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Old 18-01-2016, 06:26   #31
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Re: Buying a yacht abroad, then sailing it home

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Originally Posted by Chrisgo View Post
Honestly, it's been explained to me many times, and I don't get it either. Seriously. Not trolling.
The earth rotates around the barber pole next to santa's workshop.
- True North always points to the barber pole no matter where you are.

The north pole of earths magnetic field doesn't line up with the barber pole.
- Magnetic North lines up with the direction to the magnetic field.

So your magnetic compass doesn't neccessarily point to the barber pole.

Magnetic deviation is the difference in direction between magnetic north and the barber pole.

There's a bit more to it but that's the gist.
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Old 18-01-2016, 07:21   #32
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Re: Buying a yacht abroad, then sailing it home

Simple diagrams of the earth's magnietic field look like the first attachment. But they are vastly oversimplified. The magnetic field looks l=more like the second attachment.

So a compass which aligns itself along the magnetic field lines rarely points directly to the "top/bottom" of the globe. It will almost always be offset from lines of longitude.

The third attachment shows lines of equal deviation, i.e. anywhere along a given line the deviation from true north will be the same.
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Old 18-01-2016, 07:53   #33
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Re: Buying a yacht abroad, then sailing it home

So, what would the electronic nav instruments be reading? True or Magnetic? Do they account for the deviation, (declination?). Does anyone ever adjust the ball compass according to local deviation so it reads true?
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Old 18-01-2016, 08:06   #34
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pirate Re: Buying a yacht abroad, then sailing it home

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Originally Posted by Chrisgo View Post
Does anyone ever adjust the ball compass according to local deviation so it reads true?
Make for a lot of work.. sail a bit.. calculate.. adjust.. sail a bit.. calculate.. adjust..
Mind.. be ideal for the OCD tinkers amongst us..
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Old 18-01-2016, 08:13   #35
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Re: Buying a yacht abroad, then sailing it home

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So, what would the electronic nav instruments be reading? True or Magnetic? Do they account for the deviation, (declination?). Does anyone ever adjust the ball compass according to local deviation so it reads true?
Electronics can be set to show either one. Depends on what you ask it to do.

No it's not standard practice to adjust the compass to read true north because as soon as you travel a little ways, the deviation changes and you have to reset it. Plus anyone else coming on your boat who knows what they are doing will assume your magnetic compass reads magnetic north and therefore tend to set an incorrect heading.
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