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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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  #41  
Old 01-31-2014
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Re: Magnetic Variance

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Originally Posted by DRFerron View Post
The problem with that (pre everyone with a GPS) was that the CG and Navy plot in true (still do). If you give them a bearing in magnetic, they will not have your exact location if they had to come looking for you. They have no way of knowing, also, how much deviation is throwing off your compass.

Without a GPS (when, perhaps, your boat gets hit by lightning?), it would be in your best interest to learn how to do the calculation and to take a manual fix so that you can give them and you a better chance.
Yes, that is exactly what I was taught. The teacher was Navy and he said any communication with Navy or Coast Guard should be in true, but for his own navigation he used magnetic because it was his opinion that it was simpler and had fewer opportunities for error.
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  #42  
Old 01-31-2014
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Re: Magnetic Variance

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Originally Posted by Minnesail View Post
Yes, that is exactly what I was taught. The teacher was Navy and he said any communication with Navy or Coast Guard should be in true, but for his own navigation he used magnetic because it was his opinion that it was simpler and had fewer opportunities for error.
That confuses me. If I'm using a paper chart I'd rather do the conversion when I'm not under stress, as, presumably I would be if I'm at the point where I'm giving my rescuers my position.
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  #43  
Old 01-31-2014
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Re: Magnetic Variance

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Originally Posted by Minnesail View Post
Yes, that is exactly what I was taught. The teacher was Navy and he said any communication with Navy or Coast Guard should be in true, but for his own navigation he used magnetic because it was his opinion that it was simpler and had fewer opportunities for error.
I can see your instructors point. Perhaps the simplicity of using magnetic while coastal cruising or island hopping where the distance from help is not so great and course changes are likely more numerous would be advantageous while using true on long crossings for communicating position in the same format as ships encountered or emergency situations would be a reasonable compromise for the sake of speed and convenience.
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Re: Magnetic Variance

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Originally Posted by DRFerron View Post
That confuses me. If I'm using a paper chart I'd rather do the conversion when I'm not under stress, as, presumably I would be if I'm at the point where I'm giving my rescuers my position.
That makes sense. If you are always converting to true it will be second nature to you, so if you are in distress you won't be struggling to remember whether you're supposed to add whiskey to virgins or if there is the least yeast in the east or what.

I suppose another benefit to working in true is that most people most of the time will be navigating off their GPS, and that will always be true.

But for whatever reason, my teacher taught us to work in magnetic.

It's all strictly theoretical for me, as all my sailing so far has been on Lake Superior and the variation there is only 1ļ so you can pretty much ignore it.
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Re: Magnetic Variance

Dean--

I do not know the size of your boat or what room you have but a tool I have been using since the mid-80's, now on two different boats, is an inexpensive Plotting Board with a Pantograph fitted with an adjustable compass rose (see Weems & Plath Chartkit Plotter). With this, one can adjust the compass rose on the parallel rule for the amount of variation shown for each chart one might use and easily plot one's courses/headings in magnetic but easily translate them to True headings. The board can be laid out below, and, when necessary or desirable, taken into the cockpit (assuming one is using water proof/resistant charts). This equipment is very useful, particularly, for example, with Explorer Charts of the Bahamas, which report some significant differences in variation from locale to locale based upon local magnetic anomalies (which may account for some of the Bermuda Triangle mysteries). With this, you only need to work out your deviations and keep a little table, perhaps laminated in plastic, on a lanyard at your helm as we do.

FWIW...
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Re: Magnetic Variance

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Originally Posted by Minnesail View Post
... If you are always converting to true it will be second nature to you, ...
That, too. But the general rule of thumb is to plot your fix every half hour. That way you can look at your last known position on your chart and say "At 0800 I was here". The CG can then plug that into their computer, calculate set and drift and know where you *should* be within a matter of seconds. But a SAR operation will start at your last known position.
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Re: Magnetic Variance

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Originally Posted by svHyLyte View Post
Dean--

I do not know the size of your boat or what room you have but a tool I have been using since the mid-80's, now on two different boats, is an inexpensive Plotting Board with a Pantograph fitted with an adjustable compass rose (see Weems & Plath Chartkit Plotter). With this, one can adjust the compass rose on the parallel rule for the amount of variation shown for each chart one might use and easily plot one's courses/headings in magnetic but easily translate them to True headings. The board can be laid out below, and, when necessary or desirable, taken into the cockpit (assuming one is using water proof/resistant charts). This equipment is very useful, particularly, for example, with Explorer Charts of the Bahamas, which report some significant differences in variation from locale to locale based upon local magnetic anomalies (which may account for some of the Bermuda Triangle mysteries). With this, you only need to work out your deviations and keep a little table, perhaps laminated in plastic, on a lanyard at your helm as we do.

FWIW...
I don't currently have a boat. I use to have an Endeavour 32 and I kept my chartbook in the cockpit with me. When I purchase my next boat, one of the features it must have is either a fixed table or preferably a dedicated nav station. That portable chartboard looks handy though. Even with having a nav station, I could see me using it in confined or crowded waters when I need to remain in the cockpit. Thanks for the tip!
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Re: Magnetic Variance

As a resident of the True North Please allow me to extend an apology to all who struggle with magnetic variation .Apparently it's caused by the wandering magnetic pole up in NE Canada. Our noble gov has set the senate on the job of alignment with the true north pole but in truth they are not up to that task either .(see Peter Principle )
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Re: Magnetic Variance

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As a resident of the True North Please allow me to extend an apology to all who struggle with magnetic variation .Apparently it's caused by the wandering magnetic pole up in NE Canada. Our noble gov has set the senate on the job of alignment with the true north pole but in truth they are not up to that task either .(see Peter Principle )
They should form a committee. That would probably help.
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Re: Magnetic Variance

Rarely do I ever hear anyone give a position in anything but lat/long. In Chesapeake Bay people will often say things like "off Lookout Pt" or "a mile east of red 34" but never have i heard anyone give an exact bearing in either true or magnetic. It might be important in theory and yes, I learned to do it like everyone else but it just doesnt come up very often in real life unless you are still firmly in a paper chart only universe. Most of us arent any more.

Sure, it is a good thing to be ready for disaster to strike but do you REALLY keep a dead reckoning going on a long passage when you can simply read off the lat/long whenever you like and write it in the log? I think a lot of folks are piously repeating what their lessons told them was "proper" but which is a lot of work for a less accurate result.

It is a good idea to remember WHY our teachers told us to do all these calculations. It was to know our position with as much accuracy as possible. Again, I can do it, but why would I waste time with that when I have 4 redundant GPS's which will tell me my position with 10ft. accuracy? It is especially pointless when I am hundreds of miles from land and seem to only rarely have a wind fair enough to head directly toward my goal.
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