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Old 09-03-2015, 19:57   #46
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Re: True or Compas

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Originally Posted by Ex-Calif View Post
I am sure we would get along fine on a boat. Too many of these discussions become a challenge of "right" and "wrong" and you are right that they drift and pretty soone we forget what we are talking about.

You clearly know how to plot a course. So do I. Our conversation is one of clarification and really (partly) is so the lurkers can see that this can be complex.

It also demonstrates the importance of communication, specificity and clarity.

A lot of new folks ask why things on a boat are called something "ambiguous" like a halyard is not a rope. Well it's not ambiguous. A Halyard is a halyard. Pretty specific.

True is True. Magnetic is magnetic. Heading, Course, Bearing. Not ambiguous but specific. So it needs to be learned.
I agree, I don't think we're teaching each other anything. We're just dissecting a question- which I don't think there is harm in- the OP asked a question, he's getting an educated debate that he and others can consider when making their own decisions.
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Old 09-03-2015, 20:12   #47
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Re: True or Compas

Well you did teach me that there are places where mere mortals sail that have hella variation.

I know you know why the variation increases closer to the poles. But I've never considered there are populated (commonly navigated) areas that have to consider it.

I never would have to because those latitudes are too freakin' cold and I wouldn't be caught dead frozen in one of those places - LOL...

(I'll stick to +-4*)
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Old 10-03-2015, 00:15   #48
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Re: True or Compas

The Grand Banks are as good a place as any I know of to prove your seamanship.
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Old 10-03-2015, 01:04   #49
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Re: True or Compas

Dutton's Navigation and Piloting, Dead Reckoning chapter

DR terms defined

Heading ... 0 through 360 degrees from a reference direction--true, magnetic, or compass...

Course ... Course may be designated as true, magnetic, compass, or grid ...

Labeling DR plot ... Should course be stated with reference to another base direction, an appropriate letter is added following the digits, such as M for magnetic...


(Dutton's states default requiring no letter is true angle.)


Haven't taken a fancy course, just my club's course and I got the book to go by. I have made do with navigating local magnetic, ocean true. Most the boats I've sailed the deviation is less than my course wandering, not to mention the 0.5 to 10 knot currents making a hash of the direction you're going.
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Old 10-03-2015, 04:07   #50
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Re: True or Compas

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....... Most the boats I've sailed the deviation is less than my course wandering, not to mention the 0.5 to 10 knot currents making a hash of the direction you're going.
I once swung the compass on my (glass) yacht and made up a deviation card.. max deviation was about 4 degrees. As you say that is neither here nor there in the scheme of things where compass cards are typically only marked to 'half points' ie about 5 degrees.

Back to the OP.... working on a paper chart.... do your work in 'true'. If you lay off a course on the chart read it off in true degrees , apply variation and -if you must - deviation. Taking a bearing? Convert it back to true before applying to chart.

Further down... a question re steel boats. Adjusting a compass on a steel yacht? A good compass for a steel boat would have Lord Kelvin's Balls and maybe even a Flinder's Bar... on a smaller yacht compass maybe not but even the Plastimo 135(?) has the means for adjustment through those little holes in the binnacle. You should be able to adjust the compass down into reasonable levels on all headings but a 'Deviation Card' would be a must.

A copy of Alan Pickles' 'All about Marine Compasses' ( ISBN 0 7316 3280 X )is a handy reference work.

Re variation worldwide.... off the top of my head... on passage from Fremantle to Bass Strait the variation will go from about 20W to 15E on what would be about a 15 day passage for many.
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Old 10-03-2015, 05:23   #51
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Old 10-03-2015, 05:55   #52
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Re: True or Compas

always plot in true,
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Old 10-03-2015, 07:36   #53
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Re: True or Compas

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Originally Posted by Time2Go View Post
I have always ploted using True but I have recently had a few conversations
Where a few said no plot in magnetic,
Are UPS still used
In thereal world of navigating with GPS and Chartplotter Magnetic becomes irrelevant.
When one steers by compass the numbers are only a reference. You will still be navigating by chartplotter.
If you are in an area of high deviation (here its 13degrrees) you can add o subtract it at the compass on the go, if you so desire.


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Old 10-03-2015, 07:54   #54
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Re: True or Compas

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In thereal world of navigating with GPS and Chartplotter Magnetic becomes irrelevant.
When one steers by compass the numbers are only a reference. You will still be navigating by chartplotter.
Concur, and it's also true with paper charts. When sailing just trying to get the best long run WMG to the true course whatever the magnetic compass reading might be..
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Old 10-03-2015, 09:03   #55
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Re: True or Compas

If it only were that simple.

Now one elects to draw his lines off the N portion of that rosie. OK. Fine. BUT that N portion of that rosie will drift away at a rate and within a couple of years your reference line is way off anything you have on the chart. Are you going to draw a new reference line then?

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Old 10-03-2015, 12:02   #56
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Re: True or Compas

I cant understand why anyone would recommend plotting in Magnetic , most charts dont have current variation figures and you need to do allowances, it might have had uses in the days of widespread use of parallel rulers, but today most everyone I know uses breton style plotters and hence can use any vertical or horizontal line on the chart for a reference.

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Old 10-03-2015, 12:17   #57
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Re: True or Compas

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I cant understand why anyone would recommend plotting in Magnetic , most charts dont have current variation figures and you need to do allowances, it might have had uses in the days of widespread use of parallel rulers, but today most everyone I know uses breton style plotters and hence can use any vertical or horizontal line on the chart for a reference.

dave
Breton plotters? I'm curious now, because my parallel rulers are kind of a pain on my chart table. Is this a useful product?

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Old 10-03-2015, 12:22   #58
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Re: True or Compas

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Breton plotters? I'm curious now, because my parallel rulers are kind of a pain on my chart table. Is this a useful product?

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also called Portland plotters

very handy on small chart tables where the chart is often folded
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Old 10-03-2015, 12:29   #59
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Re: True or Compas

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also called Portland plotters

very handy on small chart tables where the chart is often folded
I've seen these. Interesting. Never tried one though. I can see why it would be useful on a boats tiny chart table. Thanks.

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Old 10-03-2015, 12:32   #60
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Re: True or Compas

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I've seen these. Interesting. Never tried one though. I can see why it would be useful on a boats tiny chart table. Thanks.

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Portland plotters are the only way to go on a small boat. One advantage is that you can 'mark' your variation on the plotter and lay compass bearings directly onto the chart as true bearings.... if you still do stuff like that.
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