

23012013, 23:46

#196

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Re: Inaccurate RYA Teaching : CTS  Quest For a New Method
Quote:
Originally Posted by bewitched
It seems I have a significantly different understanding from you on the function of the RYA method.

Sorry, yes you do misunderstand the RYA method . The RYA method does not result in a ground track along the course line unless the current is constant for the entire journey.
Quote:
Originally Posted by bewitched
Firstly, the RYA call it the course line, not the rhumb line for a very good reason. It is the course, over the ground, that it is intended that the boat will travel. Sure it is a straight line between 2 points in the examples which have been put forward, but that is only because the RYA method is very simplistic and ignores many variables. If the course line was plotted by a routing software, the course line would be far from straight and could never be confused with a rhumb line.

No, you are unfortunately incorrect. Maybe that is why confusion is arising. Maybe the RYA should stop calling it the course line and start calling it the rhumb line!!!
The course line is only your ground track if the current is constant for the entire journey (forgetting about leeway at the moment).
Quote:
Originally Posted by bewitched
It is definitely the intention to follow this course line during the the passage, whether straight or not, as closely as possible. That is why it is put on the chart and our progress checked against it. I'm afraid if I came up on watch and my helmsman had taken us 12 miles off the course line he would not be in my favour, regardless of any protest he may have for wanting to maintain a single CTS.

No, the intention is not to follow that course line at all. Think about it. How on earth could you follow one constant CTS with the current varying all over the place and still remain on the rhumb line?
Quote:
Originally Posted by bewitched
I believe, the intention of the RYA tidal method is to provide a simple means to keep you close to your course line during a passage undertaken without the ability to determine position. The single course to steer, rather than several courses to steer is simply a mechanism to reduce potential error caused by the varying tide rates. It is one small step in the effort to navigate by dead reckoning.

No sorry, it is not. That is not the point of the RYA method. Sometimes it even selects D as being a beyond the destination. In fact this will occur about 50% of the time if you follow the method well (it seems this is not what they teach though if they did teach it it would make the technique more accurate in some circumstances ).
Check out example 1 early in this thread. The point D was selected as being well beyond the destination by the RYA instructor in the previous navigation thread.
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24012013, 00:03

#197

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Re: Inaccurate RYA Teaching : CTS  Quest For a New Method
Quote:
Originally Posted by bewitched
What I believe that both the RYA and your methods aim to achieve is to get the boat back on to the course line at some point in the future. The RYA in x hours and your method when you arrive at the destination.

The fact that the RYA tries to get you on the rhumb line before you reach the destination is the major source of the problem with the method.
My method has you arriving at B (the end of the rhumb line). Unlike the RYA, I do not aim to be in the rhumb line at any point other than at departure and arrival (except if I am coicidentally taken there along the way  both methods may have you doing occasionally and that is irrelevant).
Quote:
Originally Posted by bewitched
I really have no strong opinion on which is preferable, I think both have merits.

The RYA method will frequently get you roughly there, but it could let you down very badly. In the second example I posted the RYA result was 14 degrees out (the SWL method gave you the right result).
In the next example I give, the RYA method will be even worse (mine will be correct following my procedure exactly).
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24012013, 00:25

#198

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Re: Inaccurate RYA Teaching : CTS  Quest For a New Method
Quote:
Originally Posted by conachair

Hi Conachair
I am rereading the posts to see if there is anything I missed before I sit down to think up and draw out the next example, and I see that I had missed this post of yours!
You were the first person to add a voice to mine about the accuracy of the RYA method, and this occurred well before anyone else said anything.
Sorry I did not acknowledge this earlier.
I thank you deeply, it made me feel not entirely isolated. Ten thousand odd views of the thread and no one other than you was chiming in for a long time
Many thanks, it was very appreciated and helped keep me going
Those were a dozen roses going your way
If the RYA send me a case of fine single malt Scotch in gratitude, I promise to actually send you a bottle.
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24012013, 00:35

#199

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Join Date: Feb 2012
Posts: 2,441

Re: Doctrine of the Imperative Triangle
Quote:
Originally Posted by Lodesman
Of course SWL's method still has the 'rhumb line'. She starts at point A and goes to point B  and measures the distance between them. The only thing she doesn't do is draw the pencil line between them. It is still part of the tidal triangle that is the basis of her method. I really can't believe this argument.

(sigh) Deep Frz says the same thing about measuring A to B:
If SwL measures the distance, it's for interest only. It does not play any part in her method, nor does the line joining them.
In straightforward situations, measuring it can help with estimating how many hours of tide to allow before starting to check for being within 'striking distance', saving unnecessary premature checks.
In strong tides sweeping the boat far out to one side, it's not even helpful for that.
You can assert the contrary, but your assertion is based on supposition or assumption.
I suggest you ask SeaWorthy if she uses the rhumb distance for her method, and if so, what for.
It's remarkable that several individuals have suddenly developed more expertise in her method than she has herself.
How many times have you tried applying it using neither the rhumb line or triangles?
Unless you've tried, assiduously, and failed, you are in no position to claim they are indispensable.
As for the "tidal triangle", you are mistaking the representation for the thing.
Triangles are a tool to represent vectors.
One can also, if one prefers, use them to solve vector arithmetic. It's a fine method, one of several. (refer previous elaboration)
One could choose to draw a triangle as a representation of the tidal vectors in SeaWorthy's procedure as an extra and superfluous element, but it would not include the rhumb line. From the endpoint of those vectors, only one line is drawn. It does not form a side of a useful, let alone indispensable, triangle of which the rhumb line is the other side.
(resigh)
I have no interest in triangles, I only brought the subject up to refute DH's contention that the rhumb line must be an essential element of her method, because it was one leg of an essential triangle.
No, nope, nopity no. On both counts.
    
I don't see much merit in travelling further along this path because I can only reiterate the content of this and earlier posts.
It's was hoping to establish that the rhumb line is a key element in the RYA method and not the SwL method. It's not a controversial or difficult proposition. Surely if it was a key element SwL would draw it?
The reason I want to establish that proposition is that the dependence on the rhumb line is what (for purely geometric reasons) causes the RYA method to fail to provide a solution under some scenarios. The construction the method relies on is simply not possible.
I'm not bothered that SwL has got bogged down and is not yet addressing such scenarios.
Until a few more people have their 'penny drop' moment with the above proposition, there's not much chance of the issue being understood without another longdrawnout, shambolic and energy sapping argument.



24012013, 01:31

#200

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Re: Doctrine of the Imperative Triangle
Quote:
Originally Posted by Andrew Troup
If SwL measures the distance, it's for interest only. It does not play any part in her method, nor does the line joining them.
I suggest you ask SeaWorthy if she uses the rhumb distance for her method, and if so, what for.

I keep repeating I don't use the rhumb line for anything other than measuring its length! I am almost hoarse repeating it LOL, but I think it is an important point to drive home before I present my next example.
Quote:
Originally Posted by Andrew Troup
I'm not bothered that SwL has got bogged down and is not yet addressing such scenarios.

Its coming, it's coming .
Just had breakfast and will plot up an example and post it. I am not answering any more posts until I do .
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24012013, 02:11

#201

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Posts: 888

Re: Inaccurate RYA Teaching : CTS  Quest For a New Method
Quote:
Originally Posted by Seaworthy Lass
No, the intention is not to follow that course line at all.

Forgive me, but when I was taught navigation, plotting my intended course and then navigation along it was the whole point to the exercise.
Quote:
Originally Posted by Seaworthy Lass
Think about it. How on earth could you follow one constant CTS with the current varying all over the place and still remain on the rhumb line?

.
You can't...I never said that you could. The one single, constant course to steer is a restriction that both the RYA and your method has artificially put in place to facilitate simplicity in the calculation. To enable the navigator to say to the helm; "hold this heading and in x hours we should be back on our course line....hopefully". It is an average of averages which does not reflect true life.
Let me try to explain. If the tide tables tell us:
1st hr: 4kts at w deg
2nd hr: 2kts at x deg
3rd hr: 3kts at y deg
That is not what will happen, each hour is an average. A probability within a stated deviation no doubt. This is the first averaging. What the RYA method considers now is that it is probably easier and no less accurate to consider that over the 3 hours, the tide runs on average at (4+2+3 / 3 =) 3kts at z deg. This is the second averaging. Steering a course derived from this average tidal effect will bring you back to the course line in x hours.... in theory. In practice, it won't place you directly there, but it will place you close more often than not because the averaging hopefully took out the extremes.
You could calculate a course to steer that would get you back to the course line each hour, but if there are no obstructions, there probably isn't the need to get back so regularly and it is more calculation for arguably no greater accuracy. It's only necessary to get back to the course line just before the next waypoint (RYA method), or at the next waypoint (your method).
Like I say, I think both methods have merit as long as we take them for what they are. What they are, in my opinion, is one easy calculation to determine probable tide effect, that should be considered alongside many other factors when navigating without positional information available. In other words, as a part of dead reckoning.



24012013, 02:30

#202

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Re: Inaccurate RYA Teaching : CTS  Quest For a New Method
I'm getting lost with SWL diagram as shown in post 34 http://cdn.cruisersforum.com/forums/...6&d=1358876662
Maybe I'm stuck on vector triangles, but to my mind, the 2 hour (8 mile) arc taken from the end of 2 hr current vector, should strike across the line AB, and the 3 hour (12 mile) arc taken from the end of the 3 hr current vector, should strike across the continuation of the line AB (beyond B).
If I wanted to be as precise as possible, I would use the traditional vector drawing, and check as to whether I'd end up short or beyond the destination, and then calculate a final current vector, apply that to the first current vector and re work the problem.
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24012013, 03:50

#203

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Re: Inaccurate RYA Teaching : CTS  Quest For a New Method
Quote:
Originally Posted by nigel1
I'm getting lost with SWL diagram as shown in post 34 http://cdn.cruisersforum.com/forums/...6&d=1358876662
Maybe I'm stuck on vector triangles, but to my mind, the 2 hour (8 mile) arc taken from the end of 2 hr current vector, should strike across the line AB, and the 3 hour (12 mile) arc taken from the end of the 3 hr current vector, should strike across the continuation of the line AB (beyond B).
If I wanted to be as precise as possible, I would use the traditional vector drawing, and check as to whether I'd end up short or beyond the destination, and then calculate a final current vector, apply that to the first current vector and re work the problem.

Nigel, THERE IS NO LINE AB in my method LOL.
You strike a line toward B to see if the distance travelled vector under or overshoots your destination point B NOT the line AB.
Forget about the line AB.
Look at my diagrams. there is no line drawn between A and B in any of them .
You measure the distance AB just to get some idea of the length of the journey given your speed so that you know roughly how many hours you need to initially consider (and also this info is needed accurately at the end if you want to work out your SMG).
You do not need to know the distance between A and B accurately at all to work out the CTS and the time taken. Everything is being plotted on a chart so you can see where you need to go. I have given you the distance accurately in my examples just so that you can label B on the chart/paper that you are using, not for any other reason.
Once you know roughly how many hours you need to deal with, the data collection then needs to start (the method for this is beyond the course of this discussion, I am discussing what to do once to have collected this info).
As you roughly need to know the time being considered first, you roughly measure the length of the course initially. You can measure it accurately later if you want to work out your SMG .
I am nearly finished with the diagrams and will post the next result soon.
I am warning you beforehand so that you can make sure you are sitting down and you don't fall over when you look at the figures. The RYA method gives a CTS this time that is a whopping 36 degrees off .
If that doesn't convince people to use the SWL method, I don't know what will.
I will post simple step by step diagrams later since my initial instructions were not clear .
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24012013, 04:01

#204

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Re: Inaccurate RYA Teaching : CTS  Quest For a New Method
Quote:
Originally Posted by bewitched
Forgive me, but when I was taught navigation, plotting my intended course and then navigation along it was the whole point to the exercise.

Hi again Bewitched
If you have one constant CTS and any cross current that is not constant for the entire journey, your "course over ground" if that is what you mean by your "intended course", has nothing to do with the line AB, so why draw it?
Your "course over ground" (the track you will make on your chartplotter) is what you need to mark and consider once you have finished the computations .
This is your intended course for the journey.
I think the RYA make it EXTREMELY confusing wanting the line between A and B to be referred to as the course line.
It is only the course line if you have constant current (zero included) for the entire journey .
Off to finish the diagrams. It is gusting to over 30 knots at anchor here at the moment, adding an extra challenge LOL .
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24012013, 05:02

#205

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Re: Doctrine of the Imperative Triangle
There is no mathematical operation which will give you an angle which will give you a course to steer which does not involve, directly or indirectly, implicitly or explicitly, solving a triangle. You can't solve a vector without triangles, because vectors ARE triangles. Just because you're doing it with a protractor or even with your finger without crunching any sines orsquare roots doesn't mean you're not solving a triangle. It's THE fundamental underlying mathematical principle of vectors.CTS can't be solved without distance AND bearing to destination. In fact you don't even know where the destination is without these two data, and mathematically, they amount to a line.When we get the rest of the material about SWL's method, I'll show specifically how it's done.
Quote:
Originally Posted by Andrew Troup
(sigh) Deep Frz says the same thing about measuring A to B:
If SwL measures the distance, it's for interest only. It does not play any part in her method, nor does the line joining them.
In straightforward situations, measuring it can help with estimating how many hours of tide to allow before starting to check for being within 'striking distance', saving unnecessary premature checks.
In strong tides sweeping the boat far out to one side, it's not even helpful for that.
You can assert the contrary, but your assertion is based on supposition or assumption.
I suggest you ask SeaWorthy if she uses the rhumb distance for her method, and if so, what for.
It's remarkable that several individuals have suddenly developed more expertise in her method than she has herself.
How many times have you tried applying it using neither the rhumb line or triangles?
Unless you've tried, assiduously, and failed, you are in no position to claim they are indispensable.
As for the "tidal triangle", you are mistaking the representation for the thing.
Triangles are a tool to represent vectors.
One can also, if one prefers, use them to solve vector arithmetic. It's a fine method, one of several. (refer previous elaboration)
One could choose to draw a triangle as a representation of the tidal vectors in SeaWorthy's procedure as an extra and superfluous element, but it would not include the rhumb line. From the endpoint of those vectors, only one line is drawn. It does not form a side of a useful, let alone indispensable, triangle of which the rhumb line is the other side.
(resigh)
I have no interest in triangles, I only brought the subject up to refute DH's contention that the rhumb line must be an essential element of her method, because it was one leg of an essential triangle.
No, nope, nopity no. On both counts.
    
I don't see much merit in travelling further along this path because I can only reiterate the content of this and earlier posts.
It's was hoping to establish that the rhumb line is a key element in the RYA method and not the SwL method. It's not a controversial or difficult proposition. Surely if it was a key element SwL would draw it?
The reason I want to establish that proposition is that the dependence on the rhumb line is what (for purely geometric reasons) causes the RYA method to fail to provide a solution under some scenarios. The construction the method relies on is simply not possible.
I'm not bothered that SwL has got bogged down and is not yet addressing such scenarios.
Until a few more people have their 'penny drop' moment with the above proposition, there's not much chance of the issue being understood without another longdrawnout, shambolic and energy sapping argument.




24012013, 05:11

#206

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Re: Inaccurate RYA Teaching : CTS  Quest For a New Method
Seaworthy's last reply to you is eloquently and precisely correct.
You are confused about the purpose of a constant heading course. The purpose is not at all to stay on or anywhere near the rhumb line. Its purpose is to get you there faster by sailing a straight path through the water  the course to steer IS that straight line, and the difficulty in determining the correct straight path which will deliver you through changing currents right to your destination without changing headings is the subject of this thread. The straight path through often takes you miles and miles off the rhumb line.
Quote:
Originally Posted by bewitched
Forgive me, but when I was taught navigation, plotting my intended course and then navigation along it was the whole point to the exercise.
.
You can't...I never said that you could. The one single, constant course to steer is a restriction that both the RYA and your method has artificially put in place to facilitate simplicity in the calculation. To enable the navigator to say to the helm; "hold this heading and in x hours we should be back on our course line....hopefully". It is an average of averages which does not reflect true life.
Let me try to explain. If the tide tables tell us:
1st hr: 4kts at w deg
2nd hr: 2kts at x deg
3rd hr: 3kts at y deg
That is not what will happen, each hour is an average. A probability within a stated deviation no doubt. This is the first averaging. What the RYA method considers now is that it is probably easier and no less accurate to consider that over the 3 hours, the tide runs on average at (4+2+3 / 3 =) 3kts at z deg. This is the second averaging. Steering a course derived from this average tidal effect will bring you back to the course line in x hours.... in theory. In practice, it won't place you directly there, but it will place you close more often than not because the averaging hopefully took out the extremes.
You could calculate a course to steer that would get you back to the course line each hour, but if there are no obstructions, there probably isn't the need to get back so regularly and it is more calculation for arguably no greater accuracy. It's only necessary to get back to the course line just before the next waypoint (RYA method), or at the next waypoint (your method).
Like I say, I think both methods have merit as long as we take them for what they are. What they are, in my opinion, is one easy calculation to determine probable tide effect, that should be considered alongside many other factors when navigating without positional information available. In other words, as a part of dead reckoning.




24012013, 05:14

#207

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Re: Inaccurate RYA Teaching : CTS  Quest For a New Method
Example 3
The RYA instructor in the previous navigation thread requested a couple of times that I submit an example with oblique tides, varying in strength, so here goes .
Boat speed is constant at 4 knots throughout the journey.
You are motoring in flat water.
Destination is 11 nm due east.
Current:
1st hour: 8 knots 135 T
2nd hour: 6 knots 150 T
3rd hour: 2 knots 170 T
4th hour: 2 knots 10 T
What is the CTS?
Using the RYA method
Computations are plotted in the diagram below (note, these are usually made on a chart)
CTS = 46 degrees true
Perhaps Instructor Jackdale could confirm that I have plotted this correctly and that I come up with the same CTS that he does?
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24012013, 05:24

#208

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Re: Inaccurate RYA Teaching : CTS  Quest For a New Method
For Andrei:
Seaworthy's problem is laid out in seven propositions.
The course line is proposition 3.
QUOTE=Seaworthy Lass;1138070] Example 3
The RYA instructor in the previous navigation thread requested a couple of times that I submit an example with oblique tides, varying in strength, so here goes .
Boat speed is constant at 4 knots throughout the journey.
You are motoring in flat water.
Destination is 11 nm due east.
Current:
1st hour: 8 knots 135 T
2nd hour: 6 knots 150 T
3rd hour: 2 knots 170 T
4th hour: 2 knots 10 T
What is the CTS?
Using the RYA method
Computations are plotted in the diagram below (note, these are usually made on a chart)
CTS = 46 degrees true
Perhaps Instructor Jackdale could confirm that I have plotted this correctly and that I come up with the same CTS that he does? [/QUOTE]



24012013, 05:30

#209

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Re: Doctrine of the Imperative Triangle
Quote:
Originally Posted by Dockhead
There is no mathematical operation which will give you an angle which will give you a course to steer which does not involve, directly or indirectly, implicitly or explicitly, solving a triangle. You can't solve a vector without triangles, because vectors ARE triangles. Just because you're doing it with a protractor or even with your finger without crunching any sines orsquare roots doesn't mean you're not solving a triangle. It's THE fundamental underlying mathematical principle of vectors.CTS can't be solved without distance AND bearing to destination. In fact you don't even know where the destination is without these two data, and mathematically, they amount to a line.When we get the rest of the material about SWL's method, I'll show specifically how it's done.

As I don't use the distance between A and B in any of my calculations (only for gathering tide data) and since the number of degrees I need to steer relative to my rhumb line does not depend on the angle of the rhumb line, I am not sure I will wave the white flag yet LOL.
Sure, I would have to draw triangles to compute this mathematically, but I am not computing the angle mathematically at all. I am simply charting it with, unlike the RYA method, not a triangle in sight.
The destination can be anywhere as long at the current is acting on it in the same relative way .
Although that is just an argument about nomenclature LOL.
The big thing is that I do not need to draw a line between A and B for my method at all and I don't.
In the RYA method it is essential.
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24012013, 05:58

#210

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Re: Doctrine of the Imperative Triangle
Quote:
Originally Posted by Seaworthy Lass
As I don't use the distance between A and B in any of my calculations (only for gathering tide data) and since the number of degrees I need to steer relative to my rhumb line does not depend on the angle of the rhumb line, I am not sure I will wave the white flag yet LOL.
Sure, I would have to draw triangles to compute this mathematically, but I am not computing the angle mathematically at all. I am simply charting it with, unlike the RYA method, not a triangle in sight.
The destination can be anywhere as long at the current is acting on it in the same relative way .
Although that is just an argument about nomenclature LOL.
The big thing is that I do not need to draw a line between A and B for my method at all and I don't.
In the RYA method it is essential.

But you do use the distance, because that is what determines the position of B which allows you to measure an angle to it with your protractor.
You are exactly solving a triangle; you are just using an analogue method instead of crunching sines and square roots. I am sorry to be a pedant about it, but a lot of bombastic but erroneous things were said which really need to be cleared up.
This:
Is called a vector triangle. This is the "triangle in sight" in your method. It is nothing more and nothing less than that. It is a mathematical construct which is an analogue method of solving trigonometric (i.e. triangular) functions invented in the days when there were no digital means to easily crunch the numbers.
Your vector triangle consists of three lines, none of which bears any direct relationship to your passage. These three lines meet at three points. The lines are abstractions, every one. The course line defines the range and bearing between origin and destination. The tide vector line defines the sum of the individual tide vectors (themselves each a triangular function)  the line defines the sum of all your work with the individual currents, and again it consists of range and bearing  range is the net displacement, and bearing is the net direction. The water track line defines the distance you sail in the time frame of the triangle  every leg corresponds to exactly the same time frame, which is THE essential relationship between them.
So when you read off the angle of the water line, you are receiving the output from these triangular functions and you get the pot at the end of that rainbow  your CTS.
The principle you have used to draw all of this is called the "Triangle Law of Vector Addition"  see Vector Addition
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