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Old 03-06-2018, 09:24   #16
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Re: true vs magnetic bearing

In effect sailing the boat steer true off your chartplotter, applying that to the chart is easy. No matter how well your compass is swung it will always be different, but you can make practical adjustment to that like +5 etc..

And remember when shooting sights off a handbearing compass (like compass binoculars) to convert it to true on the chart.

its a cocked hat !
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Old 03-06-2018, 09:27   #17
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Re: true vs magnetic bearing

Donít forget variation. It is on the compass rose on the chart. Then check the printing date on the chart and apply the ď per yearĒ reference.
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Old 03-06-2018, 09:41   #18
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Re: true vs magnetic bearing

Yeah I still have some old paper charts (some that came with the boat 50 years ago!) and I just apply the new variation... ahh I miss the good old days when you could be off 5 or 10 miles and you were never quite sure where you were! There's just no adventure anymore!
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Old 03-06-2018, 09:46   #19
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Re: true vs magnetic bearing

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Originally Posted by Don C L View Post
Yeah I still have some old paper charts (some that came with the boat 50 years ago!) and I just apply the new variation... ahh I miss the good old days when you could be off 5 or 10 miles and you were never quite sure where you were! There's just no adventure anymore!

Hahahahahahaha.



Thanks for that. Me, too.


But, but, but --- betcha we can still do it if we set our minds to it.
My understanding of M vs T, from Cahpman's and Dutton's was that M works for "local" navigation not covering more than say 100 nm. For planning purpose for longer voyages, i.e., offshore, T is necessary. One reason, IIRC, is that long distances, usually going E/W vs. N/S is that the variation changes, so T is essential.
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Old 03-06-2018, 09:57   #20
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Re: true vs magnetic bearing

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Originally Posted by Stu Jackson View Post
Hahahahahahaha.



Thanks for that. Me, too.


But, but, but --- betcha we can still do it if we set our minds to it.
My understanding of M vs T, from Cahpman's and Dutton's was that M works for "local" navigation not covering more than say 100 nm. For planning purpose for longer voyages, i.e., offshore, T is necessary. One reason, IIRC, is that long distances, usually going E/W vs. N/S is that the variation changes, so T is essential.
Variation changes is not really on any E/W or N/S lines. Check this chart
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Old 03-06-2018, 09:57   #21
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Re: true vs magnetic bearing

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Originally Posted by Stu Jackson View Post
Hahahahahahaha.



Thanks for that. Me, too.


But, but, but --- betcha we can still do it if we set our minds to it.
My understanding of M vs T, from Cahpman's and Dutton's was that M works for "local" navigation not covering more than say 100 nm. For planning purpose for longer voyages, i.e., offshore, T is necessary. One reason, IIRC, is that long distances, usually going E/W vs. N/S is that the variation changes, so T is essential.
Oh yeah, I'll have to dig out the charts but pretty sure you can see different variations on different compass roses on the same chart in my neighborhood. But still I remember as a kid sailing down the coast at night, my buddy's boat had no light in the compass so I'd shine a flashlight on it once in a while. I got tired of that and used the stars... which of course MOVE! By morning we were far out of sight of land with no idea of position... now that was adventure!
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Old 03-06-2018, 10:00   #22
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Re: true vs magnetic bearing

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>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>I got tired of that and used the stars... which of course MOVE! By morning we were far out of sight of land with no idea of position... now that was adventure!

Now that's a great story. We should start a new thread: "How to Navigate - NOT!"


Paul, thanks for the clarification, great graphic, too. Those pesky variations...
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Old 03-06-2018, 10:05   #23
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Re: true vs magnetic bearing

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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.
My mistake thank you for the info....
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Old 03-06-2018, 10:06   #24
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Re: true vs magnetic bearing

i think that we need to get back to first principles here. You must always plot courses on a chart in true bearings as that is what the chart is made up from. Mr Mercator spent a lot of time proving his point.
A magnetic compass points towards magnetic North which is somewhere in the North of Canada, but throughout the world the lines of magnetic force vary cnsiderably. At the moment in the UK variation is close to zero degrees, it having changed over the 50 years that I have been using charts from 7 degrees West. Variation can be as much as 30 degrees or more so trying to navigate on a chart using magnetic bearings is going to get you nowhere, or somewhere that you do not want to be!
Remember that you do not always know exactly where you are, but you must always know exactly where you are not!
You should always mark places on your chart with a cross hatch to point out those places that you must avoid - this is basic chart reading.
Now the next complication is that your compass on the boat is influenced by the metal around it, ie electric cables carrying current, and metal objects. Carelessly placed penknives close to the compass, or even binoculars when underway can affect what the compass is reading. That is why there are usually transits around ports so that when you leave you can check your headings and confirm that nothing untoward has happened to your compass.
The compass itself must be swung so that you do indeed know if it has significant errors. If it does then you will need to put corrective magnets close by - a skilled job. Even after that, you must then draw up a list of corrections for each heading from 0-360 degrees.
If you are lucky and the discrepancies are minor then, the corrections that you apply when helming will hopefully be small. As has been said, in a seaway you are only going to be able to steer +/- 5 degrees, but the skilled helmsman will make sure that he will average out his swing so that the average course is close to that which he wants to achieve.
Unless you have an ECDIS standard chart plotter then you will need to be able to plot your course, DR and EP on a chart as yachtie chart plotters are not good enough to be 100% reliable and accurate. Just read how many professional skippers make errors and cause their vessels to run aground using electronic plotters. I think that the US minelayer which went aground in the Philippines was 10 miles off course using just electronic plotters.
So you need to know the variation and deviation for each heading, and combined together they are the error of your compass. Fog and nightfall can make your systems very much in doubt if you are not plotting your DR and EP.
Always remember the basics of Navigation. It has a greater chance of keeping you out of trouble, and make you confident that you are making the right decisions.
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Old 03-06-2018, 10:12   #25
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Re: true vs magnetic bearing

Chartplotter. Done.
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Old 03-06-2018, 10:14   #26
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Re: true vs magnetic bearing

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Originally Posted by mikecambrai View Post
A magnetic compass points towards magnetic North which is somewhere in the North of Canada
Yes, please remember that Canada owns the north pole. So be nice to Canadians...we let you use our pole for free...for now.
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Old 03-06-2018, 10:22   #27
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Re: true vs magnetic bearing

I use my chartplotter but I still plot and track my course on papercharts. Using the declination diagram allows you to position the chart / map correctly to offset the magnetic variation of wherever you are. I still carry my old military lensatic compass wherever I go. Itís damn accurate and I can generally come out right where I need to be when on land.
Of course itís a little hard to use when thereís no land to shoot an azimuth to, but when I can see land and get at least two points of referance, I know right where I am. Then I go back the plotter and confirm how bad Iíve gotten at LandNav......itís a perishable skill if not used routinely.
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Old 03-06-2018, 10:23   #28
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Re: true vs magnetic bearing

On smaller sailboats, hand steering and using a magnetic compass introduced its own set of variables but over time, changing watches, and using the sextant, it was still expected to be very close for the last segment. Using DR and the magnetic compass, usually got you to the right neighborhood which was OK on a small boat in open waters. Around shallow water and shores "bell buoys" provided sound assistance if you were in an area where they existed and were maintained, as did lights (on coasts, on buoys, and even light ships, not to mention buoys with numbers and markers. Oh, and a sharp lookout was also vital. You younger sailors have it so easy! There must be some who cannot even use a pencil.
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Old 03-06-2018, 10:43   #29
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Re: true vs magnetic bearing

[QUOTE=xxhat;2643902]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 ?

Due to the planet preparing to flip magnetic polarity in the near future, the magnetic pole that used to sit stable in the northern archipelago of Canada has taken a mad dash in the last decade and is moving faster and faster away from where it used to be. I don't know how long it takes for the north magnetic pole to end up at the south pole, but I do know the poles reverse every two million years. You will be shocked to see how far the magnetic pole has traveled in the last 10 years. Charts are going to have to be really current from now on, if you want to get any accuracy from them. Good Luck.
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Old 03-06-2018, 11:04   #30
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Re: true vs magnetic bearing

I'd guess at most lat. it doesn't make much difference. Maybe if you are sailing to a pole.
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