Inaccurate RYA Teaching : CTS - Quest For a New Method

[Seaworthy Lass]

This fixation with the rhumb line (course line) is truly the source of the problems with the RYA method.

[DeepFrz]

SeaWorthy, what you are doing is exactly the same thing, same method. The only thing is that you don't draw in the rhumb line, but it is inferred by the points. The error was not in the RYA method but in the teaching, as others have said. Again, I agree that if you shift the destination point past your wanted destination, or waypoint, you will not arrive at your waypoint.

[Seaworthy Lass]

Originally Posted by DeepFrz

SeaWorthy, what you are doing is exactly the same thing, same method. The only thing is that you don't draw in the rhumb line, but it is inferred by the points. The error was not in the RYA method but in the teaching, as others have said. Again, I agree that if you shift the destination point past your wanted destination, or waypoint, you will not arrive at your waypoint, using these examples.

I hate to disagree with you DeepFrz, but the RYA focus on the course line and this is why the method is inherently wrong.

I don't focus on the course line AT ALL (why do you want to?). I focus on getting to B.

[DeepFrz]

Focusing on your ground track will keep you away from dangers. If the navigator of the USS Guardian had focused on his ground track and allowed a decent amount of room to go around a world heritage site he would not have hit the reef.

[Seaworthy Lass]

Originally Posted by DeepFrz

Focusing on your ground track will keep you away from dangers. If the navigator of the USS Guardian had focused on his ground track and allowed a decent amount of room to go around a world heritage site he would not have hit the reef.

I fully agree, but your ground track does not have to be a straight line for you to focus on the dangers.
Determine your CTS to give you the most efficient time (it may be necessary for lots of reasons even when leisurely cruising), then see where the plot of your ground track is and check there are no dangers and if all is OK, follow the CTS .

[DeepFrz]

Proof does not come from convincing someone of something, proof is in the proving that your hypothesis holds up to scrutiny. How can it be that by steering a course that puts you on a direct, straight ground track from point A to point B is less efficient than steering a course that takes you out of your way in a curved line. The curved line adds time and distance to your journey. Time and distance means fuel wasted if motoring. But perhaps you can convince me.

[Seaworthy Lass]

Originally Posted by DeepFrz

Proof does not come from convincing someone of something, proof is in the proving that your hypothesis holds up to scrutiny. How can it be that by steering a course that puts you on a direct, straight ground track from point A to point B is less efficient than steering a course that takes you out of your way in a curved line. The curved line adds time and distance to your journey. Time and distance means fuel wasted if motoring. But perhaps you can convince me.

DeepFrz, please go back to the other navigation thread and see for yourself. Or perhaps Dockhead and AndrewTroup can step in at this point and point you to the relevant posts in that thread.

This has already been proven in the other thread.

You will save fuel following a single CTS NOT following a straight ground track.

[Dockhead]

Originally Posted by DeepFrz

Proof does not come from convincing someone of something, proof is in the proving that your hypothesis holds up to scrutiny. How can it be that by steering a course that puts you on a direct, straight ground track from point A to point B is less efficient than steering a course that takes you out of your way in a curved line. The curved line adds time and distance to your journey. Time and distance means fuel wasted if motoring. But perhaps you can convince me.

This is Capt Force all over again.

You don't sail over ground, so your track and distance over ground it not relevant.

What is relevant is the distance you sail through water. That is evident from this proposition: time of passage is a function of speed through water, which is constant, not speed over ground. To solve time of passage you need distance through water. Since the shortest path between two points is a straight line, it means the quickest passage through moving water is a constant heading -- that's going straight through the water.

That is the whole purpose of CTS calculations.

Another formulation of the situation:

If the water is moving, your ground track and water track will be different. If the water is moving at other than a constant rate and direction, either your ground track, or your water track will be crooked. You can sail a straight ground track by putting your pilot on "track" mode and constantly correcting your heading to keep you crabbing along the rhumb line. In this case, however, your heading will change constantly and you will be sailing a crooked path through water.

OR, you can sail a straight path through water on a constant heading. In this case, your ground track will be crooked.

But since your speed is speed through water, the fastest way to get there is with the straight water track and crooked ground track.

Does that make sense?

We had a very long, vigorous argument with Capt Force about this (and we are all greatly indebted to him for forcing us to think through all of the concepts). He posed a problem -- a boat on a four hour passage over water, motoring at a constant 5 knots, subject to a one-sided current varying continuously from 0 to 4 knots and back to 0. What's the fastest way to get there -- "track mode" over the rhumb line, or on a constant heading? We proved to his satisfaction that the fastest way is on a constant heading. Here is the calculation:

captforcescenario_distancesliced1.xls

You will see that not only does the boat sailing a constant heading get there faster, but when you calculate the GPS track boat's passage -- the smaller the vector triangles, the slower it goes. This is the result of the fact that if you roughly follow the rhumb line but change course less often, you can sail something close to a small, ideal CTS passage between your course corrections -- little sinusoidal ground tracks which are, however, straight water tracks, which speed you up comparing to sticking closer to the straight ground track. It's another proof of the constant heading principle.

[David_Old_Jersey]

Originally Posted by Dockhead

You can sail a straight ground track by putting your pilot on "track" mode and constantly correcting your heading to keep you crabbing along the rhumb line. In this case, however, your heading will change constantly and you will be sailing a crooked path through water.

OR, you can sail a straight path through water on a constant heading. In this case, your ground track will be crooked.

But since your speed is speed through water, the fastest way to get there is with the straight water track and crooked ground track.

Does that make sense?

I am trying to keep up with this thread (many of the posts have made me brain hurt - including on simply working out WTF the "problems" that are being discussed actually are - but am perservering).........but the above caught my eye .

For me that is patently obvious and I was surprised it is (apparently?) a point of contention..........But it also made me think through why I think it is patently obvious! The answer to that is simply because it is what I was taught (by me father who was taught by a mate, with an ex RAF compass, a compass, a watch and a pencil!), and proven by use over decades (lots of tides and currents around here - so always a factor)............I mention that because I can't coherently get my head around the maths to "prove" the reason why it works, let alone explain it coherently to others! which I suspect is where many others also are!, including on simply (same as me) accepting a method taught on the basis that it works (or at least seems to), wherever that "teaching" came from.

Of course the Chartplotter "Crabbing Method" (along a direct line) also works, and as long as the GPS is all good would be more accurate - just more inefficient (slower).

As others have said, methinks the "problem" with the RYA Method (which may actually be better than my own WAG Method ©! - I will let others cast judgemnt once I can puzzle out a way to explain myself!) is more in the teaching omitting to make clear that the accuracy of the RYA Method itself is not always consistent (and can sometimes be wildly(?) out)......and if the RYA instructors were never taught about the potential for errors in the first place then RYA Method being seen as Gospel self-perpetuates. Albeit am not knocking the RYA Method as methinks that it looks pretty good and IMO wins on being fairly simple (me likes simple! and that very useful to have when cold, wet and tired)........am still a bit short on the brain cells "getting" the SWL Method though . give me time............

[Seaworthy Lass]

Originally Posted by David_Old_Jersey

I am trying to keep up with this thread (many of the posts have made me brain hurt - including on simply working out WTF the "problems" that are being discussed actually are - but am perservering).........but the above caught my eye .

For me that is patently obvious and I was surprised it is (apparently?) a point of contention..........But it also made me think through why I think it is patently obvious! The answer to that is simply because it is what I was taught (by me father who was taught by a mate, with an ex RAF compass, a compass, a watch and a pencil!), and proven by use over decades (lots of tides and currents around here - so always a factor)............I mention that because I can't coherently get my head around the maths to "prove" the reason why it works, let alone explain it coherently to others! which I suspect is where many others also are!, including on simply (same as me) accepting a method taught on the basis that it works (or at least seems to), wherever that "teaching" came from.

Of course the Chartplotter "Crabbing Method" (along a direct line) also works, and as long as the GPS is all good would be more accurate - just more inefficient (slower).

As others have said, methinks the "problem" with the RYA Method (which may actually be better than my own WAG Method ©! - I will let others cast judgemnt once I can puzzle out a way to explain myself!) is more in the teaching omitting to make clear that the accuracy of the RYA Method itself is not always consistent (and can sometimes be wildly(?) out)......and if the RYA instructors were never taught about the potential for errors in the first place then RYA Method being seen as Gospel self-perpetuates. Albeit am not knocking the RYA Method as methinks that it looks pretty good and IMO wins on being fairly simple (me likes simple! and that very useful to have when cold, wet and tired)........am still a bit short on the brain cells "getting" the SWL Method though . give me time............

Yes, it does seem to preached that it is a good approximation (or by some instructors even that it is a precise way of getting you to B). It can have significant errors. I will make my next example one were it is about 30 degrees out .

Will have to draw step by step diagrams at some stage to try and make my method more comprehensible.

If anyone is capable of doing the RYA method, they would be capable of doing mine I think. I probably just haven't explained it well .

[LJH]

Originally Posted by Seaworthy Lass

Yes, it does seem to preached that it is a good approximation (or by some instructors even that it is a precise way of getting you to B). It can have significant errors. I will make my next example one were it is about 30 degrees out .

Will have to draw step by step diagrams at some stage to try and make my method more comprehensible.

If anyone is capable of doing the RYA method, they would be capable of doing mine I think. I probably just haven't explained it well .

The RYA method will get you nicely to D, which may be a significant distance from B. Your method would probably be easier to illustrate if you end with a significant crossing current so that the vector through S is well separated from the vector through L.

[DeepFrz]

Maybe I should have added, in a classroom situation. Often, as in the Vendee Globe, the participants have to because of wind, waves or some other situation deviate far from the Great Circle route to find a more efficient means of getting to a specific point. However if circumstances allowed the great circle route would have been much more efficient. In a shorter track like we are talking about a straight line (rhumb line) will be the most efficient way to get from point A to point B.

Anyway, it is 02:00 here and I have to get up early. Ciao...

[cal40john]

Originally Posted by DeepFrz

Maybe I should have added, in a classroom situation. Often, as in the Vendee Globe, the participants have to because of wind, waves or some other situation deviate far from the Great Circle route to find a more efficient means of getting to a specific point. However if circumstances allowed the great circle route would have been much more efficient. In a shorter track like we are talking about a straight line (rhumb line) will be the most efficient way to get from point A to point B.

Anyway, it is 02:00 here and I have to get up early. Ciao...

One of the unrealistic examples from the other thread to prove the point was you have a 4 knot current perpendicular to your rhumb line for 2 hours, then it becomes 3 knots for an hour, then 1 for an hour. You can do 4 knots boatspeed. You spend 2 hours motoring straight into the current to maintain yourself on the rhumbline making zero progress towards your destination. The other boat has done a multi vector CTS calculation to steer the shortest distance through the WATER, not the shortest over the bottom, he is steering at some angle but he is making progress to the destination.

The English channel examples were good also. Say it takes 12 hours to cross the channel, 6 hours of tide nearly perpendicular one way, then 6 hours the other. If you crab into the current you travel a greater distance through the water to stay on the rhumb line and make less forward progress. If you chose the original compass course and held it the tide would carry you off rhumb one side then the other, but you wind up where you wanted to go while traveling at best speed.

Hope that was easier than finding 2 posts out of 800.

John

[Andrew Troup]

Dockhead:

My comments are threaded through your recent post to me:

Very well explained -- yes, I get it all and agree with it.

Thank you, and ! Whew ! , in that order.


The only thing you left out is the "inflation" (or "deflation") of the vector triangle which gets you conceptually to your destination according to the RYA method.

I do plan to address that, but I wanted to make sure we were on the same page on the how and why of getting to that point.

It will work perfectly if the average tide during the uncalculated partial hour equals average tide of the calculated part of the passage.

That's what I acknowledged in the post you replied to: under that singular circumstance, I can well imagine it is correct.

I think you sell the RYA method just a teeny bit short by leaving this out, although it's basically a rhetorical point -- you do describe the practical effects correctly, I think.

See above

So you can describe it either as a shifted destination, or a fudge, with equal accuracy, I guess.


I agree: to me, conceptually, it's a shifted destination; functionally, it also works out to be a useful fudge, in the typical case.

I think it's probably more correct to think of it as a fudge, but probably there is no real difference in the consequences of this frame of reference.

I think maybe if you think of it as a fudge, you can more easily avoid mistakes like Andrew's.

Huh? and which Andrew moight that be, to be sure, so it is? <wink>
And when you've cleared that up, which mistake?

One other small point: I really don't think that it's any "obsession with the rhumb line" which causes the flaws in the RYA method. I bet you'll agree with me if you think about it.

<grin> how much do ya wanna bet?

If you're not going to finish the calculations, as the RYA would have us do,

And I've underlined your word 'not' - I'm hoping you'll tell me it was a frayed not. I mean, a superfluous not. I mean, not a not.

... you have to have the course line to calculate some vector triangle.

OK, here you need to throw me a rope. "Course line" ? Please clarify what this means, and why I have to have it.

Using the course line to calculate some vector triangle is the best way to do it if we consider it impractical or useless to calculate the last partial hour.

Extending your answer to my previous question should clear this up too (pretty please?)

And that's the next battle we will have to fight -- it's Dave's defense (now, I think) that it's useless to calculate the last partial hour because we can't get close enough anyway.

I'll leave Dave to you guys, he's above my pay scale.

I think you have brilliantly disproven this showing a nearly 10% error of the RYA method in a real case.

Are you getting me mixed up again, this time with the Worthy SeaLass?

- - - - -

Now I need to remind you that, once you've answered my questions and we've tidied up the matters arising, I will still want you to address my points a) and b) from an earlier post, insofar as they still seem relevant.

OK?

[Dockhead]

Originally Posted by Andrew Troup

Dockhead:

My comments are threaded through your recent post to me:

Very well explained -- yes, I get it all and agree with it.

Thank you, and ! Whew ! , in that order.


The only thing you left out is the "inflation" (or "deflation") of the vector triangle which gets you conceptually to your destination according to the RYA method.

I do plan to address that, but I wanted to make sure we were on the same page on the how and why of getting to that point.

It will work perfectly if the average tide during the uncalculated partial hour equals average tide of the calculated part of the passage.

That's what I acknowledged in the post you replied to: under that singular circumstance, I can well imagine it is correct.

I think you sell the RYA method just a teeny bit short by leaving this out, although it's basically a rhetorical point -- you do describe the practical effects correctly, I think.

See above

So you can describe it either as a shifted destination, or a fudge, with equal accuracy, I guess.


I agree: to me, conceptually, it's a shifted destination; functionally, it also works out to be a useful fudge, in the typical case.

I think it's probably more correct to think of it as a fudge, but probably there is no real difference in the consequences of this frame of reference.

I think maybe if you think of it as a fudge, you can more easily avoid mistakes like Andrew's.

Huh? and which Andrew moight that be, to be sure, so it is? <wink>
And when you've cleared that up, which mistake?

One other small point: I really don't think that it's any "obsession with the rhumb line" which causes the flaws in the RYA method. I bet you'll agree with me if you think about it.

<grin> how much do ya wanna bet?

If you're not going to finish the calculations, as the RYA would have us do,

And I've underlined your word 'not' - I'm hoping you'll tell me it was a frayed not. I mean, a superfluous not. I mean, not a not.

... you have to have the course line to calculate some vector triangle.

OK, here you need to throw me a rope. "Course line" ? Please clarify what this means, and why I have to have it.

Using the course line to calculate some vector triangle is the best way to do it if we consider it impractical or useless to calculate the last partial hour.

Extending your answer to my previous question should clear this up too (pretty please?)

And that's the next battle we will have to fight -- it's Dave's defense (now, I think) that it's useless to calculate the last partial hour because we can't get close enough anyway.

I'll leave Dave to you guys, he's above my pay scale.

I think you have brilliantly disproven this showing a nearly 10% error of the RYA method in a real case.

Are you getting me mixed up again, this time with the Worthy SeaLass?

- - - - -

Now I need to remind you that, once you've answered my questions and we've tidied up the matters arising, I will still want you to address my points a) and b) from an earlier post, insofar as they still seem relevant.

OK?

Sorry, I was unwittingly aggregating answers to you and answers to a SWL post -- a bit of a mess. I edited my previous post a bit.

So your points a) and b), if I can correctly identify them:

I guess that would be these?

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

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

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

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

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


OK, here's my comment -- I think you don't quite get the method (which as I said I am sympathetic to -- I also couldn't get it for a d*mned long time of breaking my head over it).

The "rhumb line" (let's better call it the "course line", as the RYA do, because it has a different function) cannot be "swung", or you lose the vector triangle. It is an essential element of the calculation, even in SWL's case (she just doesn't need to draw it).

The course line is one leg of your vector triangle -- don't forget. With a better method like SWL's, you don't much use it for anything, and don't need to draw it on the chart, because the lines converge on the destination. But the RYA needs this line, and in fact, you are instructed to extend it out beyond the destination, even onto land. That is so the vector triangle can be formed out of whole hour calculations where the water track line will not intersect with the destination.

So the destination is not "shifted to some arbitrary place on the rhumb [course] line". You know the length of your water track line -- it's a function of time as a whole number of hours and your assumed average boat speed. You swing THAT until the end of it touches the course line. That spot of intersection is not arbitrary -- that's your whole hours vector triangle. Got it now? The essential output of the RYA method is the angle of the water track line after you've swung it from the end of all your tidal vectors to the point where it touches the course line. That's your CTS (subject to further adjustment for leeway and variation, of course).


So with the RYA method you end up with a perfectly (mathematically) constructed vector triangle, only it is not for your exact passage, since it has only counted whole hours (and it might count through the whole hour BEYOND your destination, if that is closer). It does not interpolate the uncalculated last partial hour, it merely "inflates" or "deflates" the whole hour vector triangle until it corresponds to your destination. So the uncalculated last partial hour is basically whitewashed over -- as if it corresponds to the average of the calculated parts of the passage. Since you will round up or down to the nearest whole hour, the maximum size of the uncalculated period is 30 minutes. If the uncalculated period is only 15 minutes or less, then the result will be pretty good in any case.

Do you get it now? It's actually very clever. It is designed to be "close enough for government work"; however Seaworthy has forcefully proven that it is not, actually, close enough.


Perhaps another way of looking at it is this: Maybe users of the RYA method, instead of "inflating" the vector triangle, if it is smaller than the real one, just sail to that point on the rhumb line and then "track mode" steer from there. It will be less than an hour of sailing and with less than an hour to go, no one is steering the constant heading anyway -- you're already homing in on your destination by other methods. If it is used in real practice like this, then I guess it's somewhat usable.

However, I still don't like to be at the mercy of random outcome of the uncalculated partial hour, the size of which can be anything from 1 minute to 30, and variation from the average tide can be anything. I prefer to have an exact CTS which I calculate not to the destination, but a mile or so uptide, to center the possible outcomes there with a mile or so on either side as perfectly acceptable outcomes, rather than right at the destination where a few cables downtide is already a problem, even while a couple of miles uptide is fine.

The destination is the edge of the circle of acceptable outcomes, so it seems to me to be a fundamentally wrong aiming point. You need to rather be aiming at the center of that circle of acceptable outcomes, and that will necessarily be someplace uptide of the destination. So looking at it again from the point of view of practical, usable, practice, I am still convinced that an exact CTS calculation is very useful, and I don't like at all the wide range of random outcomes from the RYA method.