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Old 13-10-2010, 16:27   #16
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Start with true, apply the variation, arrive at magnetic, apply the deviation (if needed, adding westerly or subtracting easterly).

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The point is that by the time you have the course plotted you've accounted for everything. It also means that any bearings you take will be in magnetic (since you took them with your compass).
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Old 13-10-2010, 16:35   #17
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Quote:
Originally Posted by rebel heart View Post
Start with true, apply the variation, arrive at magnetic, apply the deviation (if needed, adding westerly or subtracting easterly).

True
Virgins
Make
Dull
Company
(so add Whiskey)

The point is that by the time you have the course plotted you've accounted for everything. It also means that any bearings you take will be in magnetic (since you took them with your compass).
Yup. have taken coastal navigation classes...which is why i noticed the problem
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Old 13-10-2010, 16:41   #18
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Quote:
Originally Posted by phantomracer View Post
true..which is why you should have your compass turned to account for deviation on board.. and the affects of mag variation on it as well.

When I was on the phone with lowrance.. once the tech got my question..there was dead silence..he got it! still havent heard from them about they plan on fixing it.. I just never heard of anyone following a true course on a boat..since so few compasses point true
Well there must be some new techniques or something as I've yet to come across a compass thats spot on through 360... and if you want to find True from Magnetic you just reverse the procedure...
I have a little card which lists the deviations at various points of the compass... as little as 1/2 to 3 degrees.
As for variation... thats a calculation you have to make with the info on hand...
eg; Variation 3.35'W decreasing 7'annually(2001)...
turning your boat wont solve that equation
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Old 13-10-2010, 16:44   #19
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Since I always plot in "true”, I have to uncorrect (TVDMC) to get a compass (steering) course.
“True” direction is relative to true north (north pole).
“Uncorrecting” is converting from true to a magnetic direction; hence TVMDC
“Correcting” is converting a magnetic (M) direction to true (T); hence CDMVT.
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Old 13-10-2010, 16:59   #20
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Well there must be some new techniques or something as I've yet to come across a compass thats spot on through 360... and if you want to find True from Magnetic you just reverse the procedure...
I have a little card which lists the deviations at various points of the compass... as little as 1/2 to 3 degrees.
As for variation... thats a calculation you have to make with the info on hand...
eg; Variation 3.35'W decreasing 7'annually(2001)...
turning your boat wont solve that equation
What I mean is what you said. Turn the boat to make a card to account for variation and deviation. Just as you say. I just didn't say it clear enough..sorry.
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Old 13-10-2010, 17:14   #21
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I do my chart work in magnetic since that is what the compass points to on the boats I've sailed and bearings to features are also magnetic. I made a habit of using the inner compass rose with paralell rules so that removes having to think about and adjust for variation during plotting.

On larger vessels, especially Navy, gyrocompasses are used which point to true north, they operate on a different principle than magnetic compasses. Consequently formal instruction for navigation watch-standers on those vessels is geared to true.
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Old 13-10-2010, 17:17   #22
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No... maybe its my fault for being a bit casual...
Variation for an area is found in the Compass Rose on a Chart and has to be calculated by you on the spot so to speak...
50 - 100 miles away it could be up to 3degrees more West or less... in some extreme areas it can be upto 10 degrees... its an ever changing equation that cannot be fixed by turning your boat.
That only accounts for the magnetic effects in the boat on your compass...
The varying magnetic effects of the earth cannot be factored into it...
Much as I may wish they could be...

OK... thats my lot....
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Old 13-10-2010, 17:30   #23
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Turning your boat,in a circle,in itself, does not allow you to calibrate for deviatation. you must have another source of heading information , such as a hand bearing compass or use your GPS.

However the circle turning does work for electronic fluxgate compasses

Dave
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Old 13-10-2010, 18:20   #24
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Turning your boat,in a circle,in itself, does not allow you to calibrate for deviatation. you must have another source of heading information , such as a hand bearing compass or use your GPS.

However the circle turning does work for electronic fluxgate compasses

Dave
right.. there is a procedure for calibrating your compass.. you can't use the compass you are calibrating for the calibration You need another one that is (hopefully) in a magnetically clean zone..so you can account for the variation/deviation as the boat is turned
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Old 13-10-2010, 18:28   #25
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.... are we being 'trolled'...lol
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Old 13-10-2010, 18:36   #26
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Take a close look at your charts for the region you are cruising. This summer my time has been spent on Chesapeake Bay and the east coast up from florida. Find a chart with range marks on it for ships to use in order to stay in the channel. You will find a dashed line through the pair of marks. Along the dashed line you will also find the course stated as magnetic and the distance between stated points on the chart. You want to be consistent with the chart and with the equipment you plan to use.
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Old 13-10-2010, 18:53   #27
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???

True on the chart, if going my compass then converted to compass (not 'magnetic') for the driver.

I hope this addresses the question.

How can one plot magnetic? (Other than ploting the magnetic N-S on the chart first).

b.
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Old 13-10-2010, 19:08   #28
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I'd write down whatever the compass shows, preserving the "raw data." You then can show "true" magnetic and/or actual direction after adjusting for variation and deviation. If you don't record the actual compass reading, you can't go back and recheck calculations. Good luck with the drift.

If navigating by GPS, I'd simply use true direction being made good.
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Old 13-10-2010, 19:13   #29
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After spending over 28 years in the Coast Guard I always put both true and magnetic on all of my charts. If one of your compasses fails you do not have to convert to go from true to magnetic. Also as you are following your track and you see a large change on your gps that will show true and your magnetic compass you will know something is wrong with one to compasses. Could be as simple as putting something magnetic next to you compass. Helmsmen were always required to know both the magnetic and the true course they were steering on so if one of the compasses failed he could switch to the other and let the conning officer know. Always remember to correct you course for deviation and variation. And notate your courses with ie 010T or 010M. Have fun out there.

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Old 13-10-2010, 19:27   #30
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Boatman61 makes the key point. A true course may be constant from point A to point B, but the magnetic course changes depending on the local variation. This fact is important over a long distance, or when sailing in extreme latitudes. So, write down the magnetic course, because that's what the helmsman needs, but recognize that this course might change as you proceed on your route.
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