don't have a deviation card per se, but then being a river based mobo most of the tijme this is of limited sgnificance What I have done while out in the Estuary with usual cruising gear aboard, is to compare plotter, H/H and compass on many headings and make note of the differences and work out an indication of deviation thereby. I factor into this my own inability to hold a course within 10 degrees for any length of time without extreme concentration and have arrived at the premise that most of the time a deviation card is of academic significance. Having said all that, I rather enjoy navigation as an intellectual exercise in itself so sorting it out as an approximation was quite fun.

And the real bonus?

"I think the compass needs checking dear, I need to get down beyond Southend. I'll take [beer buddies] as crew and a couple of days leave."

 

[peteandthira]

Are there any issues to be wary of by NOT having swung yer main compass? (Admiral, for instance!!)

I can't be ar$ed and consider it to be accurate enough for my ability to steer in any straight line for any length of time.

If "George" is steering, then his deviation is only 1 degree anyway according to Raymarine's elecrtronics.

 

[Pye_End]

My first boat, I discovered, was in excess of 15 degrees out when I bought her. Bringing her round from Hythe to the Blackwater was surprisingly hard with this sort of error (pre electronic navigation days). It was also my first big trip so took me a while to twig.

Getting to the nearest one degree may be considered ireelevant, but it is worth checking once in a while that you don't have much bigger errors creeping in! I suppose with GPS's we all check these things without really thinking about it.

Raymarine is a good point. My autopilot has its compas under a berth, and am very careful to put clothing near it, but it wouldn't take much forget one day.

In answer to the post, I do mine agains a hand bearing, but double check by checking the odd transt as well.

 

[srm]

If you have a dome compass in a fairly clear area with a vertical pin in the centre of the card use the sun's shadow from the pin onto the card (early morning or late evening).

The shadow gives the reciprocal of the sun's bearing so all that is needed is a slow circle noting heading, shadow bearing and time (UTC) on as many headings as you can. (every 10 degrees is practical and improves accuracy).

Time is important as you then calculate true bearing of sun at start and finish time (it should only vary by two or three degrees if you havn't taken too long). Plot sun bearing against time on a sheet of graph paper then all the intervening bearings can be taken off the graph. Time interval is short enough to assume a straight line graph.

Now draw a graph of deviation against heading. There will be gaps where the compass card was in shadow but these can usually be filled in from the curve on the graph. Make your deviation card from the graph.

It may sound complicated, but I used this method regularly on a steel hulled yacht and found it a lot quicker, easier, and more accurate than transits, pelorus, ot hand bearing compass methods. Also, I could do it single handed. I would make at least two full swings noting bearings at 10 degree intervals in 10 to 15 minutes. Once back on the mooring it took about 15 to 20 minutes for the two calculations and graphs.

If you don't have a nautical almanac sun's GHA and dec are available on the web, as are formula for calculating true azimuth.

Only disadvantage was having to wait for a sunny morning or evening, helps if you live near the mooring / marina.

Have fun.

 

[bilbobaggins]

Few recreational yotties can be bothered with checking the accuracy of their steering compass, for a variety of reasons. OK, no big deal.

But go far offshore, or have to plot accurately in fog or driving rain, and the requirement becomes more significant. That's why the RYA insist on competence in compass error checking by Sun's Amplitude in the YM Ocean syllabus and during the submitted 'ocean voyage'. Further, all MCA 'Coded Vessels' - which include Small Commercial Sailing Vessels ....that's British Flagged sailing schools and charter operations, to thee and me.....

See here - MCA Site

- must by law comply. That means inter alia an approved and in-date Compass Error Card.

Also, all UK Flagged vessels over 13.7 metres must comply with the relevant provisions of the Merchant Shipping Act. ( Follow the Pleasure Craft link on the above page ). Consider those together with the provisions of your insurance contract...... this may well be relevant to a recent well-publicised - and well-debated - incident to a >13.7 metres boat near St Albans Head.

I'm a little surprised, on exploring his website, that Steve Clayton seems a little unaware of some of this requirement, as he clearly indicates his charter boat complies with the relevant UK legislation. Maybe I missed something..... or maybe he did.


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[Steve Clayton]

[ QUOTE ]
I'm a little surprised, on exploring his website, that Steve Clayton seems a little unaware of some of this requirement, as he clearly indicates his charter boat complies with the relevant UK legislation. Maybe I missed something..... or maybe he did.

[/ QUOTE ]
No, don't think I've missed anything. (and thank you for giving me the opportunity to reply and the free advertising!). I haven't said that I haven't got a deviation card for Seraph. Seraph is UK MCA coded, Spanish inspected, complies with their additional requirements, and I have the licence and the appropriate work permit to charter Seraph in Spanish waters. I also have an in-date deviation card for both compasses. My insurance is for charter activity. To take paying guests out on anything that does not fully comply with MCA and Spanish regs would invalidate both my insurance and public liability; it would be foolhardy and reckless of me to do anything less. With a UK flagged boat chartering in Spain I was merely pointing out that if you charter a boat in Spain then it may, or may not, by Spanish regs require a deviation card.

My other query was with regard to what is known as Portsmouth Coding; assuming that this is still in existence. A charter vessel that stays within the Solent and, as I recall between Bembridge and Hurst, has less stringent coding requirements and may not (and I don't know the yea or the nay) require a deviation card.

 

[bilbobaggins]

Glad we're all happy now. And the thanks for the free advertising is due to IPC Media plc.....

I remain quite certain that there are lots of owners of >13.7 metre UK-flagged boats who are content to remain blissfully unaware of their status, and obligations, under the Merchant Shipping Act - and some of whom are in for a rude shock if/when the MCA or MAIB choose to inspect 'em on the occasion of an 'incident'.

It is emerging that recreational boaters can expect to have 'the book' thrown at them if any marine incident causing hazard to others exposes transgressions in the MSA or SOLAS V obligations. That can have implications for insurance cover in respect of that incident.

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[starboard]

[ QUOTE ]
Have just looked at your 'bio' and, if this is not a troll, then I'm a monkey's uncle!
/forums/images/graemlins/wink.gif /forums/images/graemlins/wink.gif

[/ QUOTE ]

Hello you Monkeys Uncle....Sorry matey I have been around boats for too many years to think I should know it all!!! I find the input of others is always useful...yes ofcourse you will get the occasional no it all responding 9like your self) but to be honest when it comes to the sea you will never stop learning....never be affraid to ask, no matter how much experiance you have!!!

Paul.

 

[Mirelle]

Hire a Compass Adjuster

and remove all doubt, because he will correct the compass, re-swing and issue a signed deviation table with his firm's name on it, with residual deviation only.

Money very well spent.

 

[thefatlady]

Re: Hire a Compass Adjuster

Someone correct me, but isn’t the compass error a sine wave as in

error = c + a sinb

where
c= offset
a = amplitude
b = bearing

In which case all that is needed is two calibrated points to give two simultaneous equations to calculate c and a.

Then any other point can be calculated and/or a deviation card made up.