Compass in southern hemisphere (Auckland)

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dunedin

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My son is going to be sailing a racing dinghy in New Zealand. He has been told his racing compass won't work properly down under and needs a different one.

Is somebody trying to "sell us a left handed teapot" ?
Is there any real issue ?
 
Completely true, and has been asked many times before. The needle needs to be counterbalanced so it will rotate correctly, as the needle is pulled downwards at different angles (non technical explanation)
 
I suggest that you get some advice (specific to your compasses) from the national body that supports dinghy sailing at international championship level in your country so that the advice is authorative and no unexpected worries - but I suspect that the advice you have already been given is incorrect.

Dinghy race compasses (assuming that is what your son has), as far as I am aware - but stand to be corrected-, are not of a type that need correction for dip for the hemispheric zone the boat is in (seems sensible too given the rate some of these boats compete across many zones) - many have no compensation at all, given that relative tactical use may be more important than the actual magnetic heading.

(PS I assume the boat is 420 class)

John
 
I agree with BrendanS, this topic has been discussed many times on the forums and articles on it have appeared in the magazines. I am guessing that the bulkhead compasses fitted to many cruising yachts might suffer only minor problems ( although I do know that Plastimo make a different model of the Contest for the southern hemisphere ) but a large diameter racing compass might be a very different matter. However, the advice to ask the RYA is good: I guess they have some experience in Sydney, not that far away!
 
Thanks for your help. We will try to get some advice from the RYA coaches - as you say, they must have some experience from Sydney (though cost would not have been an object for that event) !

And congratulations on deducing that this is the 420 class (World Championships in Auckland in January) - finger well on the pulse here !
 
Good luck in Auckland over New Year, I will keep an eye out for the UK Open performances.

I would be interested in the outcome of the Dip question.

I have commissioned new boats in the Southern Hemisphere, for delivery to the Northern with Northern corrected compasses, in a region of very high magnetic Southern dip (around 75 degrees, that not in NZ) and even though with the large dip and also the wrong opposite correction was only sometimes a problem on a dead magnetic south heading when the angle of dip and the large exaggeration of the incorrect correction tipped the card enough to occasionally foul.

So I cannot imagine an uncorrected type compass having any problems whatsoever (especially as they are made to work most everywhere) but would be interested in the outcome.

John
 
I would have thought that you need to be pretty close to a magnetic pole for dip to have a serious effect. Auckland is only 37deg S.

I seem to remember that a compass should be swung if there is a large change of latitude, but as I presume super accurate compass navigation is not required this is probably not an issue.
 
Magnetic dip in Auckland is quite a bit greater than 60 degrees (comparatively large for its latitude due to its relative proximity to the South magnetic pole when compared to geographic latitude) - so about the same as in the northern part of Bay of Biscay and quite a bit more than the Med. with which it shares, roughly, the opposite latitude.

What I think most people think is that the card inclines to the extent of the dip or think that the error introduced by dip is important. The earth's magnetic field is very weak so the vertical component of that to the earth's surface is even weaker, even when dip is large. Compasses are normally pivoted with their centre of gravity below the pivot centre so that the pendulum effect largely offsets dip in so far as the tilt of the card is concerned.

As far as I know any reputable compass uncorrected for dip will work anywhere (except to the extent of getting dizzy close to the poles) - at least I have not heard of any that don't and have never heard of it being a problem. Dip in Scotland is around a comparatively high 70 degrees, so quite high, and I wonder if anyone has had problems with a reputable uncorrected for dip compass there.

The error from magnetic dip is only apparant in turns and during acceleration so is not of concern in a sailboat. But is in aircraft, for example, and for fast commercial MoBo's, 40 knotsish, have usually specified dip correction - hence my comment before about sea trialing for commissioning boats with Northern dip corrected compasses in the Southern hemisphere.

John
 
The compass will work fine so long as he stands on his head.
Captain Cook first discovered this phenomenon back in 1769 before he had special inverted compasses installed which he flipped over as he crossed the equator (like an egg timer).

How close can this boy hold a compass course anyway?

Man with watch always know time, man with 2 watches is never sure.

follow my advice? ... I wouldn't so why would you?
 
[ QUOTE ]
Dip in Scotland is around a comparatively high 70 degrees, so quite high, and I wonder if anyone has had problems with a reputable uncorrected for dip compass there.


[/ QUOTE ]

The b@stards seem to be able to find their way south easy enough. /forums/images/graemlins/wink.gif
 
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