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Old 02-06-2018, 18:45   #1
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true vs magnetic bearing

I would appreciate it if someone explains how they use true or magnetic bearing for navigation.


I believe to use magnetic bearing I need to find the deviation of my compass ?


Thanks,


jim
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Old 02-06-2018, 18:47   #2
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Re: true vs magnetic bearing

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Originally Posted by xxhat View Post
I would appreciate it if someone explains how they use true or magnetic bearing for navigation.


I believe to use magnetic bearing I need to find the deviation of my compass ?


Thanks,


jim
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Old 02-06-2018, 19:07   #3
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Re: true vs magnetic bearing

In the days of using paper charts, using childbearing compass for fixes and steering by the magnetic compass I used magnetic headings for most everything. Now with a chartploter giving me a continuous super accurate fix, I work in true readings.

If I wanted the approx bearing of say a flair I saw, then I'd look over the compass and get a magnetic bearing. I would convert to True if I needed to plot it.
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Old 02-06-2018, 19:24   #4
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Re: true vs magnetic bearing

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Originally Posted by xxhat View Post
I would appreciate it if someone explains how they use true or magnetic bearing for navigation.


I believe to use magnetic bearing I need to find the deviation of my compass ?
This is one of those topics where people can have strong opinions, but in truth it matters little--as long as you understand the difference and are consistent about it.

Historically, recreational sailors in Europe used TRUE headings, and in North America MAGNETIC was more popular. Part of the reason for this is the variation between the two is much smaller in most of the sailing waters of Europe than on the coasts of North America. The result is that in most of Europe, the biggest mistake you can make is just 4 or 5 degrees. In NA, one missed minus sign during the conversion process could have you 30 degrees off course. (Don't skip the whiskey with the virgins!)

Professional sailors have long since switched to using TRUE because ships have transitioned to using gyro compasses and they therefore see true north on all their instruments.

In a modern boat with all electronic displays, you might as well just use TRUE because you can tell your fluxgate compass to do the corrections for you.

If you are using a magnetic compass as your primary steering compass, I stick with magnetic all the way through the process because it reduces the possibility of conversion error.
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Old 02-06-2018, 19:27   #5
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Re: true vs magnetic bearing

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, using childbearing compass for fixes
This I gotta see!
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Old 02-06-2018, 19:38   #6
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Re: true vs magnetic bearing

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Originally Posted by xxhat View Post
I would appreciate it if someone explains how they use true or magnetic bearing for navigation.


I believe to use magnetic bearing I need to find the deviation of my compass ?


Thanks,


jim
Hi Jim,
Yes, you do need to do what is called "swinging your compass" to adjust it to take correct magnetic readings for your boat, since your compass will be influenced by stuff on your boat. These days with a cell phone that can tell you your true heading or magnetic heading, and chartplotters, things are sinfully easier. Here are a couple of good articles about swinging the compass:
How to swing a compass - Practical Boat Owner
https://www.skippertips.com/members/9197.cfm

Do you have a hand bearing compass?
Welcome aboard, good luck.
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Old 02-06-2018, 19:38   #7
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Re: true vs magnetic bearing

On my boat I only use magnetic. It's what you steer by and is easy to check. Your helm's compass and flux gates are impacted by metal object around it. So verify installation, your steering compass may or may not have deviation, if more that 3-5 degrees it might need to be professional calibrated (to have it swung).

To verify you should get a good handheld compass, looks like a hockey punk. The compass in binoculars, usually isn't accurate enough. While out on the water get a fix on land (no buoy) of something like a lighthouse a couple miles away, best in no current. Remember the reading, next verify with a bearing with the GPS cursor (setup for mag). Then take the helm and swing bow thru the mark, going both ways. Note the steering compass readings on the mark. Best done with two people, one marking the reading on the steering compass and one with the handheld. Try this on a couple different points of sail (bearings) and that's your deviation.


Autohelm and flux-gate compasses are different and should refer to the owners manuals.
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Old 02-06-2018, 19:42   #8
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Re: true vs magnetic bearing

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Historically, recreational sailors in Europe used TRUE headings, and in North America MAGNETIC was more popular. Part of the reason for this is the variation between the two is much smaller in most of the sailing waters of Europe than on the coasts of North America.
Another reason, I suspect,is that US charts usually printed a magnetic compass rose as well as the true rose, and European charts typically only showed the true. Meant that it was easy for us Yanks to plot using magnetic.

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Old 02-06-2018, 20:41   #9
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Re: true vs magnetic bearing

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This I gotta see!
Pretty sure I meant handbearing compass, but I'll check with my wife .She ought to know as she's a Midwife.
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Old 02-06-2018, 20:54   #10
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Re: true vs magnetic bearing

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Another reason, I suspect,is that US charts usually printed a magnetic compass rose as well as the true rose, and European charts typically only showed the true. Meant that it was easy for us Yanks to plot using magnetic.

Jim
.. don't think I've every actually seen a chart that didn't have magnetic variation on the compass rose.... Learning in the UK it was all chart nav in true then converted to compass for the helmsman.

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Old 03-06-2018, 08:45   #11
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Re: true vs magnetic bearing

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This is one of those topics where people can have strong opinions, but in truth it matters little--as long as you understand the difference and are consistent about it.

Historically, recreational sailors in Europe used TRUE headings, and in North America MAGNETIC was more popular. Part of the reason for this is the variation between the two is much smaller in most of the sailing waters of Europe than on the coasts of North America. The result is that in most of Europe, the biggest mistake you can make is just 4 or 5 degrees. In NA, one missed minus sign during the conversion process could have you 30 degrees off course. (Don't skip the whiskey with the virgins!)

Professional sailors have long since switched to using TRUE because ships have transitioned to using gyro compasses and they therefore see true north on all their instruments.

In a modern boat with all electronic displays, you might as well just use TRUE because you can tell your fluxgate compass to do the corrections for you.

If you are using a magnetic compass as your primary steering compass, I stick with magnetic all the way through the process because it reduces the possibility of conversion error.
just 4 or 5 degrees in miles your talking 60 miles to a degree times 4 or 5 degrees 240 to 300 miles thats one hell of a miss....
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Old 03-06-2018, 08:50   #12
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Re: true vs magnetic bearing

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just 4 or 5 degrees in miles your talking 60 miles to a degree times 4 or 5 degrees 240 to 300 miles thats one hell of a miss....
How accurately do you think you can steer to a compass. Navigators on small boats used to always round the course to steer for the helmsman to an even 5 degs, as that is the best you could read the compass and attempt to steer to.
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Old 03-06-2018, 09:08   #13
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Re: true vs magnetic bearing

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just 4 or 5 degrees in miles your talking 60 miles to a degree times 4 or 5 degrees 240 to 300 miles thats one hell of a miss....
Am I misunderstanding you here? To be 60 miles off on a 1 degree steering error you would have to run a course of 3437 miles ... not many people DR for that long.

A 5 degree error on a 100 mile course will put you almost 9 miles off target ... but still within sight of an island with any topography.
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Old 03-06-2018, 09:11   #14
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Re: true vs magnetic bearing

To summarize, the difference between magnetic and true is variation. Variation changes dramatically for different locations. In some areas where the variation is less than, say 4 degrees, for practical purposes they are much the same. However other areas can have extreme variation, so corrections are required. Charts invariably show variation for the area.

Remembering the sign for variation:

East is least, West is best.
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Old 03-06-2018, 09:19   #15
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Re: true vs magnetic bearing

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To summarize, the difference between magnetic and true is variation.
Also worth noting that on land "variation" is called "declination" ... same thing just a different name.
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