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Old 11-05-2011, 04:11   #46
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Re: What Heading Am I Really On ?

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Originally Posted by bewitched View Post
Isn't there a significant point being missed here?

Neither the binacle compass nor the fluxgate compass (whether displaying true or magnetic) actually tell you where the boat is going. They only tell you which.way the bow is pointing. The HEADING It's fairly irrelevant information.
That is why it is called “Heading” or “Ship’s Head” and not “going”


It is not irrelevant if you loose GPS and need to Dead reckon using one of your compasses


As a back-up...don't you think it should be as accurate as possible?
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Old 11-05-2011, 04:28   #47
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Re: What Heading Am I Really On ?

Well I'm glad I have provided some sport, and I'll leave it at that!
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Old 11-05-2011, 05:44   #48
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Re: What Heading Am I Really On ?

The fluxgate when calibrated will make it's own table of anomalies of the ship when you calibrate in circles. The course computer assumes the magnetic field is uniform and any variations are a result of the ship. It's possible it's not true. It computes the circle as 360 degrees and divides up the circle. It computes a set of predicted vs actual values stored in one degree increments and so you should have a magnetic reading corrected for the boat based on magnetic north as output by the course computer NOT the raw magnetic bearing from the fluxgate. To complicate it more the fluxgate can only measure 90 degree segments so the course computer does a few tricks to make it all work. It should be close to the values if you used the ships compass with the corrections computed when you swing the ship.

The GPS will compute the planetary magnetic variation using it's internal memory. A GPS can't read magnetic north it can only compute it. Your compass don't know nothing about planetary variation nor does the autopilot. The GPS COG is far more complicated since it is a moving average. I find in flat water I can get the autopilot and GPS to come to an agreement within a 1 or 2 degree offset. In any chop the GPS course is all but impossible to pilot the boat and the dampened ships compass provides the easiest to use readings even as it swings.

That is about as much as there is as to why they won't ever really agree but the other factor is they don't have the same accuracy and that accuracy will change based on other factors. The anomalies on the boat won't be the same when comparing where the fluxgate is to the ships compass assuming both were calibrated, but they might agree sometimes.

This all is most complicated when attempting to compute for a specific moment in time what the heading value really is. The three don't display a result in real time. There is some variable amount of delay in all of them. It's possible they are all correct just not at the same instant in time.

Time and space conundrums are most vexing. Using your dead reckoning plots just eliminates all this confusion. The course behind you is the most accurate.
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Old 11-05-2011, 06:05   #49
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Re: What Heading Am I Really On ?

Quote:
Originally Posted by bewitched View Post
Isn't there a significant point being missed here?

Neither the binacle compass nor the fluxgate compass (whether displaying true or magnetic) actually tell you where the boat is going. They only tell you which.way the bow is pointing. The HEADING It's fairly irrelevant information.

You.could do the TVMDC thing and then allow for current and leeway to get the COURSE

But i think most would look at the COG from the gps to figure out how the boat is moving relative to their destination.

The binacle compass to me has become little more than a backup should the fluxgate fail. The only time I look at it is for a quick and very rough reference for gybe angles, bearings etc.

I can't remember the last time I actually worked out which direction the boat was moving in using the binacle compass. Does anyone with a gps actually do this any more?

Heading can be a crucially important datum.

We just got back Sunday from an English Channel crossing, where the tide sweeps back and forth every six-odd hours at up to four knots and more.

If you steer by Course Over Ground you'll never get there, or, if that is a slight exaggeration, you'll certainly experience a world of frustruation trying to.

The way to cross a body of water with cross currents is to calculate, using a tidal stream atlas, how far you will be pushed off course along the way first in one direction, then in the other. You add all this up to get a result like -- if I steer directly towards my waypoint, I will end up 12 miles east of there. Then you calculate how many degrees different you need to steer in order to correct out that 12 mile error. Then you steer that heading for the whole passage, ignoring COG completely, and not paying attention to the fact that you are being swept way over this way, then way over that way. Your course looks like a big "S" on your chart plotter. You can check how accurate your calculations were by watching the XTE function on the plotter hour by hour.

COG becomes relevant ONLY in the last hour or two. Meanwhile, you MUST know your heading in order to navigate successfully. I guess this is pretty basic information for anyone who sails in tidal waters, but for anyone who actually thinks that heading is a useless piece of information -- here you are.
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Old 11-05-2011, 06:17   #50
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Re: What Heading Am I Really On ?

Quote:
Originally Posted by Pblais View Post
It should be close to the values if you used the ships compass with the corrections computed when you swing the ship.

. . .

That is about as much as there is as to why they won't ever really agree but the other factor is they don't have the same accuracy and that accuracy will change based on other factors. The anomalies on the boat won't be the same when comparing where the fluxgate is to the ships compass assuming both were calibrated, but they might agree sometimes.
My compass and my fluxgate agree to within the degree of accuracy I can read the compass. Like others, I keep everything set to Magnetic and do not use True for anything unless I am referring to paper charts.

Ironically, it is paperless navigation where Magnetic is seamless and perfect. If you put the plotter cursor on a place you want to go, you will get its Magnetic bearing instantly (if your plotter is set up to use Magnetic for everything), and you don't need to calculate anything. You really don't need True for anything until you start working with paper charts. And I don't really use paper charts for calculating courses -- I use them for study, preparation and orientation.
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Old 11-05-2011, 06:21   #51
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Re: What Heading Am I Really On ?

Quote:
Originally Posted by bewitched View Post
Isn't there a significant point being missed here?

Neither the binacle compass nor the fluxgate compass (whether displaying true or magnetic) actually tell you where the boat is going. They only tell you which.way the bow is pointing. The HEADING It's fairly irrelevant information.

You.could do the TVMDC thing and then allow for current and leeway to get the COURSE

But i think most would look at the COG from the gps to figure out how the boat is moving relative to their destination.

The binacle compass to me has become little more than a backup should the fluxgate fail. The only time I look at it is for a quick and very rough reference for gybe angles, bearings etc.

I can't remember the last time I actually worked out which direction the boat was moving in using the binacle compass. Does anyone with a gps actually do this any more?
I do all the time probably for the same reason I carry a couple of oil lamps Just in case.
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Old 11-05-2011, 06:44   #52
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Re: What Heading Am I Really On ?

I'm afraid with all the discussion about errors in the compasses, and corrections between True and Magnetic, and application of set and drift corrections to get course over ground (COG) we've missed a couple of very important points.

1. The steering compass is the helmsman's best friend, and is by far the best way to steer any given course. The GPS doesn't react fast enough to provide reliable heading information. The steering compass is dampened and, even without corrections, still the best device to use for hand steering the boat.

2. The GPS used together with the steering compass can be a powerful combination. A modern GPS will compute the bearing to the next waypoint with pretty good accuracy. If that bearing changes, then the boat's heading must be changed to compensate for set or drift. This is a very powerful technique to stay on the rhumb line, because the helmsman gets repeated feedback as to whether he's steering too high or too low.

Using this technique, you don't really have to know the actual values of set or drift, or variation or even deviation of the steering compass. All you're doing is correcting the heading slightly in one direction or another to maintain the same GPS bearing to the next waypoint.

I've used this technique extensively over the years, first with Loran, then with GPS as the GPS receivers got better and faster. Once, crossing the Albemarle Sound in absolutely terrible winter conditions (very strong and gusty cross winds, near freezing temperature, strong cross currents) this technique was invaluable. All that was necessary was to watch the bearing to the waypoint. If the bearing increased, I'd steer a bit to port. If it decreased, I'd steer a bit to starboard...just enough to compensate and maintain the same bearing to mark (track on the rhumb line).

Note that under these conditions of strong crosswind, lots of leeway (drift), and strong cross currents (set), an autopilot linked to a GPS would not have maintained a rhumb line course. Instead, it would have scribed an arc, continually changing the boat's heading toward the mark while sliding farther and farther off the rhumb line.

Bill
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Old 11-05-2011, 07:07   #53
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Re: What Heading Am I Really On ?

btrayfors, I'm so happy to read this. Finally someone has posted exactly what I do every time offshore.

I seemed like the natural approach to me, but I was beginning to wonder if I was going about it correctly.

I sail a bay and coastal near shore only so far. It will be some time before I'll need cross distant open waters. So I've got time to figure out the other techniques mentioned. Crossing from Cedar Key to Apalachicola in Florida will be my first over night sail, coming up in a few weeks.
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Old 11-05-2011, 07:10   #54
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Re: What Heading Am I Really On ?

Might as well chime in now.

I use the steering compass most of the time as a reference for where to steer to (along with, say, distant shore objects, stars, clouds etc) and making adjustments to keep the COG matching the bearing to the next waypoint.

FWIW.
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Old 11-05-2011, 08:06   #55
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Re: What Heading Am I Really On ?

hopefully we all know true virgins make dull companions.

Quote:
Originally Posted by btrayfors View Post
1. The steering compass is the helmsman's best friend, and is by far the best way to steer any given course.

2. The GPS used together with the steering compass can be a powerful combination. A modern GPS will compute the bearing to the next waypoint with pretty good accuracy. If that bearing changes, then the boat's heading must be changed to compensate for set or drift.
I wholly agree with point 1, with some stars/clouds thrown in to broaden the concentration.

Point 2 can work well and certainly has its uses, such as a transit through hazzards etc., but for any situation other than constant set/drift (rare as hens teeth in this neck of the woods), it will take longer/use more fuel. Of course, that might not mean the end of the world but it should be understood that you will have to travel further through the water.

Cheers.
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Old 11-05-2011, 08:39   #56
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Re: What Heading Am I Really On ?

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.......Point 2 can work well and certainly has its uses, such as a transit through hazzards etc., but for any situation other than constant set/drift (rare as hens teeth in this neck of the woods), it will take longer/use more fuel. Of course, that might not mean the end of the world but it should be understood that you will have to travel further through the water.
Cheers.
Nope, in most situations it will mean that you'll travel on a more direct (rhumb line) course and take a shorter time with less fuel. The whole idea is to stay as close to the rhumb line as you reasonably can do, by making small corrections whenever you see the GPS bearing-to-mark change. This takes into account shifts in drift and set.

And, as Four Winds said, it's intuitive. Very easy to do once you get the hang of it, and no unnecessary math. Just follow a given compass heading and make small corrections to that heading as needed to stay on the rhumb line.

I've found the technique to be equally applicable to offshore voyaging and to inland piloting. Whether on a river or on the open ocean, it works well.

And, in fact, it's the way I normally navigate these days. My GPS is attached to a computer chartplotter, which shows both the rhumb line to the next mark and my boat's position. It's really uncanny how tiny corrections can keep you right on the rhumb line, even on long passage legs. On a recent trip across the Gulf of Maine, we chided a crew member/airline pilot for staying absolutely on the rhumb line to Penobscot Bay, telling him that there was no land for a hundred miles to the west of us and for several thousand to the east of us! No matter, he stuck to that line like glue :-)

By the way, assuming the rhumb line has been correctly laid out free of obstructions, this technique is actually safer than having an autopilot steer the course, since you know the rhumb line is clear of hazards while the autopilot-gps course may stray considerably from the rhumb line (see above).

Bill
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Old 11-05-2011, 08:55   #57
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Re: What heading am I really on?

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wonder if the mounting of the autopilot and GPS a foot away from the comprass could effect it this much (was a little closer when the seas settled down some) both these should both be measuring the same magnetic heading
I personally would mount instruments, lights and any other electrical equipment more than a foot from the compass.

Most autopilots measure the magnetic field using a flux gate. Most fluxgates used for ship's compass have a procedure for spinning the boat at a constant rate to establish what the deviation is. You would have to look in your owner's manual to determine if there is a procedure for doing that with your autopilot.

The GPS does not read anything magnetic. It checks your position against satellites. By checking 2 or more positions over time it calculates your speed over the ground and TRUE direction. It if has the ability to convert to magnetic it does so by using position and an internal formula or an internal map to determine what variation is. I doubt the user's manual will tell you how it determines variation, but it should tell you how to determine what is being displayed and how you can toggle between true and magnetic.
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Old 11-05-2011, 09:53   #58
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Re: What Heading Am I Really On ?

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And, in fact, it's the way I normally navigate these days. My GPS is attached to a computer chartplotter, which shows both the rhumb line to the next mark and my boat's position. It's really uncanny how tiny corrections can keep you right on the rhumb line, even on long passage legs. On a recent trip across the Gulf of Maine, we chided a crew member/airline pilot for staying absolutely on the rhumb line to Penobscot Bay, telling him that there was no land for a hundred miles to the west of us and for several thousand to the east of us! No matter, he stuck to that line like glue :-)

By the way, assuming the rhumb line has been correctly laid out free of obstructions, this technique is actually safer than having an autopilot steer the course, since you know the rhumb line is clear of hazards while the autopilot-gps course may stray considerably from the rhumb line (see above).

Bill
I let the autopilot crab the boat to stay on the rhumb line (using XTE).

Just completed a 400nm passage, only touched the wheel when playing chicken with a freighter in the middle of the night.
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Old 11-05-2011, 10:27   #59
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Re: What Heading Am I Really On ?

DotDun,

The rhumb line is the direct, constant bearing course from where you started to the next waypoint.

Not all autopilots implement XTE (cross-track error) corrections the same way. Most older ones, I believe, just reset the heading toward the mark. One newer one tries to get back onto the original rhumb line before resuming course and, in so doing, sometimes will drive you crazy with sudden substantial turns.

There's an excellent article on this in the April Cruising World: The XTE issue, autopilot behavior & electronics dollars | Cruising World

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Old 11-05-2011, 11:04   #60
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Re: What Heading Am I Really On ?

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Nope, in most situations it will mean that you'll travel on a more direct (rhumb line) course and take a shorter time with less fuel. The whole idea is to stay as close to the rhumb line as you reasonably can do, by making small corrections whenever you see the GPS bearing-to-mark change. This takes into account shifts in drift and set. Bill
Quote:
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I let the autopilot crab the boat to stay on the rhumb line (using XTE)... Just completed a 400nm passage.
Navigating any passage over about one hour in duration in tidal waters by staying close to the rhumb line (and/or using XTE) without considering the wider strategical situation will undoubtedly mean you will; travel further/ use more fuel/take longer.

Of course, I'm not saying you shouldn't do it - it has its uses, but you should at least be aware of the wider implications... in tidal waters, the rhumb line is rarely the shortest distance through the water.

Making corrections to remain close to the rhumb line as the tidal stream changes means you are always working against the present tidal stream, whereas the shortest course to steer considers the wider situation and corrects against the net or compound tide for the whole passage.

If you're unsure of this or would like to discuss further I am more than happy to post more detailed info/examples etc. It might just save you a few hours/gallons in the future
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