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Old 23-01-2013, 02:28   #136
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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?
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Old 23-01-2013, 02:32   #137
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by Wotname View Post
Over 130 posts in total and more than 100 posts in the past 24 hours.
......
Fight the good fight Seawortly
Thanks Wotname
I have not been worn down by the resistance.
Give me a good night's sleep and I bounce back refreshed and eager to make my point understood.

I will keep posting examples daily until then. Could make up hundreds, so be prepared for the long haul .
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Old 23-01-2013, 02:43   #138
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Regarding the question of picking scenarios which expose inherent flaws in a method

- bearing in mind that, perhaps unlike Seaworthy, I'm not personally so concerned about accuracy, but more about the fact that the method relying on the rhumb line cannot be used at all, in some scenarios:


I recently wrote something which, had I spelled it out, would have run along these lines:

"I don't really care how well my anchor holds under ideal circumstances, in its favourite sort of bottom.

What I care about is how badly it holds in the worst conditions I might want to use it in -- in comparison with viable alternative anchors, under those same horrible conditions.

-- and, more importantly: what ARE those conditions - in other words - what are its weakest suits?"


Now, I'm going to add these next lines for the portion of the readership who might be wondering what the $%#@& this has to do with the topic at hand:


Anchor manufacturers like telling us how well their anchors can hold, under ideal circumstances.

Nav instructors like telling us how well their methods can work, ditto.


But that's not actually what we need to know, if we're planning on sailing hard, going to bed wet, and living to our allotted span.
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Old 23-01-2013, 02:52   #139
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Interestingly (to me at least) is that the problems of the water moving while trying to get from point A to point B is almost (if not identical to) the same problems that aircraft have when the air moves about while they fly (i.e. wind). Basic pilot navigational training covers these problems in a far less complicated way. I suspect it is not so important for the average sailor to really know this stuff as mostly it doesn't really cause a problem whereas it is a serious problem for the pilot if he/she doesn't arrive at a suitable air field while there is still enough fuel.
I bet it's because unlike tidal currents, winds are not very predictable. So probably pilots do a single vector with solutions for COG and SOG and work out from that whether or not they will arrive or crash, out of fuel

If they are on a multi-hour flight and know more or less that they will have an hour of these winds, then an hour of those winds, it might still be very profitable for them to calculate a single CTS. I would be surprised if this art does not exist in the body of air navigation disciplines.

As to whether or not sailors need it or not -- it depends on where you sail.

Most people couldn't care less -- they don't have any real situations where you need it.

For English Channel sailors, it's a matter of life and death, which is why we all know this stuff, at least the simplified version of it.

The classical Channel crossing is Needles to Cherbourg, 60 miles on the rhumb line. The tides sweep back and forth at up to 6 knots and more (they get stronger on the French side). Cherbourg is on the end of the Cotenin Peninsula of Normandy, an iron-bound coast if there ever was one, rocky and forbidding, with a 6 meter tidal range, and no alternative harbours or shelter at all, if you don't lay Cherbourg.

In a small boat you may not be able to make headway at all against the tide on the French side, if you screw up your CTS calculation and end up downtide at peak flow at springs. In this case you are really stuffed, because to the West there is no shelter until Alderney, and to get there you have to get through the horrific Alderney Race, which runs at up to 12 knots and which is unnavigable in many conditions. On the other side, there is no all-tide port until Fecamp, although you can duck around the corner to St. Vaast IF you can make it through or around the Barfleur Race, which is nothing like the Alderney Race, but can still smash your boat and kill you, if you attempt it in wind against tide in a gale. St. Vaast is NOT an all tide port (it's locked), but you can anchor in the bay to get out of a Westerly gale.

So if you're sailing this kind of passage, you really have to get it right, or else. That partially explains the wide range of knowledge about constant heading steering among sailors.
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Old 23-01-2013, 03:14   #140
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

OK. don't want to derail SWL thread so will keep this brief and then get back to regular programming

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I bet it's because unlike tidal currents, winds are not very predictable. So probably pilots do a single vector with solutions for COG and SOG and work out from that whether or not they will arrive or crash, out of fuel

Forecast winds should be good but aren't always. They predicted hour to hour at various levels; you use the most recent forecast and update as you go.

If they are on a multi-hour flight and know more or less that they will have an hour of these winds, then an hour of those winds, it might still be very profitable for them to calculate a single CTS. I would be surprised if this art does not exist in the body of air navigation disciplines.

Yes, almost always you calculate a single CTS. Not up to date but I guess it is now done with small hand held programmed computer / calculator. I learnt on and used the E-B6 whizz wheel, no doubt it would handle the the problem under discussion. . If I was keen (and I'm not), I would break it out, relearn it and adapt it to SWL examples. Here is a good description of the device E6B - Wikipedia, the free encyclopedia
As to whether or not sailors need it or not -- it depends on where you sail.

Most people couldn't care less -- they don't have any real situations where you need it.

For English Channel sailors, it's a matter of life and death, which is why we all know this stuff, at least the simplified version of it.

The classical Channel crossing is Needles to Cherbourg, 60 miles on the rhumb line. The tides sweep back and forth at up to 6 knots and more (they get stronger on the French side). Cherbourg is on the end of the Cotenin Peninsula of Normandy, an iron-bound coast if there ever was one, rocky and forbidding, with a 6 meter tidal range, and no alternative harbours or shelter at all, if you don't lay Cherbourg.

In a small boat you may not be able to make headway at all against the tide on the French side, if you screw up your CTS calculation and end up downtide at peak flow at springs. In this case you are really stuffed, because to the West there is no shelter until Alderney, and to get there you have to get through the horrific Alderney Race, which runs at up to 12 knots and which is unnavigable in many conditions. On the other side, there is no all-tide port until Fecamp, although you can duck around the corner to St. Vaast IF you can make it through or around the Barfleur Race, which is nothing like the Alderney Race, but can still smash your boat and kill you, if you attempt it in wind against tide in a gale. St. Vaast is NOT an all tide port (it's locked), but you can anchor in the bay to get out of a Westerly gale.

So if you're sailing this kind of passage, you really have to get it right, or else. That partially explains the wide range of knowledge about constant heading steering among sailors.
Yes I accept the importance in your backyard and agree fully. I also accept that is a mark of any good seaman to know this stuff even if they don't use it; however I expect most recreational sailors don't care unless they need to use it in their backyard.

Full marks to you and the other posters for strongly debating the issue.
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Old 23-01-2013, 03:22   #141
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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As to whether or not sailors need it or not -- it depends on where you sail.

Most people couldn't care less -- they don't have any real situations where you need it.

For English Channel sailors, it's a matter of life and death, which is why we all know this stuff, at least the simplified version of it.

The classical Channel crossing is Needles to Cherbourg, 60 miles on the rhumb line. The tides sweep back and forth at up to 6 knots and more (they get stronger on the French side). Cherbourg is on the end of the Cotenin Peninsula of Normandy, an iron-bound coast if there ever was one, rocky and forbidding, with a 6 meter tidal range, and no alternative harbours or shelter at all, if you don't lay Cherbourg.

In a small boat you may not be able to make headway at all against the tide on the French side, if you screw up your CTS calculation and end up downtide at peak flow at springs. In this case you are really stuffed, because to the West there is no shelter until Alderney, and to get there you have to get through the horrific Alderney Race, which runs at up to 12 knots and which is unnavigable in many conditions. On the other side, there is no all-tide port until Fecamp, although you can duck around the corner to St. Vaast IF you can make it through or around the Barfleur Race, which is nothing like the Alderney Race, but can still smash your boat and kill you, if you attempt it in wind against tide in a gale. St. Vaast is NOT an all tide port (it's locked), but you can anchor in the bay to get out of a Westerly gale.

So if you're sailing this kind of passage, you really have to get it right, or else. That partially explains the wide range of knowledge about constant heading steering among sailors.
I would choose to take the crossing any day with you Dockhead, rather than with an senior RYA instructor travelling on a CTS he calculated using the RYA method.

It would be a bit like playing Russian Roulette going with the RYA instructor
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Old 23-01-2013, 03:54   #142
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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I would choose to take the crossing any day with you Dockhead, rather than with an senior RYA instructor travelling on a CTS he calculated using the RYA method.

It would be a bit like playing Russian Roulette going with the RYA instructor .
Well let's do it if you ever get up to this part of the world!
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Old 23-01-2013, 04:17   #143
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

This thread has taken off quickly. A couple of days away and I am almost out of date! Most of my sailing has not been in locations where tidal streams have been important, but as I am venturing to the UK, including Portsmouth, this discussion is very timely.

First with out a doubt, I agree that a CTS is far better than following the rhumb line. I had been studying the RYA method, also agree that it is lacking at the destination. I have tried your method on a few examples - much better.

Bravo on a good explanation as well.
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Old 23-01-2013, 04:24   #144
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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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............
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Old 23-01-2013, 04:24   #145
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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This thread has taken off quickly. A couple of days away and I am almost out of date! Most of my sailing has not been in locations where tidal streams have been important, but as I am venturing to the UK, including Portsmouth, this discussion is very timely.

First with out a doubt, I agree that a CTS is far better than following the rhumb line. I had been studying the RYA method, also agree that it is lacking at the destination. I have tried your method on a few examples - much better.

Bravo on a good explanation as well.
Thanks LJH!
You are the first person to have used the method and reported back.

It is really not that difficult.

If the current at the end is zero or close to zero (as in instructor Jackdale's example that I still need to get around to doing) then it is in fact extremely simple - you just join the end of the sum of the current vectors to B and instantly this is the CTS and the length of that line divided by boat speed is the distance travelled.

Many thanks for reporting back .
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Old 23-01-2013, 04:31   #146
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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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 .
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Old 23-01-2013, 05:02   #147
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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The speed of tidal flows is, at best, known to 2 significant figures. Therefore, any calculation with the speed of the tidal flow as an input can never have more precision than 2 significant figures.
Not exactly true, it depends on the relative magnitudes as well. Here's a stupidly simple example:

Sailing at exactly 9 kts, with a 1 kt (+/- 10%) favorable current, how long to travel one mile? With a 1kt current, 6 minutes. With a 1.1 kt current, 5.941 minutes. A 10% change in current results in approximately a 1% change in the result. You can obviously change the proportions up and down, and get different results.

I have no idea how this plays into the competing calculations.
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Old 23-01-2013, 05:32   #148
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Not exactly true, it depends on the relative magnitudes as well. Here's a stupidly simple example:

Sailing at exactly 9 kts, with a 1 kt (+/- 10%) favorable current, how long to travel one mile? With a 1kt current, 6 minutes. With a 1.1 kt current, 5.941 minutes. A 10% change in current results in approximately a 1% change in the result. You can obviously change the proportions up and down, and get different results.

I have no idea how this plays into the competing calculations.
Exactly right.

"x significant digits in your inputs; no more than x significant digits in your outputs" -- that is a rule of thumb for simple arithmetical relationships -- a helpful warning against False Precision. We used it in 9th grade chemistry, IIRC. It doesn't (necessarily) work for more complicated mathematical relationships. It especially doesn't work here when you are aggregating possibly a large number of lower precision numbers, as we are here -- the Central Limit Theorem kicks in to increase the mathematically probable precision and accuracy of the aggregated (here, averaged) numbers, compared to the precision and accuracy of any given one of them. In simple terms, the Central Limit Theorem says that random errors increasingly tend to cancel each other out with increasing sample size.

Another good example is the fairly common case of some calculation which requires a guess. A guess is an input with practically no accuracy or precision. But the precision of such calculations is not limited to the same number of significant digits as the guess -- a good illustration that the relationship between the precision (which can be expressed, among other ways, as a number of significant digits) of some input and that of the result depends on the function.
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Old 23-01-2013, 05:51   #149
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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the Central Limit Theorem says that random errors increasingly tend to cancel each other out with increasing sample size.
Uncorrelated random errors. They usually are in the cases we are considering.
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Old 23-01-2013, 06:43   #150
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Uncorrelated random errors. They usually are in the cases we are considering.
Yes, you are right. An important point.
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