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Old 07-02-2013, 04:49   #211
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Re: Single CTS or follow the Courseline?

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Have a look at my example above. If you follow the CTS computed using the RYA method (53 degrees true in this case) you will NOT ever arrive at B. You arrive at D, 2.5 nm short of your destination. And if you continue at 53 degrees you will NEVER reach B.
Only if you assume that the current is as you state it, with rigid time quantum's and your general examples that have currents going from 3 knots instantaneously to 1 knot. Real life is very different and hence the RYA in general does not produce the errors you attempt. to demonstrate , why not get a tidal stream atlas and have a look at real data and the real extraction methods.


Secondly NO one uses a single CTS for the duration of the voyage in real life, No one but one departs on a single CTS and expects to just drive into the marina berth at the end of a 12 hour journey. Its all theory nonsense

IN reality you recompute, so in real life , even if you get to D ( ie the one before B, not after) you will be recomputing CTS a number of times across, including perhaps near the end. In practice at 2-3 miles out you are within visual range

Its all angles ( angels !!) on a pin really this debate.


Quote:
If they have "no real life use" why are they included in the RYA Navigation Handbook? And why are the calculations taught on RYA courses and examples worked through in examinations and why are students told they will fail if they don't use a sharp enough pencil as that will affect their accuracy? .
The RYA theory course takes place typically over 16 week nights. It covers lots of different chart work techniques for power and sail. The exam does not count in any way towards the Actual YM ticket. For the actual YM ticket students must demonstrate practical passage planning methods, Any examiner would fail a student that just applied that theory above without demonstrating passage safety and a realisation in real life of what to do.

The books often make a poor effort at multi CTS, the classroom does it a bit better and its up to the instructor to point out the drawbacks etc, some do that well others do not.

Comments about pencil accuracy are just nonsense. Only a silly instructor makes those remarks. ( The YM theory exam is basically a course completion cert and is examined by the instructor. The actual YM is a far harder process carried out on the boat and contains lots of things not taught in the theory class.

In my experience multi CTS workups on a chart are an area that students frequently get wrong. IN any classroom I have certainly outlined my views on Theory based CTS.


Quote:
I said that if the average cross current was negligible compared to boat speed (in Jackdale's English Channel example the average current was only 0.2 knots for a predicted boat speed of 6 knots) then I would personally prefer to crab and follow the rhumb line. If the boat speed was expected to be lower or the current greater, I most certainly would compute a CTS.

Regardless of the perfection of the data, we need a reasonably accurate method for determining this, not one that that may leave us several nm short of the destination as the RYA method does, without any regard for the current on arrival at that point.
No theory can exist without an analysis of the applicability of the underlying data. Your theory works best when you

(a) Assume the tidal quantum are rigid accurate boundaries
(b) You take tides that are comparable to the boats speed
(c) You postulate rapid changes in tides from hour to hour.

FOR your assumptions ,and taking your examples, YOUR method is a better method.

HOWEVER the test is not taking your examples and comparing them to the RYA theory ( which is based on slightly different assumptions of tide data) , It sis taking your method and comparing it to real life.

In practice as Ive said before , the error circle on a multi hour single CTS sailed in real life is so large , that it easily contains both the RYA and SWL methods theoretical error circles.

Hence either has poor accuracy in determining where you will end up in real life, ( if you wish why not do the Bowditch EP error circle approach to see what I mean.

In real life no one computes large multi hour CTS and then just sails off on some theoretically correct but intuitively nonsensical direction . NO one does it. Everyone recomputes a CTS as they go along for a time quantum based on their own experience and concerns. Hence all passages are multi CTS. Hence these arguments are just angels on a pin.

Its been an amusing one. Could I suggest that given you obvious competencies you consider doing the Actual Practical YM exam , its a very useful process. You quite comfortably demonstrate that there certainly no need for you to do the Theory class.

Dave
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Old 07-02-2013, 05:23   #212
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pirate Re: Single CTS or follow the Courseline?

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Originally Posted by Seaworthy Lass View Post
Sorry, no, I do not think this is correct.
SOG is not the same as SMG.
SMG is relative to the destination, SOG is not.
Damn.... I call it AWTY.... it works...
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Old 07-02-2013, 06:03   #213
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Re: Single CTS or follow the Courseline?

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Originally Posted by boatman61 View Post
Interesting stuff for sure... I use the PAG method myself... results vary...
Quote:
Originally Posted by boatman61 View Post
Damn.... I call it AWTY.... it works...
Finally Wot bites:

PAG ???
AWTY ???
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Old 07-02-2013, 06:05   #214
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Re: Single CTS or follow the Courseline?

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Originally Posted by Wotname View Post
Finally Wot bites:

PAG ???
AWTY ???
Im a fan of PFM, Im told is also explains heavier then air flight,

Dave
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Old 07-02-2013, 06:14   #215
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Re: Single CTS or follow the Courseline?

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Originally Posted by boatman61 View Post
Damn.... I call it AWTY.... it works...
Quote:
Originally Posted by Wotname View Post
Finally Wot bites:
PAG ???
AWTY ???
OK, OK, I'll take a nibble as well.
Boaty, put us out of our misery LOL.
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Old 07-02-2013, 06:30   #216
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RYA or SWL method for determining the CTS?

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Originally Posted by goboatingnow View Post
Only if you assume that the current is as you state it, with rigid time quantum's and your general examples that have currents going from 3 knots instantaneously to 1 knot. Real life is very different and hence the RYA in general does not produce the errors you attempt. to demonstrate , why not get a tidal stream atlas and have a look at real data and the real extraction methods.
Yes, real life current does not move in hourly jumps, but hourly averages do move in jumps. And two knots between hourly averages is not impossible. The fact that I am only examining the average current in the last portion of the journey, means that there are practical inaccuracies in my method as well, but it is at least a whole lot better than not closely examining that data at all. The RYA just extrapolates the current experienced for the journey up to the point they get you to D, when there is no reason to think the current you have experienced up until then will continue - it may be rapidly rising or dropping once you arrive at D.

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Originally Posted by goboatingnow View Post
Secondly NO one uses a single CTS for the duration of the voyage in real life, No one but one departs on a single CTS and expects to just drive into the marina berth at the end of a 12 hour journey. Its all theory nonsense.
No, of course the CTS needs recomputing, but when you recompute it why not use a method that gives the best accuracy? Why select a flawed method?

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Originally Posted by goboatingnow View Post
The books often make a poor effort at multi CTS
The RYA Navigation Handbook is one of these. In my opinion, it needs to be modified.

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Originally Posted by goboatingnow View Post
Comments about pencil accuracy are just nonsense. Only a silly instructor makes those remarks.
Well, these remarks are certainly being made currently by RYA instructors, so these instructors are under the impression the RYA method for determining CTS with variable current is "mathematically precise".

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Originally Posted by goboatingnow View Post
FOR your assumptions ,and taking your examples, YOUR method is a better method.
WOW, thank you Dave .

Up to now all I have heard from instructors here on CF (RYA and otherwise, yourself included), is how flawed my method was and how it would not get me the destination and how "confused" I was.

I agree the current RYA method will give a reasonable result in many variable cross current situations, for example for small amounts of current relative to boat speed, particularly if the journey is very close to a whole number of hours and can be easily estimated, or if the average amount of current for the trip is roughly the same whether or not the final amount of current encountered in the journey is included in the average.

This is, however, a very limited set of conditions for a method to work. It bothers me that there is no warning at all in the RYA Navigation Handbook about errors in the computed CTS if these conditions are not met. The value determined for the course to steer can be in error 10 or 20 or even 30 plus degrees in certain circumstances when computing it using the RYA method. I know you say these are for unusually high amounts of tidal stream, but strong currents do exist, for example in some parts of Canada and Australia, and I feel any method used should be able to handle any data thrown at it.

From people I have spoken to who have recently undergone RYA courses, there is also no warning about this during the courses either. Students are taught that the RYA technique is a "mathematically precise" way of getting to the destination, limited only by the data input.

The outcome of any method to determine CTS is only as good as the data we have or can estimate, but it is also only as good as the method we use for computations.

If there is a better method for determining CTS than the one the RYA is currently using, why not adopt it?

Quote:
Originally Posted by goboatingnow View Post
In real life no one computes large multi hour CTS and then just sails off on some theoretically correct but intuitively nonsensical direction . NO one does it. Everyone recomputes a CTS as they go along for a time quantum based on their own experience and concerns. Hence all passages are multi CTS. Hence these arguments are just angels on a pin.
When you initially compute or later recompute though, do you not want an accurate method regardless of the limitations of the data? Why compound errors?

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Originally Posted by goboatingnow View Post
Its been an amusing one.
I have enjoyed the debate too

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Originally Posted by goboatingnow View Post
Could I suggest that given you obvious competencies you consider doing the Actual Practical YM exam , its a very useful process. You quite comfortably demonstrate that there certainly no need for you to do the Theory class.
Dave
Thank you for the compliment (blushing here). Despite the fact that I have had several decades sailing experience and have spent over five of the last five and a half years cruising full time (winter as well as summer), I believe there would be all sorts of useful things I would learn from an RYA course. If I had the opportunity, I would love to do one.
I have just been too busy sailing the last few years to take a break .
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Old 07-02-2013, 07:16   #217
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Re: Single CTS or follow the Courseline?

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Yes, real life current does not move in hourly jumps, but hourly averages do move in jumps. And two knots between hourly averages is not impossible. The fact that I am only examining the average current in the last portion of the journey, means that there are practical inaccuracies in my method as well, but it is at least a whole lot better than not closely examining that data at all. The RYA just extrapolates the current experienced for the journey up to the point they get you to D, when there is no reason to think the current you have experienced up until then will continue - it may be rapidly rising or dropping once you arrive at D
We have no real data comparisons. SO this element of the debate is moot. Tides are given at a single point in time at a geographical position, thats all we know, after that is all assumptions In practice the error circles associated with the real life EPs and hence the final EP (using a single CTS) is massively greater I would contend then any error circle from the RYA or the SWL method. In this respect we are confusing precision with accuracy.

My 4.5 digit multimeter is very precise, unfortunately Ive just discovered it isn't accurate. The SWL method may provide a more precise answer, it may or may not be more accurate. You sail based on accuracy not precision unfortunately.


Quote:
No, of course the CTS needs recomputing, but when you recompute it why not use a method that gives the best accuracy? Why select a flawed method?
In reality you are computing often a lot, hence the methods converge, If you recomute every hour as you go and I do the SWL and the RYA methods in essence give the same results and the CTS will vary. in real life thats what tends to happen.

Hence in real life the RYA method isnt flawed. In theory IT may BE, but only based on YOUR assumptions about tide data. ( and the usefulness of single CTS in real life)

Quote:
I agree the current RYA method will give a reasonable result in many variable cross current situations, for example for small amounts of current relative to boat speed, particularly if the journey is very close to a whole number of hours and can be easily estimated, or if the average amount of current for the trip is roughly the same whether or not the final amount of current encountered in the journey is included in the average.
Yes given that most journeys care little about the +- 15 minutes then the error is very little, equally the error in tide prediction, extraction and real life sailing make +- 15 minutes irrelevant. ( in reality you could be whole tides out, or the ground track takes you into different tides than the ones you predicted),

Quote:
This is, however, a very limited set of conditions for a method to work. It bothers me that there is no warning at all in the RYA Navigation Handbook about errors in the computed CTS if these conditions are not met. The value determined for the course to steer can be in error 10 or 20 or even 30 plus degrees in certain circumstances when computing it using the RYA method. I know you say these are for unusually high amounts of tidal stream, but strong currents do exist, for example in some parts of Canada and Australia, and I feel any method used should be able to handle any data thrown at it
Thee are places with very large tidal ranges , but most tides are 25% of a modern boat or less. Above that most people wait for the turn of the tide rather then getting into CTS. My home port has an 8 knot ebb, I certainly dont CTS my way home.

In real life the RYA has nothing like the errors you list , the method compared to yours may show CTS that are that different, ( but they use your data, and you assume your method has no error) but you can only make thoses statements comparing each method to real life, a process that is very complex.


Quote:
From people I have spoken to who have recently undergone RYA courses, there is also no warning about this during the courses either. Students are taught that the RYA technique is a "mathematically precise" way of getting to the destination, limited only by the data input.
The RYA has Exams, ( Day Skipper, Coastal Skipper, YM ) the only actual course it has is for Day Skipper ( and it doesnt do these scenarios) . CS and YM are practical exams where you can demonstrate and use any method you wish to get from A to B.

AGain I differ with you in the use of mathematical word "precise". The vector addition of the RYA is correct , for its underlying assumptions hence it is a precise method, whether it is an accurate one is another argument and I have advanced many reasons why single multi hour CTS is not a good navigation methodology in real life. The exact same issues of precision and accuracy may be levelled at your method.

Only on the school desk, is this argument valid, in real life a skipper uses in effect combinations of RYA,( or any other method) may proportion by eye the last tide, may eyeball his or her way in etc.

Quote:
The outcome of any method to determine CTS is only as good as the data we have or can estimate, but it is also only as good as the method we use for computations.
Correct hence we cannot make any real statement on accuracy at all. ( for either method), both are based on assumptions of the way the data models the real life , we do not have any data to make actual error comparisons.

Quote:
If there is a better method for determining CTS than the one the RYA is currently using, why not adopt it?
I would contend its not a better way , its a different way based on different assumptions, it may be more precise, it may not be more accurate. the RYA method is based on many years of practical feedback , yours is based on what?

The fact is SWL, your method was developed to attack the RYA one , not to actually better model the underlying data and its applicability to real life. Neither method makes it any easier to get to the destination.

Quote:
From people I have spoken to who have recently undergone RYA courses, there is also no warning about this during the courses either.
To my knowledge only the YM theory course covers multi hour CTS and only in a quite limited way. YM candidates in the real practical onboard exam are required to demonstrate ( by any means ) how they would complete typically journeys, doing so by crabbing, by one hour recomputed CTS, by any method that the examiners thinks is safe, understandable and doesn't require your head in the charts for hours is acceptable, More YM candidates have failed because of time spent at the chart table the any other fault.

Do the YM Exam, courses are for newbies, given your experience, why sit in with people trying to learn how to read a chart. You need 3000 miles mostly as designated skipper and the exam takes about 14 hours, some at nighttime.
Then you get the most respected leisure certificate in the world. The RYA invented the "Yachtmaster" , it unfortunately at the time never thought to trademark it!!!



dave

ps

Quote:
Up to now all I have heard from instructors here on CF (RYA and otherwise, yourself included), is how flawed my method was and how it would not get me the destination and how "confused" I was.
I have never contested your mathematical reasoning for your final method, you worked out some bugs as you went along!.

What I have contended and still do so , is the method confers no more demonstrable accuracy in real life or such accuracy is meaningless in the face of the actual process of sailing to that destination
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Old 07-02-2013, 07:27   #218
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Re: Single CTS or follow the Courseline?

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Originally Posted by Wotname View Post
Finally Wot bites:

PAG ???
AWTY ???
I'm gonna guess PAG = Plot And Go
AWTY = Are We There Yet?
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Old 07-02-2013, 07:41   #219
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Re: Single CTS or follow the Courseline?

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This: http://www.navcen.uscg.gov/pdf/loran/handbook/APP-C.pdf

defines VMG like this:

"Velocity Along Route (VAR). Alternate name for velocity made good."

and

"Velocity Made Good (VMG).

Component of vessels ground speed in the direction of the waypoint in use. In general, VMG is less than or equal to the vessels ground speed. This will equal the ground speed, in the absence of current, whenever the vessel is on course and heading directly toward the waypoint."

I don't know why they say "in the absence of a current" -- I think the writers had a little brain f*art. Current won't make any difference to the relationship between VMG and "vessel's ground speed" (SOG).

What they are saying is that if the vessel is not heading directly towards the waypoint, a component (vector) of it's course over the ground can be realized as making progress towards the waypoint. Think of it like the tidal triangle - there's the COG line, which is the boat's actual path; the VMG which points at the waypoint; and the third line perpendicular to that, connected to the end of the COG line - this comprises the unknown components of set, drift, leeway, steering the wrong way, etc. that put you off course.

The COG line is the hypoteneuse, so it will always be equal or greater than the VMG line.
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Old 07-02-2013, 08:46   #220
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Re: Single CTS or follow the Courseline?

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What they are saying is that if the vessel is not heading directly towards the waypoint, a component (vector) of it's course over the ground can be realized as making progress towards the waypoint. Think of it like the tidal triangle - there's the COG line, which is the boat's actual path; the VMG which points at the waypoint; and the third line perpendicular to that, connected to the end of the COG line - this comprises the unknown components of set, drift, leeway, steering the wrong way, etc. that put you off course.

The COG line is the hypoteneuse, so it will always be equal or greater than the VMG line.
Absolutely agree with everything you said, but it does not contradict at all what I said about current being irrelevant to relationship between SOG and SMG (or speed of advance, or whatever you prefer to call it) . Purely a mechanical function of SOG & COG versus SMG and heading to waypoint.
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Old 07-02-2013, 10:41   #221
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Re: Single CTS or follow the Courseline?

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I am not sure of that either but I had just assumed GPS.

They use Speed, VMG and Distance calculated for 1 hr. and 24 hr. periods so these must be reported by the boats when they poll each boat.

Basically they use Distance instead of SMG (which is after all, distance).
I doubt if they are using the roll call for that information. Last summer we used a Yellowbrick to track us from Maui to Vancouver. It automatically provided our position and speed (which had to be SOG, as it was not connected to the knotmeter).

Vic Maui 2012 Return - Powered by Yellowbrick Tracking

If you slide the bar on the bottom right to the left you can back up one of the boats and then seen what information was provided every 8 hours. VG might be able to use hourly updates.

I was on Turicum.

The Yellowbrick was used to track the race and they asked if we wanted one for the return. It allowed our friends and family to track us, which was a boon since our email did not work properly.

The Yellowbrick can be seen in this photo on the port side of the stern rail.

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Old 07-02-2013, 10:43   #222
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Re: Single CTS or follow the Courseline?

Interesting discussion of VMG as it relates to ETA.

http://www.thesailinggps.com/VMGarticle.pdf
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Old 07-02-2013, 21:56   #223
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Re: Single CTS or follow the Courseline?

Any idea about when we can get a real scenario to test?

I have question about how we will construct a set drift line using 5 minute intervals. How should that be done on a 12 hour crossing of the Channel? That is 144 individual plots on the set drift line.
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Old 07-02-2013, 22:41   #224
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Re: Single CTS or follow the Courseline?

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Any idea about when we can get a real scenario to test?

I have question about how we will construct a set drift line using 5 minute intervals. How should that be done on a 12 hour crossing of the Channel? That is 144 individual plots on the set drift line.
I'll do it later today.

Neptune will do that calculation on the computer.
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Old 07-02-2013, 22:49   #225
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Re: Single CTS or follow the Courseline?

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Any idea about when we can get a real scenario to test?

I have question about how we will construct a set drift line using 5 minute intervals. How should that be done on a 12 hour crossing of the Channel? That is 144 individual plots on the set drift line.
The tidal stream is unlikely to all be conveniently perpendicular, and the time interval is too small to draw accurately, so you would need to mathematically work out the components of the current parallel to and perpendicular to your rhumb line. You could cancel out or add up what you could first, but this is an additional source of error.

The formulas used are those given by HappySeagull in post#556 of Inaccurate RYA Teaching : CTS - Quest For a New Method
I attached a diagram illustrating this in post #594 of the same thread.
(These threads are getting too long LOL, it is harder and harder to find posts I want to refer to).

Using an excel spread sheet with columns to automatically work out these components and the distance made good along the rhumb line would be the simplest way. I will post one today if I have time (market day in the village so I am heading ashore later).

Anyone see an easier way?

Using such small increments of time there is no need for either the RYA or SWL methods to determine the CTS. The CTS calculated mathematically would have you arriving within a couple of minutes if the data was correct .

PS I see Dockhead posted while I was busy typing . The spreadsheet would still be useful for anyone who has access to 5 minute data who does not have Neptune, so I will draw one up anyway.
Dockhead, when I have done this, could you please post the 5 minute data you actually use and I can see how close the spreadsheet calculations are?
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