Question about Navigation Course

Re: hm possible error in axes notation

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I think perhaps the vertical axis should be labelled "likelihood of nav instructor topping himself" and horizontal axis "number of irrelevant quasi-mathematical posts about nav which is v simple anyway and was explained 2 days ago on this thread", no?

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TCM - the above quote from you looks like you were trying to tell me to shut up - it's all very easy, so there's nothing to discuss. How else should I interpret this? Then you acuse me of trying to tell you to shut up???

Cliff.
 
Re: hm possible error in axes notation

er yesh, i suppose it is.

Soon they might get on to stabilty calculations, and you definitely CAN do some differential calculus with that stuff. Where is this course i wonder? Are there any spaces left?
 
Re: Very nice and all that ....

[quoteAlistair, you got my point exactly. I can scale the graphs down to smaller deviations but then they will have to deviate over an even smaller range of compass angles. Presumably you'd object to this even more strongly. So, the question remains as in my previous post - why can't this happen physically? If you have the physical insight to tell us why, please do.

[/ QUOTE ]

I wasn't objecting strongly at all! I was interested in the the graphical depiction and the apparent anomaly it presented.

Right, I'll have a go, get some paper and a pencil out. Imagine flux lines. The compass needle lies in these, parallel to the locally distorted pattern. Imagine the distortion is caused by a bit of magnetic material due east of the compass, but which rotates with the compass, like everything in a boat. The lines may be "bulged" away from or towards the distorting influence. I'm imagining away. The needle remains pointing north, parallel to the local lines. Rotate the boat clockwise 45 degrees. The flux lines are distorted still, but slightly differently, as the clock-wise rotation moves the magnetic material to the south-east, the bulge moves south. The needle stays parallel to it's immediate flux environment, which means it points slightly to the east, but still parallel to the local lines. There cannot be two different possible directions in which the needle can lie. If you think in a flux-line sense, not in an "Attraction to magnets" sense, you'll see it can't happen.
I hope.

Does that work?
 
Re: Very nice and all that ....

Alistair, it'll take me a while to get to grips with the flux pictures - one thing that worries me though is this. As you rotate the boat, is it simply the case that the bulge just rotates? I would have thought that the earth's magnetic field could interact with that of the boat in a possibly more complicated way than just rotation. I think I'll need to look up some stuff on magnetic fields before I have a hope of understanding.

Cliff.
 
Re: Very nice and all that ....

Would it make sense to get an ordinary compass and magnet and calculate the deviation table experimentally? Or would that wreck the compass? Maybe that doesn't make sense, not sure actually?

Cliff.
 
Re: Very nice and all that ....

<ul type="square"> [*]A compass would not be affected by a small exposure to an external magnetic source. [*]An empirical approach may resolve this conundrum. [*]It will be of no practical use and won't move the understanding of navigation forward one jot. [*]But if it makes you happy WGAF. [/list]
 
Although this started as a paper exercise and drifted from there.
Here is a sideways approach to Deviation to ponder:

I have a Autohelm 5000 part of the setup is to do a 360 to allow the AH to calculate the fluxgates deviation.
Once done I can read the ships heading from the AH, note the compass heading, then construct a deviation card from the difference in readings.

As for Variation, at the moment it is about half a degree around the UK so it gets ignored!
 
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As for Variation, at the moment it is about half a degree around the UK so it gets ignored!

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That'll be "My bit of the UK" then. It's about 6 degrees west here. Significant.
 
It's encouraging that Cliff_Nolan and others take an interest in the complexities and confusions in Magnetic Compassery. I'm certain that the professionally-trained among the forumeers will remember interminable lectures - and exams - on 'Compasses and Magnetism'.
The RYA course content is intended only to provide a simplistic but useable intro, for yotties, sufficient to keep 'em out of trouble.

For those of us who become fascinated and want more, try here:

Bowditch Online

And for those who choose to ignore such refinements as Residual Compass Error - and perhaps Height of Tide, Strong Wind Warnings, vessels Restricted in Ability To Manoeuvre, Harbour By-laws, and Notices To Mariners - here's a little tip.

When chartering a Coded Boat and doing the handover checks, have a look at the date on the mandatory Compass Deviation Card above the chart table. If this easily-visible data card is missing or out-of-date, then go and have a really close look at the dates of last inspection on the fire extinguishers, the lifejackets, and the liferaft - which are not quite so readily seen.

You might get your charter for a significant discount! /forums/images/graemlins/smirk.gif
 
Re: Very nice and all that ....

Cliff, the situation you postulate can easily be made to happen. All you need is a deviating magnetic field with a strength greater than the earth's field. As the boat turns the compass reading will at first seem to change normally, but the swing will stop and reverse as the two fields oppose each other. For any one boat direction there will only be one compass reading, but for a particular compass reading there may be two possible boat headings.

In practice no one would use a boat with such a strong deviation, so the situation is unlikely to arise. Deviation is unlikely to exceed 5 degrees; if it does, then the compass needs adjustment.
 
Re: Very nice and all that ....

[ QUOTE ]
Cliff, the situation you postulate can easily be made to happen. All you need is a deviating magnetic field with a strength greater than the earth's field. As the boat turns the compass reading will at first seem to change normally, but the swing will stop and reverse as the two fields oppose each other. For any one boat direction there will only be one compass reading, but for a particular compass reading there may be two possible boat headings.

In practice no one would use a boat with such a strong deviation, so the situation is unlikely to arise. Deviation is unlikely to exceed 5 degrees; if it does, then the compass needs adjustment.

[/ QUOTE ]

Peter, I was asking about the opposite situation - two compass readings associated to the same true / magnetic heading (due to deviation effects). Have a look at the graph I posted before, which illustrates this possibility. The only way to settle it is by experiment I think.
 
Re: Very nice and all that ....

Cliff

I don't think it is possible for the compass to give two different readings at one boat heading (other, perhaps, for deviations caused by heeling errors).

The compass is just an indicator showing the direction of the magnetic field. The magnetic field will be the vector sum of the earth's field and the boat's field. For any given direction of the boat there can only be one such vector sum, and hence only one deviation.

Heeling error is quite different, though. It's caused by the boat's field having a strong component in what would normally be the vertical direction. As the boat heels the "vertical" component heels with it, thus providing an extra horizontal component. It is quite possible for the compass reading on a given heading to vary with heel. Big ship binnacles have methods of compensating for heeling error, but I've yet to see one on a small boat.

The only time I've actually experienced heeling error was on a boat where the compass was fitted just under the main hatch, and just above the engine. 30 degrees of heel could change the compass reading by about 10 degrees, but both the amount and its direction depended on the heading. Made navigation quite difficult!
 
Re: Very nice and all that ....

Let me try to understand what you are saying about the vectors.
It looks to me (and I could be wrong, so correct me if I am) that you are thinking of the earth's magnetic field as imparting a fixed vector (force) to the compass needle and that you then add a varying deviation vector for each new heading. Your conclusion then becomes obvious.

I think this leaves out an important consideration. Everything happens in the context of the rotating frame of reference - the boat. As I understand it, if there were no deviation present at all, the compass just sits there pointing to (magnetic) north, regardles of the orientation of the boat. As the boat rotates, it looks to the helm that the compass is rotating. But of course it is not - it is just the frame of reference (the boat) that is rotating. Nonetheless, on a new heading the compass needle appears to the observer that it is experiencing a new vector force. Now if we bring deviation back into play, then it appears that there are two vectors being added. But because of the frame of reference issue, it appears that both of these vectors can vary at each heading. This opens the possibility of having the same sum for two different headings.

To put it another way, can you tell me why the deviation curve that I posted cannot happen? By the way, in that deviation curve, negative numbers mean West and positive numbers mean East.

That was interesting about the heeling influence. I'll have to think about that.

Regards,
Cliff.
 
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