True or Magnetic for Electronic Nav

[LBW399]

Originally Posted by jeremiason

Unless you are regularly plotting on paper charts and using the True Heading data from your Electronic Chart plotter to do it, it makes no sense to display true heading.

You don't steer true course, you steer magnetic course using a compass mount near the helm. So why would you display a true heading on you chart plotter?

For those of you that are concerned about the chart plotter failing and having to revert to paper, you can convert to True for plotting of your last known magnetic bearing or simply use the ship's compass and convert your current magnetic bearing.

Just my humble opinion.

Almost all my charts will be electronic. And your post now making me thinking of heading the other way and have everything read out it magnetic....what to do....guess I should choose one way and stick with it.

Having figured out the key words to "search" through old forum threads, I can now see how this and similar topics can cause a bit of a stir - unintended on my part. Still, some interesting replies.

[LBW399]

Originally Posted by GrowleyMonster

The only problem with using Magnetic is your deviation may change radically with a change in heading. If you want to split hairs, you should have a compass deviation card that you can refer to. This is especially important with steel boats, but there is often enough steel present on a glass boat to make a difference. Anyway, because of the changing deviation, as opposed to variation which will be the same on any heading for a given position, the difference between True and Magnetic will be different, depending on which way you are pointing. That would appear to present a (minor) problem, wouldn't it?

You can adjust out most of the deviation in your compass, though, but it can be a frustrating exercise if you don't know exactly how to do it, or if you have a dinky little compass and a typical small boat installation. You can download HO226, the Handbook of Compass Adjustment, in PDF though I must warn you it is a bit dry and technical. The final step is of course to construct your deviation table.

HOWEVER, we are talking about sailboats, which can have a tremendous amount of leeway into which three or four degrees of deviation can simply disappear. For taking bearings, though, it can be significant. So YMMV.

Personally, I do everything in True, because that is how it is done aboard ship. And because True North happens to point precisely to where all meridians converge at the true pole. Everything just makes a lot more sense when you look at the big picture, to me, anyway. It is simple enough to refer to your variation and deviation and convert to a course to steer, or more importantly, to convert a visual magnetic bearing to a true one. On a ship, a gyrocompass is used, and the only error is gyro error, typically a degree or less. We only use the magnetic compass to check against the gyro and in the event that the gyro quits working. So there are no issues of convenience for us, using true, while it could be seen as slightly more convenient on a sailboat to use magnetic.

But is it a bad thing, to use Magnetic? On a sailboat, not really. Especially if you cruise in one small area.

I guess it is really a personal preference thing. And it isn't an earthshaking big deal if we disagree. Do it how ya feel it.

Hey good thought provoking stuff Growley. No I won't be cruising in one small area....well at times I will be, but I also intend to sail offshore.

[David M]

If you decide to use True, you still have to convert to Magnetic in order to use your steering compass.

On ships there is still a conversion for Gyro Error. The gyro and the gyro repeaters don't always point perfectly North.

[Jim Cate]

FWIW,

I prefer to work in magnetic like many others who rely upon magnetic compasses. In this part of the Pacific the variation ranges between around 10 to more than 15 degrees, and we move around enough that it changes significantly from week to week.

Before I changed from Max Sea to OPCN I was happy to set the parameters to read out in magnetic and all was well. So far with OPCN this has not been an option, despite periodic requests for this feature to be added. Of course Ann and I have managed to continue navigating, but it is an irritation and an occasional source of error for us.

We continue to hope that the good folks who put so much effort into maintaining OPCN will someday hear our plea and add this minor feature (HINT, HINT).

Cheers,

Jim

[nhschneider]

In John Rousmaniere's book, The Annapolis Book of Seamanship, he recommends doing everything in magnetic unless you're using celestial methods of determining position.

Fair winds and calm seas.

[GrowleyMonster]

Originally Posted by nhschneider

In John Rousmaniere's book, The Annapolis Book of Seamanship, he recommends doing everything in magnetic unless you're using celestial methods of determining position.

Fair winds and calm seas.

Indeed. Another good reason, and any mariner contemplating crossing an ocean ought to be prepared to resort to celestial navigation.

[sailorboy1]

some of you much have really great eye sight and very very sharp pencils if 2 degrees of variance etc are enough to make a difference when using your charts, and if your deviation is that great maybe it is time to move the gameboy away from the compass

[capn_billl]

I would go with true. You have to convert compass headings to the chart anyway.

I.E. <-------------------------64T------------67.9M----------------------->

[LBW399]

Originally Posted by nhschneider

In John Rousmaniere's book, The Annapolis Book of Seamanship, he recommends doing everything in magnetic unless you're using celestial methods of determining position.

Fair winds and calm seas.

A slight digression into celstial but....but I must be missing something with regards celestial and true/magnetic....why would it matter to do everything in true rather than magnetic if you are using celstial (the intercept method via HO249 and plotting your lat/long fixes from that)....why would it matter?

[LJH]

Originally Posted by Kiwi399

I understand that most if not all PC or IPAD nav programmes / apps give you the option of giving you all data in true or magnetic.

If using electronic nav aids, but still preferring to steer a course via an old fashioned compass - then you should choose to have everything on your electronic gadgets read out in magnetic to save having to apply local variation for course to steer....right?

And for those of you that sail offshore, what is your peference?

Perhaps silly questions, but am just interested in whom does what.

It is a personal preference. I don't think that there is much advantage of over the other. I have only sailed with electronic charts for about 2 years. Offshore, I do all my log keeping in True and use zulu time. My chart plotter at the nav table is always in true and I let the helmsman choose mag or true for the chart plotter in the cockpit. If I ask him to steer 10 degrees to port it doesn't matter if he is in true or mag.

[Lagoon4us]

Originally Posted by Don Lucas

some of you much have really great eye sight and very very sharp pencils if 2 degrees of variance etc are enough to make a difference when using your charts, and if your deviation is that great maybe it is time to move the gameboy away from the compass

As Jim correctly said around Australia's coast the variation is quite extreme. 15deg is deadly game-boys well excluded...

Cheers

[GrowleyMonster]

Originally Posted by Kiwi399

A slight digression into celstial but....but I must be missing something with regards celestial and true/magnetic....why would it matter to do everything in true rather than magnetic if you are using celstial (the intercept method via HO249 and plotting your lat/long fixes from that)....why would it matter?

Your azimuth is True. Your Lat/Long lines on your chart which will be your reference when plotting your azimuth and intercept from your AP will be aligned with True North. Oh, and an amplitude or azimuth (for the all important regular checks of your compass) will be True. So you have to do the conversion anyhow.

No, keeping True on your chart or your charting program is not a mandatory thing. I just think it is better, for those who have to take their navigation a bit more seriously. Between changing variation and deviation which changes with your heading, a long voyage calls for using True and converting to Magnetic for your course to steer. And even this is not precisely necessary since your steered course will depend more on available and forecast winds than any arbitrary line drawn on a chart.

Put it this way... True North is the top of the globe, the reference with which your lines of Latitude and Longitude are aligned. For sailing in a small area, it doesn't matter. For long range cruising, it does. CAN you use magnetic and convert to true only when absolutely necessary? Sure. I just don't think it is the BEST way. Second best works, but best is best. Again, do it like you feel it.

[Bluewaters2812]

Originally Posted by capn_billl

I would go with true. You have to convert compass headings to the chart anyway.

I.E. <-------------------------64T------------67.9M----------------------->

Lol, I would love to see either a boat or an aircraft be steered by its pilot at exactly 64 or 68 deg, let alone 67.9. Methinks an underwater rail that guides your boat might be a consideration, albeit an utterly unusable system.

[LJH]

Originally Posted by capn_billl

I would go with true. You have to convert compass headings to the chart anyway.

I.E. <-------------------------64T------------67.9M----------------------->

Reminds me of the old saying "measure it with a micrometer and cut it with an axe!

[Bluewaters2812]

Either system will work but for me, as a private pilot I was taught to use true and apply the magnetic variation. I have always used that since and it is 2nd nature to me. I enjoy "the fun" of being aware of what the variation is wherever you are positioned in the world. As a matter of interest the variation in the Eastern part of the Caribbean is -15deg (Barbados) and -5deg (Key West). Roughly halfway between the two is the Dominican republic at -10deg. There is only about a 2 degree difference in magnetic variation between Key West and The Bahamas and only about a 1 degree difference if setting course for The Bahamas from Miami or West Palm Beach. Now, all of you in Thailand ... armed with this useless information can take on the world lol.