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Old 21-10-2010, 08:32   #61
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I haven't gone through the whole of this thread but I hope someone has pointed out that a true course holds good for any year. Just apply the 'variation' for the year that youa re sailing it. These were recorded by traders, whalers and ancient mariners for use in future years and handed from skipper to good friends and relations.
It also applies to cruisers advance planning. Never sure which year, or what boat they'll be on, but True is still True, even if the compass is false.
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Old 21-10-2010, 08:40   #62
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I haven't gone through the whole of this thread but I hope someone has pointed out that a true course holds good for any year. Just apply the 'variation' for the year that youa re sailing it. These were recorded by traders, whalers and ancient mariners for use in future years and handed from skipper to good friends and relations.
It also applies to cruisers advance planning. Never sure which year, or what boat they'll be on, but True is still True, even if the compass is false.
good point, but I can't imagine under anything resembling normal circumstances taking a heading from someone else without plotting and confirming it myself.
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Old 21-10-2010, 09:09   #63
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LOL the penny drops!!!!!!

I just worked out why some folks here are getting so toey about True and Magnetic and anual variation.............. You far northerers do get a big magnetic variation.

I have never really been in a place where its much over 10 degrees and the anual variation, corrected, never more than 1 or 2 degrees, far smaller than an olympic helmsamn could sail to.
Even the 10 degrees in the open ocean by a short handed crew not racing but hand steering, or a wind vane, would be within the limits of helmsmans error. But I see up tin the north of the USA and Canada its up in the hight 20's and 30's......

All I can tell you is to head to where its warmer
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Old 21-10-2010, 11:18   #64
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No doubt! This time of year, I start yearning for Oz, and I have told my wife that this is the last winter in Alaska. So I'll be sailing out of Kachemak bay and hanging a left for open ocean and keep the compass pointing south until I hit dry land, or not. Anyway I'll keep plotting true.
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Old 21-10-2010, 14:13   #65
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So I'll be sailing out of Kachemak bay and hanging a left for open ocean and keep the compass pointing south until I hit dry land, .
True or Magnetic you will be in a far better place
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Old 21-10-2010, 20:27   #66
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I have had a real problem with my mango crop here, and I think it might be the latitude. I certainly have the right attitude.
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Old 21-10-2010, 23:13   #67
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Coming a bit late into this discussion..... but my vote is for plotting true on the chart.

This keeps it simple because everything on the chart is true and if I need a compass course to steer to I'd do the TVMDC calc to get it:

True
Variation
Magnetic
Deviation
Compass

I can't for the life of me think why you would plot Magnetic on a chart. It's no good to steer to (for that you need Compass) and it's not True (which has to be easier to work with on the chart doesn't it?)

Surely Magnetic is just a theoretical step part way through a calculation between True and Compass or vice versa and has absolutely no value at all for either plotting or steering a course?
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Old 22-10-2010, 06:24   #68
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In Chapman's, under "The Basic Principles of Dead Reckoning:

"For DR track, use either true or magnetic courses consistently and label appropriately."
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Old 22-10-2010, 08:11   #69
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Coming a bit late into this discussion..... but I can't for the life of me think why you would plot Magnetic on a chart. It's no good to steer to (for that you need Compass) and it's not True (which has to be easier to work with on the chart doesn't it?)

Surely Magnetic is just a theoretical step part way through a calculation between True and Compass or vice versa and has absolutely no value at all for either plotting or steering a course?
As I mentioned earlier in the thread, but never got a answer,

You have to do 2 steps to go from T to C (var, dev calc). Going from M to C is one (just dev). Since variation wont change, in your area, during your journey, why always caluculate a constant? Seems like extra work to me. Unless I am missing something. If I am always going to add/subtract the exact same number, wouldn't it be easier jut to use it at the M level on the chart, and just adjust for dev for the course to steer?
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Old 22-10-2010, 11:27   #70
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Variation does change from time to time and often in the same areas it will change annually. Part of what you are doing is a process, and if you complete all the steps then most likely you won't forget one of them and put an additional error in your program. The idea is to get from one place to another hopefully with out too much adventure. You don't have to use any method, you can always take up the mantle of the old navigators that used the stars, wind, angle of the waves & time of year and navigate by pure empirical observation, or you can do like the Arctic Terns and follow the magnetic fluctuations in your head.
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Old 22-10-2010, 11:56   #71
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Variation does change from time to time and often in the same areas it will change annually. Part of what you are doing is a process, and if you complete all the steps then most likely you won't forget one of them and put an additional error in your program. The idea is to get from one place to another hopefully with out too much adventure. You don't have to use any method, you can always take up the mantle of the old navigators that used the stars, wind, angle of the waves & time of year and navigate by pure empirical observation, or you can do like the Arctic Terns and follow the magnetic fluctuations in your head.
But variation wont change during your trip, as I mentioned in my last post, which is my point.

I just know I wouldn't want my chart plotted out beforehand in true and have to tvmdc for each leg. mdc seems a bit easier to me. I think an error would more likely occour if you constantly had to do 2 calculations for each leg. On my sailboat, the compass at the helm is very accurate, and have the repeater for the fluxgate there too, and they are always about 0-1 degrees of each other (same with my handheld compass), so I am not losing sleep over it, but have been on other boats that is the dev is horrific.

I guess its all what you are used to. We were taught at the uscga coastal naviation class to plot a course in magnetic. and some of my charts come pre-plotted with often used courses, also in mag. heading. So thats what I have used for decades and is what I am used to.

I guess if I always used true, I would vote for that.
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Old 22-10-2010, 12:06   #72
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I guess its all what you are used to. We were taught at the uscga coastal naviation class to plot a course in magnetic


did you take this at a school,,, if so which one if you do not mind me asking
was it the coast guard academy??

as long as you get to your destination i guess it does not matter but with a repeater onboard the error is only 1-2 degrees off, I trust that much more than a magnetic compass because of the deviation,, also autopilots should be hooked up to a repeater not a GPS, chance of dropouts can happen with the GPS,, not saying that some are not hooked up that way but the repeater is more accurate and safer,,,

Also depends on were you are sailing to and how long,, If i was just sailing a bay i would have no problem plotting in magnetic but for anything else it is going to be true for safety,
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Old 22-10-2010, 13:09   #73
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uscg auxillary class.

my Autopilot is connected to the fluxgate for its compass heading. It can be led by the gps, but it is primarly run by the fluxgate.

whether you plot true or mag.. wouldnt the compass heading be the same in the end after doing tvmdc or mdc? Either way as long as the course legs are marked with M or T, it should be ok
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Old 22-10-2010, 15:11   #74
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Why would you plot in Mag? The chart is True and the compass is Compass. You can put the Variation and Deviation together to give a single conversion from T to C or vice versa; and you change this when you change your heading.
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Old 22-10-2010, 17:22   #75
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Plotting on a chart has two basic aims:

- or we plot what we want to achieve,
- or else we plot what we are actually making.

In the first case our plotting will be True (if measured to the meridian) or Magnetic (if measured to the N of the corrected variation rose).

PLEASE NOTE dear sailors or pretenders (hahaha) that the T and M are THE SAME LINE on the chart.

I want to say - you cannot plot Magnetic and use the meridian as a reference line. Unless variation is nil.

I mean - one can, but why should they? ;-)))

The other scenario - when we plot whatever the sea throws at us - we will be plotting our track. Once again - if measured from the meridian it will be true and if measured from the magnetic N, it will be called magnetic. Again it is THE SAME line on the chart ...

So, if you were taught to plot MAGNETIC, please come back to the course manual and note that it was supposed to be plotted from the magnetic North, not from the meridian.

Someone suggested we have a weather corner at CF, but should we not start with a navigation basics corner instead?

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