

21012013, 02:21

#646

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Boat: Aluminium cutter rigged sloop
Posts: 12,938

Re: Distinct Activities: Shackled by a Common Name?
Well, I had a good sleep in and was thrilled to find a message from a CF member this morning. Many, many thanks for the feedback .
Unfortunately he pointed out the flaws underlying the principle of my method  it was really only obvious resultwise with an extreme case, but they were serious flaws and need to be addressed.
My method still gives a more accurate result in the circumstances where the RYA method fails, but it is not good enough. A universal method needs to be able to work for ANY data thrown at it. And it doesn't at the moment.
Folks, I have exposed a serious problem in the RYA method. I will puzzle over a solution further, but it would really help if someone with better mathematical knowledge contributed to solving things.
To summarise: the RYA method will not be accurate unless D falls extremely close to B in the RYA calculations, or to allow extrapolation, the average of X number of tide displacement vectors is identical to the average of X+1 (where X is the number of hours until D can be put as close as possible to B), as it is the heading to D which is being determined.
I understand clearly now what makes the method not work most commonly and I can toss an unlimited number of examples at you. The extrapolations made by the RYA method have an inherent inaccuracy that can result in CTS measurements being greater than 10 degrees off (and probably more) just for realistic data. Ten degrees off over a 20 nm journey results in being 3.5 nm out at the end of the journey  a huge amount!
I haven't been scared to end up with egg on my face  how else do we move forward than by challenging current thoughts? Could someone please chime in and help find a solution to this?
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21012013, 02:32

#647

Nearly an old salt
Join Date: Jun 2009
Posts: 13,649

Quote:
Originally Posted by Seaworthy Lass
Well, I had a good sleep in and was thrilled to find a message from a CF member this morning. Many, many thanks for the feedback .
Unfortunately he pointed out the flaws underlying the principle of my method  it was really only obvious resultwise with an extreme case, but they were serious flaws and need to be addressed.
My method still gives a more accurate result in the circumstances where the RYA method fails, but it is not good enough. A universal method needs to be able to work for ANY data thrown at it. And it doesn't at the moment.
Folks, I have exposed a serious problem in the RYA method. I will puzzle over a solution further, but it would really help if someone with better mathematical knowledge contributed to solving things.
To summarise: the RYA method will not be accurate unless D falls extremely close to B in the RYA calculations, or to allow extrapolation, the average of X number of tide displacement vectors is identical to the average of X+1 (where X is the number of hours until D can be put as close as possible to B), as it is the heading to D which is being determined.
I understand clearly now what makes the method not work most commonly and I can toss an unlimited number of examples at you. The extrapolations made by the RYA method have an inherent inaccuracy that can result in CTS measurements being greater than 10 degrees off (and probably more) just for realistic data. Ten degrees off over a 20 nm journey results in being 3.5 nm out at the end of the journey  a huge amount!
I haven't been scared to end up with egg on my face  how else do we move forward than by challenging current thoughts? Could someone please chime in and help find a solution to this?

Seaworthy where have you shown be RYA method is that bad. You consistently refuse to accept the issue of tidal data accuracy your, deductions about CTS are simply not correct.
I've illustrated the errors in your approach with three detailed emails and I don't get a courtesy of a response.
Your method is mathematically wrong to apply the computed CtS on the last triangle to the whole journey , your method is wrong as it assumes an accuracy in tide data that isn't there
You have unfortunately invented an erroneous method that merely gives you numbers you want to believe in. The RYA method remains the best and most accurate method given the accuracy of be underlying data and the assumptions applying it
Dave
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21012013, 02:33

#648

cruiser
Join Date: Dec 2012
Location: Seattle
Posts: 1,130

Re: Distinct Activities: Shackled by a Common Name?
It would also help, for the benefit of us newbies, if someone could be kind enough to post a blowbyblow description of the RYA method.
I sail here in Puget Sound, in the United States. I'm trained in the U.S. Sailing program, and had never heard of the RYA until I read this thread.
My training in these matters consists, therefore, of the U.S. Sailing Piloting & Navigation course, and that is the first and, so far, only method I know of plotting courses and currents.
I've been keeping my mouth shut, because it seemed clear to me that everyone in the discussion understood all facets of the RYA method, but at this late date, I'd like to point out that I don't, and that means there are, perhaps, many folks following this thread who don't.
As you are willing to accept egg as makeup, (an extremely worthy stance!) so am I much too old to care about how I look or sound. So I will speak for all the clueless, curious newbies reading this thread, and give voice to all the stupid questions that only a newbie would ask.
(Those who think there is no such thing as a stupid question haven't met my apprentice. But I digress.)
So as a prelude to comparison between methods, I would like to hear both methods described as Lass has described hers.
One of the most valuable threads I've ever read on the internet. And that's saying a lot.



21012013, 02:36

#649

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Join Date: Oct 2008
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Posts: 12,938

Re: Distinct Activities: Shackled by a Common Name?
Quote:
Originally Posted by goboatingnow
Ill look, at your chart work later , its quite difficult from the pictures

As this is a really simple example, you just need a pen, paper, ruler and and calculator. No graph paper is required. Please check my figures and see if you come up with 56 degrees. If not what figure do you come up with?
Quote:
Originally Posted by goboatingnow
The RYA says estimate in hours the journey time or whatever tidal quantum you have available the , in some places you have 30 minute tides ). That estimate is the total expected passage time allowing for any tides. typically where tides are 25% or less then boat speed or the journey is short then the rhumb line can be used. But in long runs with many tides ( like across the Irish Sea ) you have to guesstimate.

The RYA video I watched said to approximate the time of the journey by dividing the course length by the boat speed and using that whole number of hours. I don't know what is in the RYA notes, but people recently doing RYA courses report that is what they are taught.
Quote:
Originally Posted by goboatingnow
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. This can be done by simplest calculating the rate of advance , ie the average SOG to D and by inspection determine how far B is away. 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.

Some instructors omit this bit? In other words it isn't taught?
It is rarely done in examples as test questions are picked to ensure it isn't needed??
No wonder this problem with the RYA method in some circumstances hasn't been detected before!
Quote:
Originally Posted by goboatingnow
If D lies in front of B , calculate the time of arrival by applying the rate of advance to D to the whole journey B, equally if D is behind reduce the time by using the SOG to B accordingly ( ie inflate or deflate the whole tidal triangle

Inflating and deflating data can lead to errors and it does. This is the source of the problem.
Quote:
Originally Posted by goboatingnow
CTS is the bearing of line CD at all times.

I am glad you agree with me now, you were hotly disputing this before.
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21012013, 02:49

#650

Nearly an old salt
Join Date: Jun 2009
Posts: 13,649

Quote:
Originally Posted by Seaworthy Lass
As this is a really simple example, you just need a pen, paper, ruler and and calculator. No graph paper is required. Please check my figures and see if you come up with 56 degrees. If not what figure do you come up with?
The RYA video I watched said to approximate the time of the journey by dividing the course length by the boat speed and using that whole number of hours. I don't know what is in the RYA notes, but people recently doing RYA courses report that is what they are taught.
Some instructors omit this bit? In other words it isn't taught?
It is rarely done in examples as test questions are picked to ensure it isn't needed??
No wonder this problem with the RYA method in some circumstances hasn't been detected before!
Inflating and deflating data can lead to errors and it does. This is the source of the problem.
I am glad you agree with me now, you were hotly disputing this before.

Please STOP telling me what you think the RYA method is. I have provided a detailed description of the complete method. I've met and talk to one of the people who designed the course You estimate the passage time using any available means.
Question
You calculate your journey CTS as the last one , and then retrospectively say thats the whole journey CTS. Isn't that right ?
Further more I have ALways argued that CD is the CTS vector. I have repeated in several posts that CB is never correct, unless by confidence D and B coincide
The bones of your argument is that inflating a say 6 hour plot by 5 minutes , is LESS accurate then what you are doing. Given the way tidal data applies , you are fundamentally wrong here. Inflating the tidal data DOES not lead to any more errors then any other method. It arguably leads to less
The RYA is the originator of the world renowned YM program, its has had its theories tested by 1000s of Yachtmen over many many years especially around the tidal waters of the UK, your not seriously telling me they wouldn't have discovered any flaws in real life by now.
I do not understand how you don't see the flaws in your application of tidal data.
Dave
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21012013, 02:50

#651

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Join Date: Oct 2008
Boat: Aluminium cutter rigged sloop
Posts: 12,938

Re: Distinct Activities: Shackled by a Common Name?
Quote:
Originally Posted by goboatingnow
Where you to assume like you did , that the tides are actually exactly constant for each hour AND the hourly, tidal boundaries are immutable your method would be accurate.

You must make this assumption to compute anything as it is all the data we have (the RYA method runs into exactly the same problem). I am not arguing about the limitations of the data, I am arguing about the limitations of the RYA method.
Quote:
Originally Posted by goboatingnow
In practice , as I have repeated over several posts , proportioning , say 4 minutes of additional vector to each hour say over a 6 hour plot in order to " advance " the whole triangle is MORE accurate then deflating the last triangle by 6024= 38.

In the last hour of passage, the boat is under the influence of the last hour of current, so if half an hour is spent in that current, then half an hour of it must be apportioned to the total, NOT a proportion of the whole tide up to that point. That is the whole thrust of my message and what alerted me that something was wrong with the RYA method.
Quote:
Originally Posted by goboatingnow
The fact is that by the nature of determining tidal vectors your method results in a less accurate result where you proportion the last tide by a large percentage of an hour.

Huh? . In every single example so far my method (that still needs modification to make it better) has been more accurate.
Quote:
Originally Posted by goboatingnow
Accepting that mis  assumption in your method , then all you have done is the RYA method.

Sorry, I have not. The CTS I determine is quite different to the one determined by the RYA in the above example, so the method is not the same. And despite the fact that my method needs more work, it is still more accurate than the RYA method..
Dave, you still have not admitted you were wrong about several points we were discussing regarding the RYA method.
Namely:
 That vectors drawn are distance not speed vectors
 That the the RYA method is designed to have you arriving initially at D before you continue on to B (you have kept insisting that you will never be at D and you said that in the hovercraft example you would end up south of D).
 that the RYA method is an approximation due to the fact that it is inflating or deflating the whole tidal triangle.
At least you have stopped telling me how wrong and confused I am about these points. It would have been polite after giving me such a hard time, to have you admitted your errors when you recognised them.
You mentioned in post #356 that you taught chartwork  lets work together on this. You also said you are an engineer, look at this problem with fresh eyes, not by simply trying to justify what the RYA teaches.
The RYA method is flawed. For all the examples I have given so far my method gives a better result, but it needs revising significantly still.
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21012013, 02:53

#652

Nearly an old salt
Join Date: Jun 2009
Posts: 13,649

Quote:
Originally Posted by Jammer Six
It would also help, for the benefit of us newbies, if someone could be kind enough to post a blowbyblow description of the RYA method.
I sail here in Puget Sound, in the United States. I'm trained in the U.S. Sailing program, and had never heard of the RYA until I read this thread.
My training in these matters consists, therefore, of the U.S. Sailing Piloting & Navigation course, and that is the first and, so far, only method I know of plotting courses and currents.
I've been keeping my mouth shut, because it seemed clear to me that everyone in the discussion understood all facets of the RYA method, but at this late date, I'd like to point out that I don't, and that means there are, perhaps, many folks following this thread who don't.
As you are willing to accept egg as makeup, (an extremely worthy stance!) so am I much too old to care about how I look or sound. So I will speak for all the clueless, curious newbies reading this thread, and give voice to all the stupid questions that only a newbie would ask.
(Those who think there is no such thing as a stupid question haven't met my apprentice. But I digress.)
So as a prelude to comparison between methods, I would like to hear both methods described as Lass has described hers.
One of the most valuable threads I've ever read on the internet. And that's saying a lot.

The Royal Yachting Association amongst other things designed and administers the Yachtmaster training program , arguably the highest skill tests a leisure Yachtmsns can attempt, outside commercial certification. With the addition of a medical cert you can receive commercial endorsement upto 200 tons , without area restriction.
As to the method if you look back at a few posts I have outlined the full method in step by step method
Dave
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21012013, 02:59

#653

Moderator
Join Date: Oct 2008
Boat: Aluminium cutter rigged sloop
Posts: 12,938

Re: Distinct Activities: Shackled by a Common Name?
Quote:
Originally Posted by goboatingnow
...... think about this. , tides are SOG based. , a boat with a water track is slowed by a tide from say 6 kn to 4kn SOG , therefore its spends more then one hour being affected by that tide. Ie lets say it was stopped by the tide. Your assumptions say the boat must actually get out of the tide in one hour ( because you apply all the corrections at the end ) but in reality if I spend 6/4 x 60 minutes in a 2kn tide I am affected by it for that time. , ie the tidal quantum is simply not abrupt., the tide does not disappear after 60 minutes. hence average back each tide for the appropriate additional time is more correct then applying all at the end.

Huh?
"TIME AND TIDE WAIT FOR NO MAN".
Tides are periodical and vary with time.
If the tide only runs for one hour then you are only influenced by it for one hour, even if you are in exactly the same spot for that entire hour. You have made this point before and I just can't see how you can think this, unless you are confusing tide with situations were there is constant current over time in one location (nothing to do with tides).
Of course the tide can disappear completely at the end of the 60 minutes (that is called slack water). It can even reverse!
Quote:
Originally Posted by goboatingnow
The RYA method has withstood the test of 50 years of application, I suspect it will withstand the next 50.

Well maybe, but how else does progress occur if not by challenging old ideas?
I would like you to work through one simple example if you could please. It will settle the argument of the accuracy of the RYA method once and for all. It doesn't need graph paper or a protractor as it is a very simple example with right angled triangles, draw it up to scale on plain paper if need be. You only need a pen, paper, and calculator (the calculator can be downloaded free online).
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
What is the CTS and the time taken as estimated by the RYA method?
Please give me the values you come up with and we can settle the accuracy of the RYA method once and for all!
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21012013, 03:00

#654

Nearly an old salt
Join Date: Jun 2009
Posts: 13,649

Quote:
Originally Posted by Seaworthy Lass
You must make this assumption to compute anything as it is all the data we have (the RYA method runs into exactly the same problem). I am not arguing about the limitations of the data, I am arguing about the limitations of the RYA method.
In the last hour of passage, the boat is under the influence of the last hour of current, so if half an hour is spent in that current, then half an hour of it must be apportioned to the total, NOT a proportion of the whole tide up to that point. That is the whole thrust of my message and what alerted me that something was wrong with the RYA method.
Huh? . In every single example so far my method (that still needs modification to make it better) has been more accurate.
Sorry, I have not. The CTS I determine is quite different to the one determined by the RYA in the above example, so the method is not the same. And despite the fact that my method needs more work, it is still more accurate than the RYA method..
Dave, you still have not admitted you were wrong about several points we were discussing regarding the RYA method.
Namely:
 That vectors drawn are distance not speed vectors
 That the the RYA method is designed to have you arriving initially at D before you continue on to B (you have kept insisting that you will never be at D and you said that in the hovercraft example you would end up south of D).
 that the RYA method is an approximation due to the fact that it is inflating or deflating the whole tidal triangle.
At least you have stopped telling me how wrong and confused I am about these points. It would have been polite after giving me such a hard time, to have you admitted your errors when you recognised them.
You mentioned in post #356 that you taught chartwork  lets work together on this. You also said you are an engineer, look at this problem with fresh eyes, not by simply trying to justify what the RYA teaches.
The RYA method is flawed. For all the examples I have given so far my method gives a better result, but it needs revising significantly still.

My hovercraft example was not correct in the regards that I didn't estimate the time right so that D was too far away from B, I was illustrating a different point. FORGET the hovercraft example , it has insufficient data , just compare your own or do two 2 hour plots.
The RYA method is not to get you to D , a point in front of B , many examples in the workbooks ( there's about 10 workbooks on the market ) have examples where D is behind B
The approximation of inflating or deflating the whole triangle is More accurate then your method , once you abide buy the rules of how tidal data is derived and how long it is valid for.
seaworthy
I am an engineer , I have taught the courses and I am a YM , your method is wrong because
( a) you apply the accuracy of tidal data incorrectly
(B) your course to steer CTS is incorrect mathematically.
Stop simply going back 100s of posts. I agree we have arrived at your method and are comparing it to the method I have laid out about 10 posts ago. Yours is not correct for the reasons stated.
Dave
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21012013, 03:06

#655

Nearly an old salt
Join Date: Jun 2009
Posts: 13,649

Quote:
Originally Posted by Seaworthy Lass
Huh?
"TIME AND TIDE WAIT FOR NO MAN".
Tides are periodical and vary with time.
If the tide only runs for one hour then you are only influenced by it for one hour, even if you are in exactly the same spot for that entire hour. You have made this point before and I just can't see how you can think this, unless you are confusing tide with situations were there is constant current over time in one location (nothing to do with tides).
Of course the tide can disappear completely at the end of the 60 minutes (that is called slack water). It can even reverse!
Well maybe, but how else does progress occur if not by challenging old ideas?
I would like you to work through one simple example if you could please. It will settle the argument of the accuracy of the RYA method once and for all. It doesn't need graph paper or a protractor as it is a very simple example with right angled triangles, draw it up to scale on plain paper if need be. You only need a pen, paper, and calculator (the calculator can be downloaded free online).
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
What is the CTS and the time taken as estimated by the RYA method?
Please give me the values you come up with and we can settle the accuracy of the RYA method once and for all!

This. Is actually not how tides are specified in real life . In real life tides are stated at a set and drift at a fixed point in time., the data is interpolated for your arrival time at that point and ASSUMED to be constant for 60 minutes , in reality it is never constant, the tide data is equally as valid for many minutes before or after 60 minutes , in practice if you examine the tidal curve you will see this is the case
But I'll work up your example ,
Dave
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21012013, 03:07

#656

Moderator
Join Date: Oct 2008
Boat: Aluminium cutter rigged sloop
Posts: 12,938

Re: Distinct Activities: Shackled by a Common Name?
Quote:
Originally Posted by goboatingnow
This. Is actually not how tides are specified in real life . In real life tides are stated at a set and drift at a fixed point in time., the data is interpolated for your arrival time at that point and ASSUMED to be constant for 60 minutes , in reality it is never constant, the tide data is equally as valid for many minutes before or after 60 minutes , in practice if you examine the tidal curve you will see this is the case
But I'll work up your example ,
Dave

Thank you for doing these calculations .
I think it will sort out our disagreement .
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21012013, 03:19

#657

Moderator
Join Date: Oct 2008
Boat: Aluminium cutter rigged sloop
Posts: 12,938

Re: Distinct Activities: Shackled by a Common Name?
Quote:
Originally Posted by goboatingnow
My hovercraft example was not correct in the regards that I didn't estimate the time right so that D was too far away from B,
I was illustrating a different point. FORGET the hovercraft example , it has insufficient data , just compare your own or do two 2 hour plots.

I am happy to forget the hovercraft example if you like. .
Quote:
Originally Posted by goboatingnow
The RYA method is not to get you to D , a point in front of B , many examples in the workbooks ( there's about 10 workbooks on the market ) have examples where D is behind B

Sorry, it will always aim to get you to D if D is before B, despite you vehemently denying this before.
Quote:
Originally Posted by goboatingnow
I am an engineer , I have taught the courses and I am a YM , your method is wrong because
( a) you apply the accuracy of tidal data incorrectly
(B) your course to steer CTS is incorrect mathematically.

Lets see. I eagerly await your results for the very simple exercise I set.
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21012013, 03:49

#658

Nearly an old salt
Join Date: Jun 2009
Posts: 13,649

Here it is assuming I haven't made any simple addition mistakes
Now we can critique it in comparison to real life
Dave
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21012013, 03:55

#659

Moderator
Join Date: Oct 2008
Boat: Aluminium cutter rigged sloop
Posts: 12,938

Re: Distinct Activities: Shackled by a Common Name?
Quote:
Originally Posted by goboatingnow
CTS is the bearing of line CD at all times.
Quote:
Originally Posted by Seaworthy Lass
[B][COLOR="Blue"]
I am glad you agree with me now, you were hotly disputing this before.

Apologies, you did say CTS was the bearing of the line CD at all times.
You were denying that you ever arrived at D. You have given so much misinformation on this thread that I forgot for a moment exactly what you had mixed up.
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21012013, 03:58

#660

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Join Date: Oct 2008
Boat: Aluminium cutter rigged sloop
Posts: 12,938

Re: Distinct Activities: Shackled by a Common Name?
Quote:
Originally Posted by goboatingnow
Here it is assuming I haven't made any simple addition mistakes
Now we can critique it in comparison to real life
Dave

Dave, could you please type out what is the CTS and time taken?
Also any other comments you have written.
The photo is too indistinct for me to read it.
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