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Old 07-11-2010, 20:53   #106
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[QUOTE=phantomracer;556017]great thread.. my original question was what you use.. not really what is right or wrong.. still very interesting.

Knowing why to use a particular method helps to explain why it works...I can wrap my mind around a concept better if I know why, though alot of people are fine with "because that is what you are supposed to do"
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Old 08-11-2010, 05:44   #107
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Originally Posted by wolfenzee View Post
There is a proper way that has been used for hundreds of years...it is proper because it is right.
Methods have changed over the past hundreds of years, so the "proper way" is the one that has evolved. We no longer sail into the uncharted abyss, we know what longitude is, and plane sailing is no longer the proper way - although it still works.
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Old 08-11-2010, 06:33   #108
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Annapolis Book of Seamanship says "most [navigators] prefer to plot in magnetic."

The Annapolis book of seamanship - Google Books
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Old 08-11-2010, 09:51   #109
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Well Mssrs Rousmaniere and Smith have obviously not polled the navigators on this forum
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Old 08-11-2010, 10:09   #110
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Quote:
Originally Posted by sneuman View Post
Annapolis Book of Seamanship says "most [navigators] prefer to plot in magnetic."

The Annapolis book of seamanship - Google Books
They used almost the same words I used earlier - they use magnetic because "it is what the compass uses".

I don't know why I would use true, if I had to adjust for variation for every leg. And then deviation. Seems like too much work to me. At least taking a constant out of the equation..and the deviation on my boat is not bad..using a magnetic heading is 'close enough'. Other factors affect the course more than the deviation (current, etc)

The chartbooks I use (Richardson) have pre-plotted magnetic courses on the charts . I must not be alone plotting magnetic.
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Old 08-11-2010, 10:17   #111
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Originally Posted by wolfenzee View Post
Considering you are steering with a magnetic compass it is best to plot magnetic and ALSO set your GPS to magnetic.
Keep everything agreeing with each other and the simplest the less chance for error
This was the point of my post. I can set my plotter (Lowrance HDS8) to show magnetic for EVERYTHING BUT the course planner, which is ONLY true! I would prefer it to be more consistant.

But don't allow me to use magnetic if you are going to force me to use true when plotting a course. Very strange behavior IMO. Took me a while to realize why my course legs were all off 17 degrees!
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Old 08-11-2010, 10:18   #112
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This thread has been an interesting read, because I learned to navigate in formally in aircraft before boats. In aircraft, and keep in mind this was before GPS, all nav aids were laid out in magnetic, not true direction (they still are by the way).
It's been quite a while, but iirc aero charts don't have compass roses on them, so for planning purposes you measure your track against meridians with a Douglas (square) protractor, which gives True; to which you apply the wind vector (also in True) to get your heading which is then converted to Magnetic (compass) for you to steer.
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Old 09-11-2010, 07:33   #113
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Well Mssrs Rousmaniere and Smith have obviously not polled the navigators on this forum
They didn't poll me either. They seem to have arrived at the correct answer entirely on their own.
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Old 09-11-2010, 14:41   #114
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Yes there is a "right" way and a "wrong" way.....but as it turns out that these even seem to differ. Personally I was taught to plot magnetic, I steer my boat by magnetic...it works for me, no reason to throw a wrench into things because someone says to plot true then convert is the right way. I admit that the method I use may not be the "proper" way.....but it works for me, that is what is important. As far as the initial question, if you are just starting out learning navigation it is best to do it right from the start, most important find something that works and stick with it.
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Old 17-11-2010, 10:03   #115
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Originally Posted by wolfenzee View Post
Yes there is a "right" way and a "wrong" way.....but as it turns out that these even seem to differ. Personally I was taught to plot magnetic, I steer my boat by magnetic...it works for me, no reason to throw a wrench into things because someone says to plot true then convert is the right way. I admit that the method I use may not be the "proper" way.....but it works for me, that is what is important. As far as the initial question, if you are just starting out learning navigation it is best to do it right from the start, most important find something that works and stick with it.
Not starting out. Been on the water since 92. My opening question was just a simple poll to see how people plotted a course..and the thread took many tangents!

My problem was my plotter (hds8 lowrance) plots ONLY in true in the course plotter on the unit, but everything else on the unit you can choose true or magnetic. This caused me problems when I planned a course on paper charts and went to enter it on the plotter..everything was 17 degrees off! Calls to Lowrance have gone unanswered to explain why. Now I know what I am up against and can correct for it. but you would think a plotter could do that for you!

I plot courses using the magnetic compass rose on the paper chart, because thats what my compass shows (got to correct for deviation, which is close to nil on my boat, which makes it easier as well as annual changes). Just was curious how the rest of the sailing community plotted.
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Old 17-11-2010, 13:27   #116
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Goodness. If you had told us that 115 posts ago we could have helped you.

Now you have split the forum and ago old buddies have to be seperated by dividers and slide rules!

Now you come and tell us you are a heathen that uses a chart<spit>plotter!

My advice it to turn the bloody thing off, chuck the paper stuff overboard and sail as the as the Vikings 2,000 years ago: "Vere ze puck you sink ve are boss?"



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Old 17-11-2010, 16:40   #117
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Quote:
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Goodness. If you had told us that 115 posts ago we could have helped you.

Now you have split the forum and ago old buddies have to be seperated by dividers and slide rules!

Now you come and tell us you are a heathen that uses a chart<spit>plotter!

My advice it to turn the bloody thing off, chuck the paper stuff overboard and sail as the as the Vikings 2,000 years ago: "Vere ze puck you sink ve are boss?"



I Mentioned it in more than one post before.

The plotter is below deck, so I dont use it regularly. Just use the paper charts and a handheld gps in the cockpit.
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Old 17-11-2010, 18:52   #118
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G'Day all,

Pedantic arguments about true vs. other plotting methods aside, the real world beckons to us.

Once away from daysailing in ones own backyard, my observation is that most folks actually sail by watching cross track error. Doesn't matter how they plotted their course, doesn't matter if they are hand steering, auto-piloting, or windvaning, the bottom line is where is the boat relative to where you intend to go. And to worry about fractional degree changes in variation, or minor changes in deviation wrought by moving magnetic masses about the boat is usually over the top. Few helmsmen, autopilots or windvanes can maintain a course with less than a few degrees of wandering about over the length of a watch, so the theoretical errors we've been ranting about really don't apply.

Yes, I believe that most cruisers in small yachts check their position every now and then, see if they are on track, and if not, change the desired course to compensate. Some do it with electronic plotters, some with XTE on their GPS, some by plotting positions on paper charts... doesn't matter! In mid ocean one might only do this every few hours, coastally or anywhere there are more impediments to progress more frequent checks make sense.

Another set of factors that affect where you actually go are leeway and drift from current. Very few of us actually know how much leeway our boats make in different sailing conditions. I suspect that it is usually more than we think! Then there are currents. For years we sorta believed the pilot charts, where there are these great broad arrows across the sea, labeled "x miles/day". The advent of GPS on board has shown that there are LOTS of eddies, whirlpools and other deviations from the general flow, that can and do make significant changes in your COG as you traverse them. These deviations are far larger than the minor errors that come from using dated variation data, etc. Incidentally, it is really educational to check one of the websites that show real-time pictures of surface currents and temperatures... they are incredibly complicated!

I expect that professional navigators, trained to operate large vessels with gyro compasses, large crews and (usually) very precise autopilots will disagree with this approach. But in the real world of shorthanded cruising in small yachts, something like what I have described is, I believe, what really goes on. At any rate, this sort of navigation has served Ann and I well for some 150K miles now, and I'm still happy with it.

Cheers to all,

Jim and Ann s/v Insatiable II lying Manly Qld Oz, southbound
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Old 06-12-2010, 04:55   #119
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In all this talk about true and magnetic bearings does anyone care much about the vessels local deviation table? This is the table that would get from swinging the ship. Our professional software allows you to incorporate this table so that you get a choice of true, magnetic or compass bearings - where compass means that this will be the actual reading off your magnetic compass.
Would anyone really like to see this in our Nuno software or is it something that is not actually ever used much?
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Old 17-12-2010, 20:43   #120
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Fascinating - does no-one set their mechanical compass to true?

Scanning the discussion was fascinating!

Everything on my boat is set to to True, including the mechanical compass at the helm station. As a result, there's zero conversion to work out a course and then set it up on deck.

The only thing you have to be really careful about is telling the helmsman that the compass course they're being told is True, and to NOT back out the known local variation. Sometimes people new to the boat, but with local knowledge, will automatically back it out and suddenly we're headed off somewhere else.

As regards swinging the compass, I can get the mechanical compass within 2 degrees of accurate around the card - and the deviation card lists that out.

From an accuracy perspective, in flat water we can maintain course within 3 degrees, in lumpy ocean swells +/- 5 degrees is the norm.

So why is everyone going through the conversion effort when all they have to do is set up the mechanical compass to work in True?

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