compass compensation

[dnv]

Hi there


My Suunto (see attached picture) deviates from 5 to 12 degrees, depending on the true heading. The compass has little magnets underneath for compensation.

I found advice on howto compensate here and there:
Compass Compensation
Ritchie Navigation - Compass Compensation
http://www.google.de/url?sa=t&source...k_4vBQ&cad=rja

Does anybody on here have other, maybe more simple, advice?

thanks for any help in advance, Jan

[Firefly]

[QUOTE=dnv;401737] Hi there


My Suunto (see attached picture) deviates from 5 to 12 degrees, depending on the true heading. The compass has little magnets underneath for compensation.

I found advice on howto compensate here and there:
Compass Compensation
Ritchie Navigation - Compass Compensation
http://www.google.de/url?sa=t&source...k_4vBQ&cad=rja

Does anybody on here have other, maybe more simple, advice?

thanks for any help in advance, Jan [/QUOTE


Compass deviation is the error introduced by external influences such as ferrous metal components, magnetic materials or electrical cables which are too close to the compass.

The proper procedure:

Once all such influences have been minimised the correct course of action is to check swing the compass. Using a calibrated hand bearing compass take readings from a position about 10 meters in front of the bow, looking down the centreline from the bow to the stern, with someone monitoring the boat compass. Take a reading on the boat compass on 0, 90, 180 and 270 degrees and write down the headings shown on the boat compass. Using the mini magnets, correct these errors remembering that that over correction will cause errors on the reciprocal. Correction with in 3 degrees should suffice.
Now do the same ever 45 degrees. Errors that can't be eliminated are entered on a "compass deviation card" fitted next to the compass. So, for example if there remains a +3 degree error on North, the deviation card will read heading 000 - steer 357 and so on. This card should start 000 and list headings every 45 degrees.

The quick procedure:
Minimise external influences. Using a GPS at slow speed in flat conditions, adjust your headings N,S,E,W and then check every 45 degrees. Remember GPS is true heading so allow for variation in your area. For any remaining small errors, if you wish, you can make up a deviation card as above.

Having said all that, if you can read your compass within 3 degrees at 30 kts in anything less than glass conditions, you're a better skipper than me.

Good luck

JW

[jwalker]

.....you're a better skipper than me.

Good luck

JW

Oi !!! Who's this JW fella?

[wavelength]

have a look at my post here some time back re adjusting
http://www.rib.net/forum/f8/compass-fitting-15771.html
the screws only turn a couple of bar things that are securely attached to the screwend rod so its no use screwing them in thinking they will adjust more as you turn the rod more-they just rotate thru 360 degrees if that makes sense--its early morning and a tad lumpy out here at sea so fully coherent sentences are not forthcoming

[wavelength]

Remember GPS is true heading so allow for variation in your area

or go into the gps menu and set it to magnetic instead of true, it will sort variation out by itself and may even occasionally agree with the compass
ho-hum 0300 hours

[Firefly]

Originally Posted by wavelength

or go into the gps menu and set it to magnetic instead of true, it will sort variation out by itself and may even occasionally agree with the compass
ho-hum 0300 hours

Wasn't sure that was possible, that's even easier.

[wavelength]

you must be on nights
We always run the gps on magnetic so as we are using the same bearings on compass and gps.
Daylight now and I can see its rough instead of just feeling that it is!

[biffer]

i've never been able to get a magnetic compass to perform that well on a rib, there are too many things to influence it in such a small area, if you have cable steering it will affect your compass when there is a larger amount of steering cable in the console at full lock, bags in the console that weren't there when you swung it, keys on the dash, throttle controller in gear(nearer the compass), the list goes on, you get it as near as you can and it will all change next year when it will need correcting again, i hate to say this being old school but electronic's are the way forward and a compass on a rib should be used for general reference

[Firefly]

Originally Posted by wavelength

you must be on nights
We always run the gps on magnetic so as we are using the same bearings on compass and gps.
Daylight now and I can see its rough instead of just feeling that it is!

That makes sense. However in my case, (small rib with small compass), in anything other than a mill pond sea, I confess to relying on the GPS hdg and trk with the compass as back-up.

Are you still at sea?

[Firefly]

Originally Posted by biffer

i've never been able to get a magnetic compass to perform that well on a rib, there are too many things to influence it in such a small area, if you have cable steering it will affect your compass when there is a larger amount of steering cable in the console at full lock, bags in the console that weren't there when you swung it, keys on the dash, throttle controller in gear(nearer the compass), the list goes on, you get it as near as you can and it will all change next year when it will need correcting again, i hate to say this being old school but electronic's are the way forward and a compass on a rib should be used for general reference

That, of course, is the reality. The theory makes interesting reading

[Firefly]

Originally Posted by biffer

i've never been able to get a magnetic compass to perform that well on a rib, there are too many things to influence it in such a small area, if you have cable steering it will affect your compass when there is a larger amount of steering cable in the console at full lock, bags in the console that weren't there when you swung it, keys on the dash, throttle controller in gear(nearer the compass), the list goes on, you get it as near as you can and it will all change next year when it will need correcting again, i hate to say this being old school but electronic's are the way forward and a compass on a rib should be used for general reference

That, of course, is the reality. I use the compass as back-up also.

[C-NUMB]

Good advices here, there is no easy cure. Even in professional shipping, magnetic compasses is an issue as various cargoes, change in draft of the vessel, type of cargo among others affects the result. That's one of the reason why gyros was invented, and magnetic compasses are used only as an emergency back up.

One option is to do only a rough deviation table, without any adjustments to the compass. That's easy to update over time. I have never sailed a smaller boat with a accurate magnetic compass and when You needs it most, the heading is going anyway +-15 degees
On paper charts its easy to note the magnetic compass reading when You are doing a specific route in controlled conditions. That is then useful data, but only for that specific course.

[wavelength]

In the great plan of things gps is a relatively new addition to boating, we had decca before that but for a long long time it was only available to commercial craft and was notorious for errors but we still got around without electronic nav.
Steering to a compass is a skill and needs practising when the vis is reasonable to gain confidence cos you really won't believe the compass when the fog drops. Of course we rely totally on gps these days, even yachtmaster exams use gps as the main mode of navigation and why not, time moves on and we should use what is available and not cling to the past....however when the gps fails (and one make we had was very good at failing as soon as it got a little damp and foggy) or your wiring lets you down (surely not ) then the compass should still be there needing no electrickery, albeit a few brain cells, to work it.
Thats why on advance courses we have folk steer to the compass at night with the gps switched off (well the one they can see ).

[Firefly]

Originally Posted by wavelength

Of course we rely totally on gps these days, even yachtmaster exams use gps as the main mode of navigation and why not, time moves on and we should use what is available and not cling to the past....however when the gps fails (and one make we had was very good at failing as soon as it got a little damp and foggy) or your wiring lets you down (surely not ) then the compass should still be there needing no electrickery, albeit a few brain cells, to work it.

Indeed. It's the same in aviation. General Aviation has embraced GPS as primary navigation, allowing vertical separation minimums to be halved in European Airspace. Lately GPS has been approved for precision approaches. Even the big boys use GPS as a primary interface to their flight management systems. I guess it's good enough for us...

[biffer]

Originally Posted by wavelength

Thats why on advance courses we have folk steer to the compass at night with the gps switched off (well the one they can see ).

i hope it's in front of the helm

[dnv]

Thanks to all for your views. Seems there is no quick fix...

regards
Jan

PS
Alternative to compensation 'd be to remove again the external speaker of the VHF, but that's no option.

[wavelength]

i hope it's in front of the helm

the compass is
but the 2nd fixed gps is on the back of the helm's seat