Compass

[copterdoctor]

We have just returned from our two weeks Headpump fixing, weather watching, over eating holiday.

Whilst looking at the choice of compasses in a swindelry in the Solent (bottom bit of England in the middle) I noticed that the compasses on special offer were marked "Southern Hemespere Australia Area". Given the ability of the average helm is limited to steering +/- 5+ degrees is the region important?

 

[john_morris_uk]

I am not suprised that they were on offer. The magnets in the compass have to be set for the correct 'dip' of the magnetic field according to the latitudes that the compass is to be used in. Northern - Equatorial and Southern from memory. It always used to make me wonder what happened when you sail round the world, but most people seem to manage! However, the compass will still work - its just better to have the correct compass for your region. If you sail to very high latitudes with the 'wrong' compass, I guess that there might be a chance of it sticking sometimes, but you are at the limit of my knowledge here.

 

[Evadne]

I think you might get away with it if you use a floating ball type, but the balanced needle ones definitely won't work. I believe the correct method is to fit adjusting magnets, or more correctly to get a compass adjuster to fit them for you, so you'd have a set for the northern hemisphere, the southern hemisphere and the tropics if you were really keen. I've never been anywhere near the magnetic poles myself, but most yachts that have are steel, and would probably have a fluxgate compass anyway.

 

[Cornishman]

Maybe they are to help their cricketers find the ball?

 

[Twister_Ken]

They're for boats with Oz drivers. Sunsail fit them on lead boats.

That's not lead boats, you unnerstand, coz they wouldn't float, but lead boats, as in boats that don't follow.

 

[StugeronSteve]

Should still point in the right direction, but it might be best to hang it upside down from the cabin lining. Just to be on the safe side.

 

[Rowana]

I thought that they had a soddin' great lump of lead on the bottom in order to keep them right way up . . .


But then again, is the right way up ar$e about tit if you're in Oz.


The mind boggles (or at least mine does), especially as the boat I sailed when I worked in Oz always seemed the right way up to me.

Confused of Aberdeen .

 

[tonyran]

Dip doesnt just exist in the textbooks. A UK/Northern Europe compass will work in Queensland (Whitsundays) but no chance in Sydney harbour unless you tilt it. The needle just jams on the housing. I dont suppose fluxgate compasses are affected so I guess that's what the circumnavigators use once they get more than 20deg South (assuming that is they start from here)

 

[trev]

What a load of twaddle ! Dip ? Southern Hemisphere? Do you think a commercial vessel has different compasses for different areas? Any well made magnetic compass will work anywhere, and as far as trying to adjust a non-centreline compass on motor cruiser is concerned - forget it - just get to know its quirks.

 

[john_morris_uk]

Its not twaddle. And I do sail on large ships all over the world which have magnetic compasses as well as Gyros. Commercial shipping might just pay a bit more than £100 for a compass!

As you are so cynical here is the zone page and details off the Plastimo website:

[ QUOTE ]
All our compasses are balanced manually : with a standard procedure for Zone A* models, and using a specific magnetic field simulator for all other area compasses.
Balancing the compass consists in adjusting the card so that it is perfectly horizontal. If you sail to far away from the area for which your compass was originally balanced, the card will dip and the precision can be affected by a few degrees.
For a long passage in a different zone, the acquisition of a compass specifically balanced for that area is a wise investment.

[/ QUOTE ]

 

[trev]

It Is mostly a load of cobblers so far as 'Dip' and area is concerned ! the differences are best just logged when you do your compass errors.
I too have spent all my life at sea and am quite conversant with the magnetic compass. We only ever bothered to 'swing' at the beggining of a voyage and after that is was largely left untouched.
It is virtually impossible to correct any off centreline compass due to a coeficient'A' (assymetric) effect - you can't get a sine curve of deviations. - Please don't overcomplicate a simple bit of kit, because if you really get into it it gets really involved.

 

[john_morris_uk]

trev - with the greatest respect I am not overcomplicating the kit - just recognising that Plastimo make compasses that are balanced according to what latitudes they are to be used in. The truth of it is that if they make a 'cheap' (in commercial terms) compass and they claim that it needs to be balanced to ensure that it works correctly, you can't dispute it. Magnetic 'dip' exists and can be measured and corrected for.

The original question was all about what does for 'Southern Areas' (or however it was phrased) mean on the Plastimo Compasses that were on sale at a bargain price. Perhaps there are lots of people who have never heard of magnetic 'dip' and for the majority it doesn't make any difference. However, I believe that I was answering the original question in as simple a way as I reasonably could. I will let others judge whether my answers are 'twaddle'!!

Whether a compass can be corrected or not is entirely irrelevant to the original question. After all the deviation curve won't change according to where the boat is in the world. Why are you being so abrasive?

 

[trev]

Cos I think most yacht type compasses don't really benefit from too much messing around. Most yachts can only steer within a few degrees and trying to correct for minor deflections is a waste of time.