

21012013, 23:53

#1

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Inaccurate RYA Teaching : CTS  Quest For a New Method
There has just been a heated discussion on this thread:
http://www.cruisersforum.com/forums/...ame94879.html
In this thread I went head to head with an experienced RYA instructor who up until proven wrong ( he was five degrees off using the RYA method on a very simple short exercise) was heatedly insisting the RYA was using a mathematically perfect model for determining the CTS (course to steer) for a passage.
The instructor had earlier provided me with a link to a YouTube video which illustrates the RYA technique:
YouTube
It is useful to view this if you haven't already.
As an aside, note in this video how the arced off distance travelled vector lies quite a distance from the destination point and no attempt is made to see if this point falls closer to the destination point B after the next hour or even the following one, where this point would end up past B. Selecting the point closest to B (whether it falls before or after B) would at least improve the technique, but that is a separate issue. The technique is essentially flawed.
I am starting a new thread as I think this is a very important issue and as it risks being lost over time in a very long thread, it needs to be continued here without the other navigational discussions that are going on concurrently and distracting from this issue.
Flaws with the current RYA method:
The RYA determine the CTS to get to a point near the destination point, not actually to it. They then extrapolate the data. Sometimes that works just fine, sometimes it will be substantially out (extrapolations are often not valid, as shown in the example I set the RYA instructor where his computed heading was five degrees out), sometimes it will be dramatically out.
The instance where it will fail dramatically is where the sum of the vector positions ends up perpendicular to the destination point and exactly above or below it. In this case the arc off of the distance travelled from the tip of the sum of the current displacement up until then does not reach the rhumb line. If the next arc off of the current is perpendicular to B (the destination) it hits the rhumb line twice, equidistant from B.
These two lots of D could be wildly different  it is even possible for them to be more than 90 degrees apart!!!! Pointless choosing either of them, the correct CST is exactly halfway between them then. These type of tidal/current situations may not crop up, but any method should work with ANY data thrown at it .
This extreme case was pointed out to me by a CF member who sent me a PM shooting holes in one of the methods I proposed for determining CTS.
By the way, would you like to come out of hiding yet and take kudos for pointing me in the right direction? (No pun intended). You were inspirational! Many thanks for taking the trouble to sit and look at my model carefully and see it was based on false principles. You were the only one who also recognised I was on to something and had a close look at what I was proposing and told me to go back to the drawing board. Many thanks
The following post will describe my method. It removes the need to draw a rhumb line at all. I am throwing it out for consideration. If anyone can pick flaws in it (please please try) it will have to be revised yet again, but I am hoping this time it is right .
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22012013, 01:34

#2

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Re: Inaccurate RYA Teaching : CTS  Quest For a New Method
My method for determining CTS (version 3 LOL, I am getting there).
This may not be the final one, feedback from anyone mathematically minded here would be fantastic, but I can't pick any flaws in it at the moment.
My method is distinctly different to the RYA one as I am ditching any reference at all to the rhumb line. Why is the RYA marking a position on it at all? As the ground track during a passage with current and can take you just about anywhere if you have a single CTS, why use the rhumb line as a reference? It is as arbitrary as any other line through B.
My proposal is:
FORGET ABOUT THE RHUMB LINE, there is no need to mark one.
Step 1: Mark the departure and destination points on a chart. These are A and B respectively
Step 2: Measure AB (= the course distance)
Step 3: Divide the course distance by the expected boat speed. This gives time of the journey if there is no current or leeway.
Step 4: You know the passage will take longer than that if there is total current against you (and shorter if it it with you), so make an educated guess how much longer by looking at the current along the way (this may actually be difficult, but that is a whole different topic).
This gives you a starting point for how many hour lots of current you need to consider (or half hour lots if this current info is provided). For simplicity I will describe hourly data here, but the technique works equally well for any segment you care to look at.
Step 5: Mark the first current displacement from A, then continue adding on hourly displacement amounts from the tip of each final one until you can arc off a speed displacement vector that is getting close to B (eg if you have marked off 3 lots of tides and your speed is 5 knots, then you are arcing off 15nm).
If the arc off from the tip of your subsequent current displacement vector then extends past B, almost all the hard work has been done. You know you will arrive sometime during this last hour of current.
(If the arc exactly crosses B, mark this off. This is your CTS. The time taken for the journey is the number of hours of current vectors you had to use.)
Step 6: Your arc is unlikely to exactly coincide, so draw a line from the start of this final current displacement vector to B. Arc off the distance vector for this time and mark S (for short)
Step 7: Draw a line from the end of the final current displacement vector through B. Arc off the distance vector for this time and mark L (for long)
Step 8: Look at the proportion between SB and LB
If SB is very short compared to LB, only a small amount of the final current needs to be applied.
If SB is very long compared to LB, most of the final current needs to be applied.
If SB = LB half the current needs to be applied.
For the greatest accuracy you could measure the two and work out exactly what proportion will apply, but you could also just eyeball it. This will give you a good approximation and it usually will be all you need. It is at least very reliable and will NEVER give you a bad result as the RYA method can.
When you decide what proportion of the last current vector applies, mark that point on the final current vector and label it K
Step 9: Join K to B, extending the line past B
This is your CTS before you have made allowances for compass variation and for leeway.
The time taken is the distance between K and B, divided by the boat speed.
Easy as that! No extrapolations (that make invalid assumptions and can gives results that have significant errors), and I think the method is based on solid principles, unlike any method utilising the rhumb line.
Diagram will follow. It will need to wait until tonight though as the Sunbrella has arrived to replace the sacrificial strips on the headsails and the hot cutter came yesterday, so going ashore shortly to commence work.
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22012013, 02:09

#3

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Re: Inaccurate RYA Teaching : CTS  Quest For a New Method
I can't evaluate the RYA method without a complete explication of it. That YouTube video is not nearly enough information. Does anyone know where to find the complete instructions?



22012013, 02:11

#4

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Re: Inaccurate RYA Teaching : CTS  Quest For a New Method
"You know the passage will take longer than that if there is total current against you (and shorter if it it with you), "
Remember that the passage will take longer in some cases even if the current is with you. If it pushes you off the rhumb line more than it pushes you ahead, you have a net negative effect on your passage time.



22012013, 02:30

#5

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Re: Inaccurate RYA Teaching : CTS  Quest For a New Method
Quote:
Originally Posted by Dockhead
"You know the passage will take longer than that if there is total current against you (and shorter if it it with you), "
Remember that the passage will take longer in some cases even if the current is with you. If it pushes you off the rhumb line more than it pushes you ahead, you have a net negative effect on your passage time.

I took it that SwL was writing shorthand for "... longer if the average current reduces your VMG to the destination (and shorter if it increases it)"
but I've been wrong before ...
and if the tide is unidirectional, maybe VMG does not capture what you're alluding to, DH .... will have to think some more but praps tomorrow.



22012013, 03:52

#6

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Re: Inaccurate RYA Teaching : CTS  Quest For a New Method
Quote:
Originally Posted by Andrew Troup
I took it that SwL was writing shorthand for "... longer if the average current reduces your VMG to the destination (and shorter if it increases it)"
but I've been wrong before ...
and if the tide is unidirectional, maybe VMG does not capture what you're alluding to, DH .... will have to think some more but praps tomorrow.

I have no doubt that SWL understands the situation. I was merely correcting the formulation.
It's not a very deep point, just a precisioning of the phrase to prevent confusing anyone.
Yes, your formulation using VMG is exactly right, I think.



22012013, 04:23

#7

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Re: Inaccurate RYA Teaching : CTS  Quest For a New Method
OK, I've been studying this:
Sailtrain: Navigation and Chartwork,
I don't know to what extent this differs from Dave's method.
I start to get it. It's cool because the tidal vector line corrects you for the direction as well as speed of the tide (those who understand the method will be saying "Duh, that Dockhead sure is a dim bulb, he only just now gets that?!").
I think the problem is the partial hour at the end, and what troubles me is that you are supposed to lay the end of the water track line onto the rhumb line, which doesn't make the slightest sense to me. This introduces a built in steering error. The water track line must point at the destination, it seems to me, or you are getting the wrong course correction. And it needs to end at the destination, or you haven't accounted for part of the passage and are not correcting for anything which happens during that unaccounted part of the passage. Not so?
For this method to work in a mathematically precise way for a multihour passage, you've got to keep growing the the tidal vectors line until the water track line touches the destination. It means the last leg of the tidal vectors line has to be a partial one corresponding to the partial last hour  drawn to match the average direction and average speed of the current during that partial hour. It will be an approximation of course, but the water track should just touch the destination if the tidal vectors were drawn right, not end on the rhumb line ahead or behind the destination. It is complicated to get the last partial hour because the length of the water track will vary and reflect back on the length of the tidal vector line. It would need an analogue fudge or better, a computer calculating it recursively. But if everything is correctly calculated, the length of the water track line must equal the distance from the end of the series of tidal vector lines right to the destination, unless I am gravely misunderstanding something (which I do not exclude).
That's an idealized method which might really not be optimally practical for the real world, but I think this is the mathematically correct one. I don't know if it has anything to do with SWL's method or not; waiting for her graph to try to fully understand it.
Perhaps the RYA method is intended to be more practically usable; nevertheless I am not confident we are applying it correctly. I can't believe that it would produce such large errors. I really doubt that this is an error in the method itself; surely it is much more likely to be an error in the application of it. I am still looking for a complete explanation of it somewhere.



22012013, 04:27

#8

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Re: Inaccurate RYA Teaching : CTS  Quest For a New Method
Here is something else interesting turned up by Mr. Google:
Course to Steer Calculator
If you input the average speed of the current (divided by the number of hours of the passage) and the average direction of the current for the entire passage, you get a valid CTS for a multihour passage.
The math in this app is exactly the same as in my own crude Channelcrossing method, removing the direction element since a perpendicular current will produce one mile of displacement per hour per knot of current



22012013, 04:38

#9

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Re: Inaccurate RYA Teaching : CTS  Quest For a New Method
You can download an an Excelbased sophisticated CTS calculator from this site: CompasscourseCalculator
It is very complete and very cool  automatically calculates a multihour passage with varying current, and considering even leeway (includes a cool separate leeway calculator). Unfortunately the formulae are hidden.
It would be interesting to see how he deals with the last hour problem.
Maybe we should invite the author to join in this discussion.



22012013, 05:06

#10

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

WOW, do they get it wrong! They call the course line "the ground track" (it only is for constant current over the entire course). .
"If we then draw this line in, it becomes the water track. It does not matter that the water track has not reached the destination, because you can assume that for a short period, conditions will remain similar, and you can continue on the same course. In practice, you would probably be able to see the destination and could approach it by eye."
A least they admit it will not get you to B (so they do not claim the same precision that Dave did about arriving at B). And what happens if D is NOT close? in my example in the last thread it was a couple of nm away. What happens if the current is significantly different for that portion of the journey? What happens if it adverse enough that you cant reach D?
Quote:
Originally Posted by Dockhead
I don't know to what extent this differs from Dave's method.

Seems similar.
Quote:
Originally Posted by Dockhead
I start to get it. It's cool because the tidal vector line corrects you for the direction as well as speed of the tide (those who understand the method will be saying "Duh, that Dockhead sure is a dim bulb, he only just now gets that?!").

Yes, it is very clever! It allows simple computation of complex currents varying in direction as well as degree .
Quote:
Originally Posted by Dockhead
I think the problem is the partial hour at the end, and what troubles me is that you are supposed to lay the end of the water track line onto the rhumb line, which doesn't make the slightest sense to me. This introduces a built in steering error. The water track line must point at the destination, it seems to me, or you are getting the wrong course correction. And it needs to end at the destination, or you haven't accounted for part of the passage and are not correcting for anything which happens during that unaccounted part of the passage. Not so?
It doesn't make the slightest sense to me either doing this! It didn't as soon as I saw the video. Why does the RYA have you heading to D (a point on the rhumb line) not B (he destination). And yes, you then you correct according to the current experienced in the past, not the current for the final part of the journey. This is the first reason errors are introduced.
For this method to work in a mathematically precise way for a multihour passage, you've got to keep growing the the tidal vectors line until the water track line touches the destination. It means the last leg of the tidal vectors line has to be a partial one corresponding to the partial last hour  drawn to match the average direction and average speed of the current during that partial hour. It will be an approximation of course, but the water track should just touch the destination if the tidal vectors were drawn right, not end on the rhumb line ahead or behind the destination. It is complicated to get the last partial hour because the length of the water track will vary and reflect back on the length of the tidal vector line. It would need an analogue fudge or better, a computer calculating it recursively. But if everything is correctly calculated, the length of the water track line must equal the distance from the end of the series of tidal vector lines right to the destination, unless I am gravely misunderstanding something (which I do not exclude).

The CD line is then moved parallel to CD until it intersects B in the RYA method. The time taken is calculated that way. The CTS is the same (lines are parallel).
My method has the vector for boat speed going STRAIGHT through B (the destination). No fudging needed .
Quote:
Originally Posted by Dockhead
That's an idealized method which might really not be optimally practical for the real world, but I think this is the mathematically correct one. I don't know if it has anything to do with SWL's method or not; waiting for her graph to try to fully understand it.

It is only correct (no matter what world you consider it in, real or otherwise) if the average of the current for the first X hours under consideration is exactly the same as the average current for X+1 hours.
This is not the only problem though. The other problem is which D to choose when one before B is is equidistant to the next one produced after B if you bothered to check where D fell after the next lot of current (this wasnt done in the video, nor was it checked at all if D was in fact closer to B after the next lots of current). I know this isn't taught speaking to someone who recently attended an RYA course.
Quote:
Originally Posted by Dockhead
Perhaps the RYA method is intended to be more practically usable; nevertheless I am not confident we are applying it correctly. I can't believe that it would produce such large errors. I really doubt that this is an error in the method itself; surely it is much more likely to be an error in the application of it. I am still looking for a complete explanation of it somewhere.

It amazingly does produce big errors. The quote from the RYA instructor was that examples for exams are chosen where D falls close to B (and maybe otherwise as well??). The instructor said:
"Perform a check to ensure that D lies approx less then 30 minutes away from B, some instructors omit this bit, ie to ensure the tidal data remains valid....
If its greater then 30 minutes (or less) redraw the plot using one more or less tide, (note this is rarely done in examples as the test questions are typically picked to ensure it isn't needed."
In other words it isn't taught by the RYA and students aren't alerted to the possibility of problems if D is a long way from B (and it could be several nm)
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22012013, 05:06

#11

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Re: Inaccurate RYA Teaching : CTS  Quest For a New Method
Getting me head around numbers etc is not my forté  I kinda got lost around step 5!, so looking forward to the Diagram!
I had a looksee at the RYA video link (and another) and they use a different method to me! One of us is ass backwards! Admittedly the method I use also substitutes a few lines on the chart for a touch of WAG on the tidal drift effect!  but in practice seems to work ok. Fortunately navigation ain't all science.........



22012013, 05:21

#12

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Re: Inaccurate RYA Teaching : CTS  Quest For a New Method
All right, I think I am stumbling towards some kind of understanding of what we are calling the "RYA method"  without much right to call it anything, since apparently no one except Dave knows exactly what it involves.
Here is another helpful resource I found: http://www.tiller.co.uk/downloads/in..._fileID=389DA5
So I finally get the geometry, and finally get why the water track does not point to the destination. It is to avoid the recursive adjustment of the passage time in order to get the lines to converge on the destination.
If we did recursively adjust the passage time, we would get the mathematically perfect answer. However, it's not really practical without using a computer.
So this method has us get it close  to the nearest hour. Then we shrink or expand the triangle proportionately until we get the lines to converge on the destination (and we don't even need to do that, unless we want to know time to destination  we already have our CTS in the previous step). The main thing is that the angle of the water track line, which is our CTS, is determined by the proportion between the lengths of the extended rhumb line and the water track, which is a function of the aggregated position of the end of the tidal vector, the distance over ground to the destination, and the length of the water track determined by average STW. So what happens in effect is that the partial hour, which has been causing all the trouble heretofore, is fudged by averaging up or down from the full hour calculation we end up with.
If there's a big difference between the real tide during the last partial hour, and the average tide, then there can be a pretty big error, I guess. I'm not sure whether or not I'm following in Seaworthy Lass' footsteps as I won't understand her method until she publishes her drawings.



22012013, 05:23

#13

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Re: Inaccurate RYA Teaching : CTS  Quest For a New Method
In post 7 Dockhead, I think your putting forward the same method as SWL, which to me is just fine tuning the traditional method of calculating a CTS.
The final tide vector will be a best guess, as tidal streams over an hour are not linear. But a best guess is probably as good as it gets.
I've got Admiralty Total Tide Running on the laptop, and stepping through a tidal vector at 5 minute intervals shows the data changing at those intervals, so given the right tools, it is feasible to get the data, but its a bit laborious, and if you manage all the calculations, and then the anchor gets fouled,and you have to start the trip an hour later, its back to the drawing board.
My preferred method is a a rough calculation followed by suck it and see
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22012013, 05:26

#14

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Re: Inaccurate RYA Teaching : CTS  Quest For a New Method
Quote:
Originally Posted by Seaworthy Lass
WOW, do they get it wrong! They call the course line "the ground track" (it only is for constant current over the entire course). .
Seems the same to me.
Yes, it is very clever! It allows simple computation of complex currents varying in direction as well as degree .
The CD line is then moved parallel to CD until it intersects B. the time taken is calculated that way. The CTS is the same (lines are parallel).
My method has the vector for boat speed going STRAIGHT through B (the destination). No fudging needed .
It is only correct (no matter what world you consider it in, real or otherwise) if the average of the current for the first X hours under consideration is exactly the same as the average current for X+1 hours.
This is not the only problem though. The other problem is which D to choose when one before B is is equidistant to the next one produced after B if you bothered to check where D fell after the next lot of current (this wasnt done in the video, nor was it checked at all if D was in fact closer to B after the next lots of current). I know this isn't taught speaking to someone who recently attended an RYA course.
It amazingly does produce big errors. The quote from the RYA instructor was that examples for exams are chosen where D falls close to B (and maybe otherwise as well??). The instructor said:
"Perform a check to ensure that D lies approx less then 30 minutes away from B, some instructors omit this bit, ie to ensure the tidal data remains valid....
If its greater then 30 minutes (or less) redraw the plot using one more or less tide, (note this is rarely done in examples as the test questions are typically picked to ensure it isn't needed."
In other words it isn't taught by the RYA and students aren't alerted to the possibility of problems if D is a long way from B (and it could be several nm)

My last post crossed in the mail with yours
When I talked about a theoretically ideal method, I was not talking about the RYA method, I was talking about my own variant, recursively adjusting the water track and tide vector lines until the water track line lays the destination.
The RYA method, if I correctly understand it, fudges the last partial hour based on average tides over the complete hours analyzed. Rather than using real data for the last partial hour.
And that seems to be the entire heart of the matter.
By the way, the RYA materials I linked to don't make a mistake with the term "ground track". They say " desired ground track". They mean the rhumb line to the destination  A OK.



22012013, 05:28

#15

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

So much wrong with this, that I don't know where to start!
Will address this later. No time now .
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