Bulkhead compass weirdness

[funlovincriminal 191]

I've just done a trip to Fiji on a mates cat, now that we are in flat calm Anchorage it has become apparent that both of his compasses are not acting correctly.

The cards on both are tilted towards the center and front of the boat. The starboard one is 5 degrees to the center and I'd say nearly 10 forward.

Port is about 5 degrees to center and 5 forward.

Both 2 months old Riviera Polare units.

Nothing ferrous near them, when removed and held level in cockpit they don't change.

We have sat a hand bearing compass in the hole when removed and it behaves properly.

Any ideas?

[K4309 353]

You sure its not just your animal magnetism?

Are they southern hemisphere compasses? Northern hemisphere ones are different, and 90% of compasses sold globally are northern hemisphere. I don't actually know how they are different, but they are different. And no, its not just the needle pointing south instead of north.

If they are brand new (2 months old) and procured in a hurry before departure, this may not have been checked.

[Leftred 45]

I'd say K4309 may have it there. It's good to remember that the Earth's magnetic field isn't parallel to the ground in most of the world (magnetic dip), and so compasses are weighted to compensate to hold them roughly parallel to the ground. There can be different weightings for different regions.

That's the reason why when flying a plane using a magnetic compass you learn to compensate for the momentum of that weight during a turn. On some heading the compass will spin ahead, and on other's it will lag. Likewise when accelerating or decelerating it's will move the compass on some headings. Trippy stuff.

[LBD 169]

the phenomena is known as "compass dip"

Magnetic dip - Wikipedia

[LE Bb 27]

see pic, instructions pretty generic 

[ex Elly 221]

What does zone A, B and C mean?

 

[harrytom 679]

learnt something new, who would of thought to ask??

 

http://www.mapworld.co.nz/global.html

 

[waikiore 445]

Yep seen it many times -that is Northern Hemisphere dip, a few years back Parallel Imported were selling Steiner Commander binoculars at a very good price so I beetled along to check them out, the dip was so bad in the built in compass as to render it unusable.

[Ed 146]

I've got a plastimo hand bearing compass someone from the uk kindly sent me for christmas, totally useless, card just binds when held horizontal

[Island Time 1,284]

4 hours ago, ex Elly said:

What does zone A, B and C mean?

 

Normally numbers, not letters. Compasses can be adjusted to suit each region. See https://www.maritimenz.govt.nz/content/commercial/safety/safety-management-systems/recognised-surveyors/recognised-compass-adjusters.asp

They come to your boat, and adjust the compass, and provide you with a deviation card. This is normal commercial/offshore boat operations...

[ex Elly 221]

Thanks, IT. So different compasses work in different zones. But according to this map, you would need a different compass for NZ North Island, and South Island.  I don't think that's correct. I have seen other maps where NZ and AUS are both in one zone.

 

[Island Time 1,284]

Nope, but yes to different corrections. The earth's magnetic field is not identical everywhere.

Mostly compasses are northern hemisphere or southern hemisphere from the supplier.  

Then a compass adjuster can correct for actual locality .

[funlovincriminal 191]

My initial thought was that they could have been Northern Hemisphere units.

They were purchased in NZ from a reputable retailer, possibly shipped from supplier in error.

I have brought them home with me and they are going to sort him out with replacement or credit.

Interestingly enough, they were only installed to meet Cat 1 and we never looked at them once on the trip. As it was 99.9% Autohelm all we used was the heading on the control unit 🙄

[Island Time 1,284]

3 hours ago, funlovincriminal said:

My initial thought was that they could have been Northern Hemisphere units.

They were purchased in NZ from a reputable retailer, possibly shipped from supplier in error.

I have brought them home with me and they are going to sort him out with replacement or credit.

Interestingly enough, they were only installed to meet Cat 1 and we never looked at them once on the trip. As it was 99.9% Autohelm all we used was the heading on the control unit 🙄

That is right for 99% of the cruises and passages out there. Until you have a complete electrical failure - it's happened to me once on a leaky boat delivery. Then the (adjusted!) compass becomes your best friend!

[MartinRF 62]

On 25/06/2023 at 8:22 AM, Island Time said:

Nope, but yes to different corrections. The earth's magnetic field is not identical everywhere.

Mostly compasses are northern hemisphere or southern hemisphere from the supplier.  

Then a compass adjuster can correct for actual locality .

How do round-the-world sailors deal with this?

/Martin

[mcp 34]

2 hours ago, MartinRF said:

How do round-the-world sailors deal with this?

/Martin

Sextant.

[Black Panther 1,692]

Sextant doesn't solve rhe problem.  You have to learn hiw to disassemble and reassemble a comoass and adjust the weights.

Or live with it till you move south again.

I discovered it is possible to fill your compass with rum and it will still work. ( the book said alcohol right?)

[Island Time 1,284]

Yep, as BP said. Learn to adjust your compass, or alternatively pay someone to do it (that's what the commercial boats do, they need valid compass adjustment docs...)

info here;

https://msi.nga.mil/api/publications/download?key=16920950/SFH00000/HoMCA.pdf&type=view

That being said, most marine compasses will still work without further adjustment, they just will have the card at the angle of dip....

[Romany 162]

1 hour ago, Black Panther said:

 

I discovered it is possible to fill your compass with rum and it will still work. ( the book said alcohol right?)

So - when it seems all is lost its not, actually. Rum makes everything good

[waikiore 445]

Of course Gin makes the card easier to read....