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post #31 of 69 Old 01-30-2014
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Re: Magnetic Variance

However when it is all said and done, the course and heading are two different things. Your heading is what you steer to arrive and the desired course. Has anyone mentioned compass deviation? (compass adjusted for magnetic anomalies caused by the boat) This really should have been the 1st step, then variation (declination), then heading (=/- currents and winds).?

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post #32 of 69 Old 01-30-2014
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Re: Magnetic Variance

Absolute simplest way to do it, IF your compass has almost no deviation?

Just lay your course out using the Magnetic compass rose, and steer magnetic.

Yes, you'll still have to adjust for leeway and current, but you won't have true virgins or dead men to deal with. ;-)
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Re: Magnetic Variance

Once you figure your COA (course of advance, or desired course), remember just round off the the calculated course (true, but preferably magnetic, or if you have a deviation table, compass) to the nearest 5 degrees and steer that course. You cannot steer a course much more accurately than +/_ 5 degrees anyway, especially in rough conditions. Then...assuming a DR plot...

Fix your position as frequently as prudent, so you know where you are at all times (every hour, four times a day, daily, depending on the passage duration).

The navigator's adage is: the art and science of continually checking and calculating and fixing your position...and finding your errors (if any), before you have placed yourself and your vessel in serious trouble.
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Re: Magnetic Variance

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Originally Posted by nolatom View Post
Absolute simplest way to do it, IF your compass has almost no deviation?

Just lay your course out using the Magnetic compass rose, and steer magnetic.

Yes, you'll still have to adjust for leeway and current, but you won't have true virgins or dead men to deal with. ;-)
Exactly! That's what my teacher said when I took a navigation class.

We were taught the TVMDC AW and all that, but the teacher said he just did everything magnetic.
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post #35 of 69 Old 01-30-2014 Thread Starter
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Re: Magnetic Variance

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Originally Posted by sck5 View Post
here's another method:

1. Set your chartplotter to show Magnetic headings instead of True. Just follow those same headings on your boat's compass.

2. Print out this thread in case your plotter gives up and you have to go back to paper charts.

Seriously, I learned the TVMDC jingles and passed the nav exam, etc. etc. but I rarely use any of that now that there is a decent plotter on the boat. It speaks magnetic. All the compasses speak magnetic. No need ever to convert T to M or M to T. (Deviation is minimal on my fiberglass boat unless I park the binocs on the pedestal. Makes the compass lie)
I'm of the mindset that good seamanship includes being prepared as best you can. I will definitely be using a chart plotter but I will also most likely be singlehanding, or short handed at best. I don't want to be learning how to do this stuff in conditions that force me to use it. I see no sense in carrying paper charts If I don't know how to use them. Kinda like carrying a liferaft and not knowing how to inflate it. It sounds like it pays to minimize the deviation. I'll still be carrying reference books on the boat.


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Originally Posted by jerryrlitton View Post
However when it is all said and done, the course and heading are two different things. Your heading is what you steer to arrive and the desired course. Has anyone mentioned compass deviation? (compass adjusted for magnetic anomalies caused by the boat) This really should have been the 1st step, then variation (declination), then heading (=/- currents and winds).?

Jerry
Thanks for mentioning it Jerry. I caught on to the deviation also. I already knew that course and heading are two different things. When I sailed my boat on Kentucky lake I had to allow for leeway and current. I just took a guess for my heading allowing for those and hope my course was close to what I wanted. In the short distances involved, that mainly came into play when making room for the occasional barge. I still need to learn how to more accurately account for them though.

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Originally Posted by nolatom View Post
Absolute simplest way to do it, IF your compass has almost no deviation?

Just lay your course out using the Magnetic compass rose, and steer magnetic.

Yes, you'll still have to adjust for leeway and current, but you won't have true virgins or dead men to deal with. ;-)
Good point using magnetic plots. True virgins and dead men could get a man in trouble in most parts of the world!

Last edited by Dean101; 01-30-2014 at 10:20 PM.
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post #36 of 69 Old 01-30-2014 Thread Starter
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Re: Magnetic Variance

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Once you figure your COA (course of advance, or desired course), remember just round off the the calculated course (true, but preferably magnetic, or if you have a deviation table, compass) to the nearest 5 degrees and steer that course. You cannot steer a course much more accurately than +/_ 5 degrees anyway, especially in rough conditions. Then...assuming a DR plot...

Fix your position as frequently as prudent, so you know where you are at all times (every hour, four times a day, daily, depending on the passage duration).

The navigator's adage is: the art and science of continually checking and calculating and fixing your position...and finding your errors (if any), before you have placed yourself and your vessel in serious trouble.
I've read in several places about the 5* thing. The compass on my old boat was marked in 5* increments. I noticed on the really windy days I tended to wander +/- 5* each side of my heading anyway. I'm thinking that keeping a DR plot charted as I go while at the same time charting my course by GPS will not only keep my mind actively engaged in where I'm at and where I'm going, it will also help me to improve my accuracy. I'm also planning on learning how to do celestial navigation later on and I'll check those positions against my GPS. Hopefully, if I'm ever in a position that my electronics get fried, it will be an inconvenience rather than something else to stress out about.
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Re: Magnetic Variance

Just move to the western Florida panhandle and don't worry about it.

Magnetic Declination Explained and Setting Magnetic Declination
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post #38 of 69 Old 01-31-2014
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Re: Magnetic Variance

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Originally Posted by Minnesail View Post
Exactly! That's what my teacher said when I took a navigation class.

We were taught the TVMDC AW and all that, but the teacher said he just did everything magnetic.
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.
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Re: Magnetic Variance

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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.

Excellent point. If you are in distress and don't have a "true" fix or DR estimated position, it may be better not to guess but rather give them your last fix/DR and let them figure set and drift from there. They are better at it than we are, much more experienced. And it's most likely that the first responder will be a merchant ship once the Coast Guard calls for position reports from any ships in the area. Most ships are in AMVER, which is a program where they agree to report their positions and respond if at all possible.

Also, if you're WAY out there and no electronics (you may have a problem but..) you have a portable radio (but no portable GPS which unlikely but humor me, maybe it's broken or soaked too?) and a ship is within sight and VHF range? Call 'em on 16, switch to another channel, and ask the Mate to give you his (her? yup, nowadays) position, which will be from GPS and their course/heading from gyrocompass will be True.

This is how the old-time trekkers would "bum a fix" after days of cloudy weather and no celestial fixes possible. So yeah, the farther offshore you get, the more most everyone is "True" ;-)


Sorry if I'm wandering off topic a little, but it's interesting.
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post #40 of 69 Old 01-31-2014 Thread Starter
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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.
That is definitely an excellent point! I did not consider that aspect of it. From a potential safety standpoint, it would be best to plot in true. I'm totally in agreement about learning how to do this stuff before it's actually needed in an emergency. I wonder how many people carry paper charts because it's recommended (or mandatory) but don't know how to actually use them other than finding a lat/long position? Like I said earlier, it's like carrying a liferaft and not knowing how to inflate it.


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Originally Posted by nolatom View Post
Excellent point. If you are in distress and don't have a "true" fix or DR estimated position, it may be better not to guess but rather give them your last fix/DR and let them figure set and drift from there. They are better at it than we are, much more experienced. And it's most likely that the first responder will be a merchant ship once the Coast Guard calls for position reports from any ships in the area. Most ships are in AMVER, which is a program where they agree to report their positions and respond if at all possible.

Also, if you're WAY out there and no electronics (you may have a problem but..) you have a portable radio (but no portable GPS which unlikely but humor me, maybe it's broken or soaked too?) and a ship is within sight and VHF range? Call 'em on 16, switch to another channel, and ask the Mate to give you his (her? yup, nowadays) position, which will be from GPS and their course/heading from gyrocompass will be True.

This is how the old-time trekkers would "bum a fix" after days of cloudy weather and no celestial fixes possible. So yeah, the farther offshore you get, the more most everyone is "True" ;-)


Sorry if I'm wandering off topic a little, but it's interesting.
I don't think you're off topic here. You're discussing the real life application of the subject and that is well within the bounds of the topic.
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