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Old 22-01-2013, 10:34   #31
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by Dockhead View Post
The correct answer seems to be 58.3 degrees, and I think it is a mathematically perfect answer, based on the assumptions, ignoring only the fact that the sea surface is not a perfect plane, something absolutely reasonable to assume over distances like these.
There is far too much imprecision in the input data to write decimal degrees. We don't even have two significant figures of precision, so the best we could do would be to say the course to steer should be about 57-60 degrees. In my opinion, it's valuable to have some idea of our uncertainty. RYA do this with "circles of error" which, while not mathematically correct, are useful approximations.
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Old 22-01-2013, 10:35   #32
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by David_Old_Jersey View Post
Getting me head around numbers etc is not my forté - I kinda got lost around step 5!, so looking forward to the Diagram!
Here you go. Pictures and arrows: Course Correction Calculator
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Old 22-01-2013, 10:47   #33
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by mcarling View Post
There is far too much imprecision in the input data to write decimal degrees. We don't even have two significant figures of precision, so the best we could do would be to say the course to steer should be about 57-60 degrees. In my opinion, it's valuable to have some idea of our uncertainty. RYA do this with "circles of error" which, while not mathematically correct, are useful approximations.
We are solving an artificial problem with assumed perfect data. Therefore we are entitled to use as many significant figures as we like, and figures even to the hundredths of a degree may shed light on the integrity or internal consistency of a particular method.

The usable level of precision is an entirely different question, which we have not gotten to. You are guessing straight from the hip, a WAG in Internet parlance, and that is not nearly good enough for me. When we've settled the question of CTS calculation method and have agreed on a mathematically perfect one - for this we will wait for a concession speech fromDave as witty and gracious and classy as Captain Force's - we will turn to that question. We can set up a very good experiment using precise 5 minute tide data and comparing precise calculations based on that with the results we get from using analogue fudges and one hour tide data. I am guessing (not entirely a WAG because I have the benefit of the experience of a couple dozen real Channel crossings) we will get within around 2 degrees on 60 mile passages. But we're going to test this empirically! Stay tuned!
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Old 22-01-2013, 10:47   #34
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

I'm back .
Chores are finished for the day and armed with a cup of coffee rather than a glass of wine, I have drawn up what would be drawn on a chart for the example I set the RYA instructor in the other thread where all this discussion started.

Details:
Boat speed is constant at 4 knots throughout the journey.
Destination is 8.5 nm due east.
Current is always from the north:
1st hour 3 knots
2nd hour: 2 knots
3rd hour: 0.5 knots

MY METHOD
Step 1: Mark A and B on the chart

Step 2: Distance from A to B = 8.5 nm (specified in this case, but you would usually measure it)

Step 3: estimate time taken by dividing distance by your speed (8.5 / 4)
It is more than 2 hours, so you know at least three hours of current vectors need to be drawn.

Step 4: determine current (specified in this example, see above)

Step 5: Mark the current vectors starting from A, adding each one to the tip of the previous. In this case nice and easy, the are all running due south)

Step 5A: Check at the 3 hour mark you have gone past B (measure 3x4= 12 nm from the end of the 3 hour current vector)
- Check at the 2 hour mark you haven't reached B (measure 2x4= 8nm from the end of the 2 hour vector)

Step 6: Draw a line from the end of the 2 hour vector and mark off 8 nm and label that S

Step 7: Draw a line from the end of the 3 hour vector and mark off 12 nm and label that L

Step 8: Calculation to determine the proportion of the final current:
Distance from S to B divided by the distance from S to L = 0.5
Position of K = 0.5 x (length of last current vector) = 0.5 x 0.5 = 0.25
Mark K at this point

Step 9: Join K to B.
CTS as measured on chart: 58 degrees
Time taken = length KB/ boat speed = 10/4 = 2.5 hours

Note: it is a bit of a messy plot as the lines are all so close close together (within a couple of degrees).
In this example you probably wouldn't go to all this trouble after drawing the lines to B. You would just "eyeball" the lines drawn from the tips of the current displacement to B and say that halfway looks right - you would be within a degree of the right answer doing this : thumb:

I will compare this the the RYA method and chart diagram in the next post.
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Old 22-01-2013, 10:51   #35
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by Bash View Post
Here you go. Pictures and arrows: Course Correction Calculator
A good resource, but it's for a single hour CTS calculation, which is uncontroversial.

The problems arise when we have a current which changes over a multiple hour time frame. The problem becomes vastly more complex.
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Old 22-01-2013, 10:57   #36
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

RYA METHOD result for the previous example:

Same data as above
Complete steps 1 to 5 above, but add course line (rhumb line)
Only 2 lots of current vectors would be selected though I think (I am not sure why the RYA instructor in the other thread used 3 vectors in this example).
Maybe someone who has done an RYA course would comment on this. Selecting the other D does not give a better result though (it gives 51 degrees as the CTS)

Mark the tip of the selected current vector as C
Join C to B
Measure the direction of the line CB.
This is 63 degrees

Estimate the time taken by marking a line parallel to D going through B.
Time taken = length CB/ boat speed = 9.6/4 = 2.5 hours

This may seem simpler but it is not much more work to add another line as I did and mark off the distances travelled on each of them.
Even if you do nothing more than compare the length of SB and LB visually and mark this off the current, the result is still better for my method.

Note where this extrapolated line goes through the current vectors in the RYA method.. At this stage alarm bells should be going off as it intersects the current line well before the end of the second hour and we know that the journey takes more than 2 hours because the course length is 8.5 nm and our boat speed is 4 knots and there is no current pushing us towards the destination point.

This highlights another problem with the RYA method!
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Old 22-01-2013, 10:58   #37
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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We can set up a very good experiment using precise 5 minute tide data and comparing precise calculations based on that with the results we get from using analogue fudges and one hour tide data. I am guessing (not entirely a WAG because I have the benefit of the experience of a couple dozen real Channel crossings) we will get within around 2 degrees on 60 mile passages. But we're going to test this empirically! Stay tuned!
That will only account for a fraction of the sources of inaccuracy which I listed above.
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Old 22-01-2013, 11:04   #38
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Bravo, Seaworthy!

I still can't quite evaluate the method for dealing with the last vector, but you get precisely the right answer. We know that because we can check it with a simple right triangle solution.

The RYA method is WAY out.

In my opinion, you have proven your case
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Old 22-01-2013, 11:07   #39
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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That will only account for a fraction of the sources of inaccuracy which I listed above.
Can you demonstrate that mathematically or devise an experiment? I think this is just a wild guess - I think you are just expressing a bare prejudice.

I HAVE devised an experiment, which we will run by and by.
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Old 22-01-2013, 11:08   #40
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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None of these methods are mathematically correct, but they are not meant to be.
I have just spent four days arguing with an RYA instructor who heatedly kept insisting that the RYA method was mathematically precise. He kept repeating I was ignorant and confused.

He said the RYA method would give a precise CTS to reach B and B is where you would arrive following this CTS and he kept repeating I was ignorant and wrong to say you would arrive at D. He kept insisting the boat would never be at D. In fact in one of the examples he said the boat would pass well south of D if it followed the RYA computed CTS in one example we were looking at.

Several times I called for help from members to chime in and support me that the RYA method was simply an approximation and would take you to D not B and there was DEAD SILENCE from everyone. Now suddenly everyone is saying "of course it is imprecise" .
Where were all these comments when I was struggling on my own?

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They are meant to be easy to use and near enough
There are many cases where the RYA method does not produce a "near enough" result. This is my concern and why I am persisting with this.

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I agree that Seaworthy Lass's method adds some precision, but it is still far from precise and it's debatable whether the added complexity is worth the added precision. For those who understand the maths, this discussion is useful but, for those who do not, I don't think it's anything to worry much about.
I will illustrate over the next few days examples were the RYA method fails badly and mine still gives a result to within a degree .
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Old 22-01-2013, 11:14   #41
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Can you demonstrate that mathematically or devise an experiment? I think this is just a wild guess - I think you are just expressing a bare prejudice.
I'm expressing a basic principle of epistemology: that the output of a calculation cannot be more precise than the least precise input.
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Old 22-01-2013, 11:24   #42
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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I have just spent four days arguing with an RYA instructor who heatedly kept insisting that the RYA method was mathematically precise. He kept repeating I was ignorant and confused. Several times I called for help from members to chime in and support me that the RYA method was simply an approximation and there was DEAD SILENCE from everyone. Now suddenly everyone is saying "of course it is imprecise" .
Where were all these comments when I was struggling on my own?
Forgive me. I didn't see that thread.

Four days is not such a long time. When I was a grad student, I had a disagreement with a professor. Because he had a Nobel Prize and I didn't, no one took me seriously until I offered the professor a large wager. It took six months before he was convinced, but he paid.

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I will illustrate over the next few days examples were the RYA method fails badly and mine still gives a result to within a degree .
There is still more uncertainty, using dead reckoning, than anyone here seems willing acknowledge.
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Old 22-01-2013, 11:33   #43
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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I am guessing (not entirely a WAG because I have the benefit of the experience of a couple dozen real Channel crossings) we will get within around 2 degrees on 60 mile passages.
There is a good reason why experience leads one to over-estimate the precision of the method, which I will try to explain. There are multiple sources of imprecision. Each of them is independent from the others and each can be wrong in either of two (or more) directions. Most of the time, the errors will not all be in the same direction, so the errors will usually tend to cancel each other, to some extent. However, occasionally, all the errors can be in the same or similar directions and compound each other. In a hundred ocean crossings, most people would never see all the errors line up in the same direction but, on rare occasions, it will happen.
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Old 22-01-2013, 11:33   #44
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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I'm expressing a basic principle of epistemology: that the output of a calculation cannot be more precise than the least precise input.
Sure, but how much is that? Hundredths of degrees? Tenths? Tens of degrees? You have no idea. This, without more, is meaningless, and you have made statements unsupported by knowledge which you actually possess. It's an empirical, quantitative question - the degree of imprecision of the data, and to what degree of imprecision that translates to in the result.
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Old 22-01-2013, 11:34   #45
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Forgive me. I didn't see that thread.
Four days is not such a long time. When I was a grad student, I had a disagreement with a professor. Because he had a Nobel Prize and I didn't, no one took me seriously until I offered the professor a large wager. It took six months before he was convinced, but he paid.
They were a very long 4 days for me and I copped a lot of abuse .

Pity I didn't have a bet with the RYA instructor though .
I may have been sipping some nice Scotch at the moment, sources are near non existent for single malts here in the Greek islands.

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There is still more uncertainty, using dead reckoning, than anyone here seems willing acknowledge.
I fully acknowledge the imprecision - tide data, estimated boat speed etc.
But it does not help if you start with a faulty method. Errors are cumulative usually!
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