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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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Old 04-25-2008
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Reading chart

confusion with a fraction after degrees on the chart rose:

On the Rose of the chart I am looking at the magnetic variation is 18 degrees 1/2 E with a yearly change of 19 minutes W. It is a 2005 chart so 3 years has past, what is the new variation?

18 deg 30 min - 3 x 19 sec therefore 17 deg 33 min

the 1/2 is throwing me off, is it 1/2 a minute or half a degree? why is the half show as 30 minutes if it is degrees?
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Old 04-25-2008
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The variation is 18 and 1/2 degrees which may be expressed as 18 degrees 30 minutes OR 18.5 degrees in decimal notation. All the same thing.
Now 19 minutes West is heading BACK towards true north and over three years that is 57 munutes. Simple arithmetic brings you back three minutes short of a full degree or 17 degrees 33 minutes EAST variation. This may be expressed also as 17 and 11/20ths degrees or 17.55 degrees.
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Old 04-25-2008
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And if either of you can steer within .55 degrees on a magnetic compass, I owe you a drink... ;-)
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Old 04-25-2008
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Quote:
Originally Posted by nolatom View Post
And if either of you can steer within .55 degrees on a magnetic compass, I owe you a drink... ;-)
Me too
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Old 04-25-2008
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Forget the inner magnetic circle and convert all of your brgs and courses to True and plot them on the chart. Then you won't have to do those secular magnetic corrections.
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Old 04-27-2008
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This brings up a good point, however: Why are charts not standardized to read 18 degrees, 30" (can't find the degree sign). 18 1/2 and 18.5 are ambiguous.

I don't mind decimal minutes on my GPS, but I find it annoying on my charts when the minutes integer is five or less.

I consider plotting and helming slightly differently in the age of GPS. Autopilot is an extension of plotting, whether by GPS or paper charts and plastic plotter, bearings, known waypoint, etc.: it's a function of what you put in. A slip of brain or finger and you can't blame the equipment.

Helming on a tiller to a bulkhead compass, on the other hand, is just the law of averages at work, and sometimes the helmsman will off a little off course to catch a puff or in anticipation of a tack/gybe. What is surprising to many, however, is that over short stretches, a good helmsman can equal or beat an autopilot, and over long stretches, a wind vane can equal or beat an autopilot.

Windvane, Autopilot, steering systems


Where the autopilot wins, of course, is while motoring to a waypoint in calm waters/light airs. What this says about the popularity of autopilots currently I can't really say.
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Old 04-27-2008
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Boasun will become famous soon enough Boasun will become famous soon enough
There use to be charts that are Degree, minutes and seconds a decade or two ago. But here in the USA the charts are Degrees, minutes and tenths of minutes. But for for a quick conversion; remember that a tenth of a minute is six seconds. So you can do the math quickly in your mind, if you have one.
Me? My mind is a dark and scary place and I don't go in there. brrrrrrrr!
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Old 04-28-2008
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Charts make great gasket paper for gearboxes.
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Old 04-28-2008
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And some Charts make great wall posters, book covers and tempt table covers for a picnic. I am talking about the old charts that are out of date. The ones with Coffee Ring Shoals and Bogger rocks.
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Old 04-28-2008
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Quote:
Originally Posted by nolatom View Post
And if either of you can steer within .55 degrees on a magnetic compass, I owe you a drink... ;-)
Give this drink to my autopilot
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