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  1. #1
    Join Date
    Feb 2004
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    Boat (500m E of) the Clyde
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    4,017

    Default Compass in southern hemisphere (Auckland)

    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 ?

  2. #2
    Join Date
    Jun 2006
    Location
    South East Coast - United Kingdom
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    7,078

    Default Re: Compass in southern hemisphere (Auckland)

    Well, I have to admit, "NORTH" will point towards the sun so he will have to be careful!
    regards David - DSW Marine Engineering
    www.dswmarineengineering.com

  3. #3
    Join Date
    Jun 2002
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    Oxonia
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    60,815

    Default Re: Compass in southern hemisphere (Auckland)

    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)

  4. #4
    Join Date
    May 2003
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    Here or there
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    74,433

    Default Re: Compass in southern hemisphere (Auckland)

    wot happens on the Equator then, or say 200 mls either side of it [img]/forums/images/graemlins/confused.gif[/img]
    Brexit: ‘taking back’ what we had never lost, in order to lose everything we had...

  5. #5
    Join Date
    Dec 2001
    Posts
    140

    Default Re: Compass in southern hemisphere (Auckland)

    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.

  6. #6
    Join Date
    Sep 2004
    Posts
    4,180

    Default Re: Compass in southern hemisphere (Auckland)

    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
    <span style="color:blue">www.sailroom.co.uk</span>

  7. #7
    Join Date
    Oct 2006
    Posts
    5

    Default Re: Compass in southern hemisphere (Auckland)

    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?

  8. #8
    Join Date
    Nov 2004
    Location
    18A, Linear North
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    1,740

    Default Re: Compass in southern hemisphere (Auckland)

    [ 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. [img]/forums/images/graemlins/wink.gif[/img]
    Rum, it's not just for breakfast anymore.

  9. #9
    Join Date
    Sep 2004
    Posts
    4,180

    Default Re: Compass in southern hemisphere (Auckland)

    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
    <span style="color:blue">www.sailroom.co.uk</span>

  10. #10
    Join Date
    May 2001
    Location
    North Wales, sailing Aegean Sea or Menai Strait
    Posts
    19,826

    Default Re: Compass in southern hemisphere (Auckland)

    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!
    Answers to some technical queries at http://coxengineering.sharepoint.com

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