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Old 03-06-2018, 15:05   #46
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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
This is going to hurt because it's over 40 years ago since my coastal navigation course.

The reason magnetic variation exists is because the earths magnetic field affects compass readings and the variation is not the same all over and it changes over time.

A relatively new paper chart (the variation for the area shown on the chart won't be out of date) will show the variation on the compass rose expressed as a number W or E. (West or East).

Other posters have talked about "West is best and East is least" which is just an easy way of remembering that Westerly variations are added to a compass reading and Easterly variations are subtracted both calculations resulting in the True bearing. If you wanted to steer a course of 100 degrees and the chart variation is 2 degrees w then the course to steer on your compass would be 102 degrees. This 102 is the true heading.

Your ships compass is something entirely separate from these calculations. Other posters have talked about swinging a compass which is the method used to establish your compass is reading correctly. Sailors of old would swing their compass everytime they left port and it's probably a good habit to get into. This wasn't done because compasses were unreliable but because they are so susceptible to out side influences. Place a drink can down next to your compass and you may end up exploring some place you never intended going to.
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Old 03-06-2018, 15:05   #47
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Re: true vs magnetic bearing

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Originally Posted by Sailorbob8599 View Post
They say 'what's in a word' but as you are responding to a neophyte who is genuinely trying to educate himself, I think it behooves all responders to at least proof read their response to be sure 'autocorrect' has not created more doubt as to the writers response. ie: childbearing compass? & flair? Come-on guys, it may sound picky to some of you but surely you can do better.
I guess a childbearing compass is one you get for cub scouts.
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Old 03-06-2018, 15:49   #48
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Re: true vs magnetic bearing

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Originally Posted by Tbonem17 View Post
...

Your ships compass is something entirely separate from these calculations. Other posters have talked about swinging a compass which is the method used to establish your compass is reading correctly. Sailors of old would swing their compass everytime they left port and it's probably a good habit to get into. This wasn't done because compasses were unreliable but because they are so susceptible to out side influences. Place a drink can down next to your compass and you may end up exploring some place you never intended going to.
Tony
And of course many modern yachts in the age of electronics dont have properly adjusted compasses.

In preparation for delivering a recently refit boat once, I was going over the boat. I noticed that much attention had been paid to cosmetic items while some important maintenance items had been neglected. So, when I noticed the big shiny new Ritchie Navigator compass...I was suspect. A quick check showed it to be as much as 10d off on some headings!, but it sure looked pretty.
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Old 03-06-2018, 16:26   #49
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Re: true vs magnetic bearing

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Sorry to correct you
Declination, is the angle of Dip of the Earths Magnetic Field, this magnetic field is only parallel to the Earth's surface at the Magnetic equator, and is vertical to the Earth's surface at the Magnetic Poles, which explains why a Magnetic Compass cannot be used at high latitudes.
Sorry to correct your correction...

Magnetic declination is the angle between magnetic north and true north.

Magnetic inclination is the dip of the magnetic field vs the earth's surface.
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Old 03-06-2018, 16:34   #50
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Re: true vs magnetic bearing

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Originally Posted by KiwiKen View Post
....
A handy mnenonic to remember this is:
Deviation East Magnetic Least (Compass North is East of Magnetic North)

Deviation West Magnetic Best


If we wish to plot the course on a Chart we must convert the Magnetic Heading to a True Heading

A handy mnenonic to remember this is:
Variation East Magnetic Least ( Magnetic North is East of true North)

Variation West Magnetic Best
All the usual mnemonics for remembering how to handle these calculations seemed to come up a little short to me, so I made up my own extended version:

"True Virgins Make Dull Company add Whisky, subtract Ethics".

A nice feature of this one is that it flows logically from True to Magnetic to Compass and in the opposite direction too. For example, if you are converting from True to Compass then you can think of this as "corrupting". Therefore to corrupt you add Whisky (West) and subtract Ethics (East)....the result is more "interesting" company and a correct conversion. If you are "purifying" (going from Compass to True for example) then you do the opposite: add Ethics, subtract Whisky.
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Old 03-06-2018, 16:42   #51
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Re: true vs magnetic bearing

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Originally Posted by Kelkara View Post
Sorry to correct your correction...

Magnetic declination is the angle between magnetic north and true north.

Magnetic inclination is the dip of the magnetic field vs the earth's surface.
Exactly
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Old 03-06-2018, 16:47   #52
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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 ?
To be absolutely precise, once you apply deviation, you'll be using compass bearings.
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Old 03-06-2018, 17:09   #53
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Re: true vs magnetic bearing

Then we also have, besides childbearing sextants ( Martha, you young hussy! Are you laying my son on the horizon?) , the dreaded deviants. Deviation is the affect your vessel has on the compass reading - which itself can vary with the magnetic bearing. The deviants lurk around your compass and can lead you away from the righteous pathway. Then there is inclination ..nothing to do with childbearing although it does involve what angle the field is laying at. Still, magnetism is all about attraction .....
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Old 03-06-2018, 17:47   #54
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Re: true vs magnetic bearing

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Originally Posted by Kelkara View Post
Sorry to correct your correction...

Magnetic declination is the angle between magnetic north and true north.

Magnetic inclination is the dip of the magnetic field vs the earth's surface.
Correct, and it is important, particularly for cruisers traveling long distances. I no longer recall detail, but it is possible, perhaps even probable, that a compass with inclination set for, say, Perth, Australia, will not work properly, or at all (?) off of Vancouver, British Columbia. The compass makers (or?) have established something like seven zones in the world for inclination settings. It all makes me want to take a nav refresher class, and sail again sans electronic instruments. Doing so would be foolish, but it is a darn good idea to be able to do so.
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Old 03-06-2018, 19:29   #55
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Re: true vs magnetic bearing

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Originally Posted by belizesailor View Post
.... For handbearing compasses, "deviation" (not "variation") is assumed to be zero. This is based on the assumption that user is smart enough to move away from significant sources of deviation aboard the boat when taking the bearings.
.........
I don't know about this. I used to have issues with my handbearing hockey puck and the metal in my glasses.
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Old 03-06-2018, 19:31   #56
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Re: true vs magnetic bearing

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Originally Posted by Sailorbob8599 View Post
OK Canucks, that being so, perhaps we shud warn Mr.Trump if he continues to screw with NAFTA we will retaliate by moving the pole! That shud get his attention.
Oh no, a typo. Other than.that I am sure it was very useful for the OP
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Old 03-06-2018, 19:39   #57
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Re: true vs magnetic bearing

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Originally Posted by Paul L View Post
I don't know about this. I used to have issues with my handbearing hockey puck and the metal in my glasses.
See, you were smart enough to figure that out!

I think another reason is that it just gets a bit impractical...who is gonna make up a deviation card for a hand bearing compass...then stand in EXACTLY the same spot to take a bearing each time. Just standing away from most influences is likely enough for all practical purposes (and maybe getting titanium frames!).
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Old 03-06-2018, 20:44   #58
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Re: true vs magnetic bearing

Very interesting thread... Old school sailors and commercial operators all became very adept at sorting out magnetic from true compass bearings depending upon where they were traveling. Even the older iron ships that had been ‘degauzed’ needed to have deviation tables.
I recall when a friend of mine bought a (new to him) 43 foot sailboat and not being experienced off shore talked the seller into accompanying him from Morro Bay to San Diego. The seller told him that one of them would have to stay up and steer because the autopilot was B/O.
After they arrived in San Diego, my buddy asked me to sort out his auto pilot. I found that the fluxgate compass on the’pilot was mounted behind the VHF!l. It still had 40 feet of cable attached and rolled up.
I moved it down below the cabin sole and duck taped it amid ships.
We went out and the autopilot performed flawlessly! He called the prior owner who had put up with the recalcitrant autopilot for years since it had been professionately installed!
My friend permanently installed his autopilot in the location that I had taped it to and still uses it to day. This all happened over 20 years ago.
Phil
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Old 03-06-2018, 22:42   #59
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Re: true vs magnetic bearing

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Originally Posted by Kelkara View Post
Also worth noting that on land "variation" is called "declination" ... same thing just a different name.


Don’t try to use declination for variation in a marine setting, declination in marine setting is used to mean the latitude of stars and other celestial bodies in the sky, used for celestial nav which seems to be coming back a little bit.
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Old 04-06-2018, 00:04   #60
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Re: true vs magnetic bearing

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Yes, please remember that Canada owns the north pole. So be nice to Canadians...we let you use our pole for free...for now.

Well put. Be nice to Antarcticans, too, as _your_ pole is pretty useless without theirs...
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