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Old 22-01-2013, 22:32   #91
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by jackdale View Post
I do not normally use the term rhumb line in coastal nav. I probably shoudl have stuck to Course made Good or Course to Make Good. Ideally they are then same.
Agreed. Some have also refered to the end product as a DR - it is not; once you throw tidal set into the mix, it's an EP.
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Old 22-01-2013, 22:40   #92
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

'Morning all .
I am ready for another round. I got a bit depondent last night at the end with another instructor chiming in that my method is "faulty".

This is a very hard slog for me, but I am back with renewed energy. You can't put a determined Aussie shiela down .

Could everyone please think about the example Jackdale set in post #73?

The journey will take three and a bit hours.
In the first two hours you are affected by the current, in the second two hours there is no current.
At the end of two hours you want to be heading to the destination, not some spot on the rhumb line and if you are on one heading for the whole trip, (which is the fastest way of getting there), then the heading you started off with should be correct so that after two hours of travel you are pointing straight to where you are going.

It doesn't matter how much of the third hour the current affects you, as it is ZERO. You need to be heading straight for the destination at the two hour mark.

The RYA method decides at the end of two hours it wants you heading to a spot on the rhumb line, not the destination, even though at you have no current then for at least a further hour (and it may be hour and a half) of the journey.

How is that logical?
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Old 22-01-2013, 22:45   #93
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by DeepFrz View Post
Otherwise by steering the CTS heading you will describe an arc (more likely an uneven curved track) or a S track if the current changes direction.
That's what I thought too, until others here proved to me that it's actually the other way around -- an arc for a symmetrically changing direction current, a sinoid S curve for a single direction current. It's weird, but true.
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Old 22-01-2013, 22:47   #94
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Dockhead

I frankly cannot discover any relationship between your post #88 and my post #56.

Given that my post #56 was to remind you of my post #51, the trail is getting faint, so here I restate the two objections I made to the choice of the rhumb line by the RYA as an indispensable component of their method (I have others, but let's keep it simple)

Please address these objections: if you understand them, please refute them; if not, please seek clarification from me.

" a) .... the RYA method ... use(s) the rhumb line as a geometric construction aid.

This isn't a big problem in the usual case, but in unusual cases, their construction technique, to interpolate* the final hour of tide, is simply not possible, using the rhumb line. In such cases, it needs to be swung through 90 degrees.

b) They .... shift ... the destination to some arbitrary place on the rhumb line.

....This ... is not a big problem in the usual case, but like most dumbed-down rules, it misleads the user by diverting them away from an understanding based on first principles. "

(end of quoted excerpts from post #51)

* the word "interpolate" assumes a sophisticated instructor and trainee: in the simple case, substitute the word "identify"

What I'm saying is, because the rhumb line is treated as an essential prerequisite for this method to work, (and Jackdale is compounding that here by agreeing with it), the way the method is taught does NOT lead students to the realisation that any line which passes through the actual destination will do (broadly speaking - qualified below).

I will have more to say about that, because I suspect that some of the times when it pays to rotate the rhumb line (as mentioned above in (a)) will coincide with times when it is highly beneficial to do so, in order to approach the destination from uptide.

In other words, to offset the destination in a more sensible direction, reflecting the tidal currents at the end of the passage, rather than the historical location of the departure point.

It sounds as though Seaworthy is planning to post some diagrams on an anomalous case of this nature, so we can come to that in due course, and I'll see if I can make a convincing argument along these lines..


As amply shown by the indelible record of this forum (unless fighting breaks out and the mods delete the thread!), aspect b) above also misleads instructors.

It's not the fault of the method, but of the instructors, if they confuse the arbitrary, shifted destination, with the actual destination. It's worse if they misrepresent the rhumb line as being the only valid choice on which to peg this shifted destination.

Any line through the true destination will serve, provided it's not too oblique to the CTS vector.

If they are confused about this, it's hard to see how most students will not be.
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Old 22-01-2013, 22:54   #95
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by jackdale View Post
The SWL method is faulty.
Jackdale, could you please tell me what makes you think my method is faulty?

In the two examples I gave above:
My method gave a CTS of 58 degrees and 56 degrees respectively.
The RYA method gave a result of 63 degrees and 48 degrees.

The RYA method result was 9% out in the first example and 14% out in the second case.

Are you saying that that the RYA CTS is correct and mine is incorrect?

I know that giving the correct result in two examples does not prove my method works all the time, but my method is based on firm principles and it WILL work with every bit if data thrown at it I think, or at least give a better approximation than the RYA method does, as it has me heading to the destination every time, not some arbitrary point.

So could you please tell me why you you think my method is faulty?

Quote:
Originally Posted by jackdale View Post
The rhumb line is crucial.
......
This thinking is PRECISELY why the RYA method has huge inaccuracies in some circumstances. I will show this in later examples.
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Old 22-01-2013, 22:58   #96
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by Andrew Troup View Post
Dockhead

I frankly cannot discover any relationship between your post #88 and my post #56.

Given that my post #56 was to remind you of my post #51, the trail is getting faint, so here I restate the two objections I made to the choice of the rhumb line by the RYA as an indispensable component of their method (I have others, but let's keep it simple)

Please address these objections: if you understand them, please refute them; if not, please seek clarification from me.

" a) .... the RYA method ... use(s) the rhumb line as a geometric construction aid.

This isn't a big problem in the usual case, but in unusual cases, their construction technique, to interpolate* the final hour of tide, is simply not possible, using the rhumb line. In such cases, it needs to be swung through 90 degrees.

b) They .... shift ... the destination to some arbitrary place on the rhumb line.

....This ... is not a big problem in the usual case, but like most dumbed-down rules, it misleads the user by diverting them away from an understanding based on first principles. "

(end of quoted excerpts from post #51)

* the word "interpolate" assumes a sophisticated instructor and trainee: in the simple case, substitute the word "identify"

What I'm saying is, because the rhumb line is treated as an essential prerequisite for this method to work, (and Jackdale is compounding that here by agreeing with it), the way the method is taught does NOT lead students to the realisation that any line which passes through the actual destination will do (broadly speaking - qualified below).

I will have more to say about that, because I suspect that some of the times it pays to rotate the rhumb line (as mentioned above in (a)) will coincide with times when it is highly beneficial to do so, in order to approach the destination from uptide.

In other words, to offset the destination in a more sensible direction, reflecting the tidal currents at the end of the passage, rather than the historical location of the departure point.

It sounds as though Seaworthy is planning to post some diagrams on an anomalous case of this nature, so we can come to that in due course, and I'll see if I can make a convincing argument along these lines..


As amply shown by the indelible record of this forum (unless fighting breaks out and the mods delete the thread!), aspect b) above also misleads instructors.

It's not the fault of the method, but of the instructors, if they confuse the arbitrary, shifted destination, with the actual destination. It's worse if they misrepresent the rhumb line as being the only valid choice on which to peg this shifted destination.

Any line through the true destination will serve, provided it's not too oblique to the CTS vector.

If they are confused about this, it's hard to see how most students will not be.
Yes, I think I understand what you're saying, and I'll try to respond.

Let's forget the term "rhumb line" for a moment. The RYA method uses an extended course line (that's what they call it). It is an essential construction for this method (not Seaworthy's!) because it is one side of the large vector triangle -- the "desired ground track". You draw it all the way through the destination; sometimes using the RYA method, the water track line intersects the course line behind the destination. This does not mean that you are sailing beyond your destination onto land. This merely means that a longer full hour of tide data was used in the calculation.

It is not an error of the RYA method -- in fact it is a clever and (somewhat) useful trick, because it allows you to construct the vector triangle without finishing the last calculation, without getting the legs to converge right on the destination. Thus you get a CTS which will in fact take you right to your destination on your constant heading, provided the last uncalculated partial hour runs according to the average of your passage. If it does not so run -- and Seaworthy's example is an especially vivid example of where it does not -- then you will come out uptide or downtide of your destination, in this case nearly 10% by degrees.

Seaworthy doesn't need a course line only because her vector triangle always converges on the destination. Yet the course line -- rhumb line -- is still there -- it is still one leg of her vector triangle. You just don't need to draw it, because you're only using one point on it -- the destination itself.

Is that more coherent?
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Old 22-01-2013, 23:00   #97
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

By the way Jackdale, thank you for providing the data for the last portion of the journey. This is perhaps one of the easiest bits of data to select correctly as you know roughly where you will be in this portion of the trip (close to the destination) and you know roughly what time you will be there.

It is very important to look at the current data for the last portion of the journey. What if you are affected half an hour in the last current? The RYA forces you to choose a whole number of hours of current to work on (if it is given on one hour blocks).

Can't everyone see the inherent problems with that if the current in the last hour significantly affects the average amount of current for the entire trip?

This is not far fetched. If the time taken is very close to the whole hour, the RYA CTS result will be close, but otherwise it is possible it will not be.

I will plot out Jackdale's example and give my course to steer after I have commented on a few posts that went in while I was asleep and drawn out the chartplotter tracks for the 4 plots (two methods for two examples).
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Old 22-01-2013, 23:05   #98
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by Andrew Troup View Post
It's not the fault of the method, but of the instructors, if they confuse the arbitrary, shifted destination, with the actual destination. It's worse if they misrepresent the rhumb line as being the only valid choice on which to peg this shifted destination.

Any line through the true destination will serve, provided it's not too oblique to the CTS vector.

If they are confused about this, it's hard to see how most students will not be.
Andrew, to elaborate a little on this -- I'm not sure that it is helpful to think of the destination as being shifted. It's not really. Imagine this -- if the intersection point is half an hour before you reach the destination, and the last half hour of tide behaves in the way favorable to the RYA method, you just keep sailing on your same heading you exactly reach the destination.

It is not so much destination shifting as a mathematical proxy for the other leg of the of the vector triangle. It does not steer you to a shifted destination necessarily, it steers you towards the real destination but using an averaged last partial hour. Using a line other than the course line will not work.
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Old 22-01-2013, 23:08   #99
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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or at least give a better approximation than the RYA method does, as it has me heading to the destination every time, not some arbitrary point.
How could this possibly be true if you are steering one heading and the current is changing speed?

Work out your position at the end of each hour. Are you on the rhumb line?
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Old 22-01-2013, 23:10   #100
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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
"Short and Wide" is part of the WAG Method© I use.
Hi David
Welcome to the discussion! Nice to see your friendly face here .

Yes, I agree wholeheartedly that short and wide is a good principle (the wide bit needs to allow for the current when you get close, though, not the current when you started).

So your destination is actually a point short and wide of wherever you finally want to end up - in other words you are setting a course with two legs.

All the calculations for CTS should aim to get you at this first point

Using the RYA method to get you there will not always give a good result. I think mine always will .

Quote:
Originally Posted by David_Old_Jersey View Post
In regard to "real world" tidal currents - in addition to the considerations (uncertainties) mentioned already, also gotta factor in when close to shore that currents can reverse, or simply speed up or lessen differently to what the Tidal Atlas says for that hour in that area (tide currents are not plotted for every square mile - usually pretty good, but never gonna be to the exact inch on every day. except by accident ).........all that means that the "short and wide" approach works well as requires the last mile or so to be a "live" calculation rather than simply a prediction from many hours past.
Yes, I fully agree currents will often not be as expected close to land. But that affects whatever method you use to compute CTS. And in circumstances like this, your initial "destination" should perhaps be a mile from where you are going and a upcurrent (the degree depends on what the current is like at the end of your journey) so that you can then decide what to do. You still need an accurate way of getting to this point though, not a method that can be 10, 20 or even more degrees off, as the RYA method can be in some circumstances .
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Old 22-01-2013, 23:27   #101
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by DeepFrz View Post
Seaworthy Lass, in order to assess the accuracy of either method you will have to plot your position at different time internals along the way. You will, if steering one heading, cut an arc through the water. By plotting your position for each hour you will be able to approximate the arc and determine the accuracy of the plot...given that all inputs are true for the problem in a classroom solution. In a classroom situation (where you can assume the data is correct with no underlying conditions) the only time you will be on the rhumb line is at your starting point and your destination. .
Hi DeepFrz
Welcome to the thread.

I will plot my position for you by marking my course over ground on the plots with a dotted red line (it is actually a very quick thing to do)

In fact I would strongly recommend that the RYA teach you to plot your position for every hour (before you start) if you have any cross current.

A huge problem brought to light by this thread
Distinct Activities: Shackled by a Common Name?
was that even for experienced navigators, the ground track may be totally counterintuitive (even I was totally wrong about where it was in one example before I really sat and thought about it).

If you are not heading in a straight line between A and B, it is essential to work out where you will be in case there are obstacles you will encounter along the way.

Determining the ground track is very easy - just add the current vector to your speed for each hour. The sum of these is where you are at any point. It does not take long to plot this .

Quote:
Originally Posted by DeepFrz View Post
The plots in the RYA problem give you CTS and Time to destination (the second being the parallel line to CTS). Because of unknowns they teach you to ignore the last section of the course so that you have time to make corrections. Plotting to the exact destination could put you on some reef in the Philippines (or other unknown obstacle).
I fully agree with that, but that is not why the RYA selects D where it does. In some cases it selects a D that is beyond the destination!

If you go back to example 1 earlier in this thread, in the previous thread the RYA instructor selected a point roughly 2 nm beyond the destination to determine his CTS, just cutting the time taken by extrapolation. He ends up exactly due south of the destination for this example, nowhere near the rhumb line .
See post #658 in Distinct Activities: Shackled by a Common Name?

Your "destination" is actually whatever spot you think is a safe place to be to then made your way to where you want to finish your trip (see my comments to David above)

I fully agree that picking a spot short and then reassessing things is an excellent idea, but then just pick that point and call that your "initial destination" (it will be a two leg journey then, with a short second leg).

You still need a reliable way of working out the CTS to get there don't you?

I put forward that my method will be reliable with any data thrown at it. The RYA one will not (the CTS was 14% out in the last example I set).
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Old 22-01-2013, 23:34   #102
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

In response to my comment:
Quote:
"or at least give a better approximation than the RYA method does, as it has me heading to the destination every time, not some arbitrary point."

Quote:
Originally Posted by DeepFrz View Post
How could this possibly be true if you are steering one heading and the current is changing speed?
I will demonstate this simply on the diagrams I post earlier using my method for two examples. Just give me 10 minutes.

Quote:
Originally Posted by DeepFrz View Post
Work out your position at the end of each hour. Are you on the rhumb line?
Why would I necessarily be on the rhumb line at any stage (other than at the start and finish)?
Why would I want to be?

For the two examples I posted I don't think I would be on the rhumb line at any stage other than at the start and finish.
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Old 22-01-2013, 23:38   #103
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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This form of argument: "it's real complicated, therefore we can't figure it out", or "the data are approximations, therefore the answer is meaningless", or indeed "the input data have only two significant digits, therefore the output cannot have more than two significant figures" are all false. The relationship between precision of data and precision of information derived from data is very complex. A good example of this complexity is digital processing of optical imagery. These days it is even possible even to produce a focussed image from a blur - to alter the focal plane in an already captured image - producing, say, readable text out of a chaotic blur, apparently lacking sufficient information.
You appear to be missing the critical difference between independent data and correlated data. The results you list in the paragraph above only work with correlated data. Do you understand the mathematical concepts of variance and covariance?
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Old 22-01-2013, 23:47   #104
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
For the two examples I posted I don't think you would be on the rhumb line at any stage other than at the start and finish.
I absolutely agree as per my previous posts.

You are right that shifting the destination point beyond the true destination you will not end up at your true destination if you have any large offset.
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Old 22-01-2013, 23:54   #105
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Your "destination" is actually whatever spot you think is a safe place to be to then made your way to where you want to finish your trip (see my comments to David above)

I fully agree that picking a spot short and then reassessing things is an excellent idea, but then just pick that point and call that your "initial destination" (it will be a two leg journey then, with a short second leg).

You still need a reliable way of working out the CTS to get there don't you?
That's excellent if one has an opportunity to take bearings (and may be fairly good with even just one bearing taken) at the waypoint.
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