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13-10-2010, 16:27
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#16
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Registered User
Join Date: Oct 2005
Posts: 6,185
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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).
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13-10-2010, 16:35
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#17
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Registered User
Join Date: Aug 2010
Location: Boston, MA
Boat: 1968 Ohlson 38 Sloop
Posts: 1,063
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Quote:
Originally Posted by rebel heart
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).
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Yup. have taken coastal navigation classes...which is why i noticed the problem 
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13-10-2010, 16:41
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#18
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Senior Cruiser
Join Date: Jan 2010
Location: PORTUGAL
Posts: 31,081
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Quote:
Originally Posted by phantomracer
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
__________________
You can't oppress a people for over 75 years and have them say.. "I Love You.. ".
"It is better to die standing proud, than to live a lifetime on ones knees.."
Self Defence is no excuse for Genocide...
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13-10-2010, 16:44
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#19
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Senior Cruiser
Join Date: Mar 2003
Location: Thunder Bay, Ontario - 48-29N x 89-20W
Boat: (Cruiser Living On Dirt)
Posts: 51,332
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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.
__________________
Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"
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13-10-2010, 16:59
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#20
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Registered User
Join Date: Aug 2010
Location: Boston, MA
Boat: 1968 Ohlson 38 Sloop
Posts: 1,063
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Quote:
Originally Posted by boatman61
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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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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13-10-2010, 17:14
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#21
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Moderator
Join Date: Feb 2010
Location: La Ciudad de la Misión Didacus de Alcalá en Alta California, Virreinato de Nueva España
Boat: Cal 20
Posts: 21,332
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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.
__________________
Num Me Vexo?
For all of your celestial navigation questions: https://navlist.net/
A house is but a boat so poorly built and so firmly run aground no one would think to try and refloat it.
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13-10-2010, 17:17
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#22
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Senior Cruiser
Join Date: Jan 2010
Location: PORTUGAL
Posts: 31,081
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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.... 
__________________
You can't oppress a people for over 75 years and have them say.. "I Love You.. ".
"It is better to die standing proud, than to live a lifetime on ones knees.."
Self Defence is no excuse for Genocide...
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13-10-2010, 17:30
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#23
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Nearly an old salt
Join Date: Jun 2009
Location: Lefkas Marina ,Greece
Boat: Bavaria 36
Posts: 22,801
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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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13-10-2010, 18:20
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#24
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Registered User
Join Date: Aug 2010
Location: Boston, MA
Boat: 1968 Ohlson 38 Sloop
Posts: 1,063
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Quote:
Originally Posted by goboatingnow
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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13-10-2010, 18:28
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#25
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Senior Cruiser
Join Date: Jan 2010
Location: PORTUGAL
Posts: 31,081
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.... are we being 'trolled'...lol
__________________
You can't oppress a people for over 75 years and have them say.. "I Love You.. ".
"It is better to die standing proud, than to live a lifetime on ones knees.."
Self Defence is no excuse for Genocide...
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13-10-2010, 18:36
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#26
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Registered User
Join Date: Jan 2010
Location: Philadelphia, PA
Boat: 1990 Caliber 38
Posts: 108
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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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13-10-2010, 18:53
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#27
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Registered User
Join Date: Aug 2009
Location: between the devil and the deep blue sea
Boat: a sailing boat
Posts: 20,976
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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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13-10-2010, 19:08
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#28
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Registered User
Join Date: Aug 2010
Location: Central California
Boat: M/V Carquinez Coot
Posts: 3,782
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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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13-10-2010, 19:13
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#29
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Registered User
Join Date: Mar 2010
Location: Coosaw Island SC
Boat: Island trader, Ketch, 40.8ft, Vagabond
Posts: 31
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ploting
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.
Charles
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13-10-2010, 19:27
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#30
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Registered User
Join Date: Apr 2008
Location: Brooklin, Maine U.S.A
Boat: Allures 44
Posts: 734
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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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