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Old 14-02-2013, 21:09   #706
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

...ok....we have CTS's!! :jackdale at 356 goin for hole-in-one! using the good ol' Classic method; my takes on RYA at 355 (fallin a wee bit astray t'wardsthe trap) and last, a slash at the SWL method givin the outside 002 - a big risk hopin' for the tidetobring that baby HOME!....(both "birdies"? Idon't golfand have nocluewhat I'm saying but the analogy is hard toshake....)
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Old 15-02-2013, 01:30   #707
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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ok. I mada a scenario from the first scanned page Dockhead had up. as above in this thread too, where I posted it.

Start->Destination (A->B)=11 miles
Boat speed =4 knots

current
0845 263 0.8kn
0945 266 1.3kn
1045 254 2.0kn

Departure Time is 0815

this is important. ...a half hour of 0.8 current=0.4 knots and boat speed vector is therefore 2 knots
Happy, I have been puzzling trying to make sense of this. If the current at 8:45 is 0.8 knots the assumption I would make is that this is roughly the average current for the hour 8:15-9:15.

So if you leave at 8:15 am the boat displacement due to current for the next hour would be roughly 0.8 nm.

Why are you applying 0.8 knots of current only for half an hour (from 8:15-8:45)?
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Old 15-02-2013, 08:07   #708
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

good for you!SWL, noticing something is wrong!

yes that is what I thought too, until I looked at the example offered in the book.
well, I'll have to redraw this
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Old 15-02-2013, 08:42   #709
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

there.this should be better. CTS=356
The AB line 337 at 11 miles is an arbitrary choice on my part which doesn't help, but the book says yellow buoy just west of Newhaven -a chart I don't have.
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so, the current vectors are HINGING on the unseen centers 0845,0945,1045
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Old 15-02-2013, 08:54   #710
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

I like that! vectors Hinging on the tide data (as center of vector) .If that's what this page means, I wish they'd said so.

oh, and beyond that, yes indeed the first pictures are pretty muddled ...at the time I had hopes that if I posted a gpx it could be easily corrected and passed back and forth...I will put this one up if anyone wants it
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Old 15-02-2013, 09:33   #711
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

so this is new and improved
Jackdales and above CTS=356 plot on opencpn.

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hey, I can upload gpx. That is news to me.
plot 356.gpx
RYAexamplefixed.gpx
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Old 15-02-2013, 09:48   #712
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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there.this should be better. CTS=356
The AB line 337 at 11 miles is an arbitrary choice on my part which doesn't help, but the book says yellow buoy just west of Newhaven -a chart I don't have.

so, the current vectors are HINGING on the unseen centers 0845,0945,1045
Yes, that's better. That's why there is no missing data for current. The journey is less than three hours and they have given us three full hours of current data.

I was wondering how you knew the distance and bearing LOL, thought it may have been on another page.

What they are describing is the RYA method. In this case D falls beyond B. I get a CTS of 357 degrees true based on this technique.

If I use the SWL method, the amount of the last hour of current used is 0.7 hours, so the displacement is 1.4 nm for this last bit. Joining this point to B gives a CTS of 355 degrees true. Time taken is 2.7 hours.

The RYA and SWL methods do occasionally give a similar result .
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Old 15-02-2013, 09:56   #713
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by jackdale View Post
My drawing



I did not change symbols.

I used the convention for establishing a EP by using a DR and accounting current.

Dave - I would appreciate any comments.

Jack
Jackdale, your diagram is incorrect.
You have plotted the full three hours of boat displacement due to current (0.8 + 1.3 + 2.0 nm) to compute the CTS, but with your ground track you have made the first displacement due to boat speed and current only half an hour.

If you were using Happy's initially incorrect estimate of half an hour of the first lot of current being applicable, then the first boat displacement due to current vector should only have been half an hours worth also (ie 0.4, not 0.8 nm).
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Old 15-02-2013, 10:19   #714
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Jackdale, your diagram is incorrect.
You have plotted the full three hours of boat displacement due to current (0.8 + 1.3 + 2.0 nm) to compute the CTS, but with your ground track you have made the first displacement due to boat speed and current only half an hour.

If you were using Happy's initially incorrect estimate of half an hour of the first lot of current being applicable, then the first boat displacement due to current vector should only have been half an hours worth also (ie 0.4, not 0.8 nm).
I redid the diagram and a CTS 354.
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Old 15-02-2013, 10:20   #715
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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The RYA and SWL methods do occasionally give a similar result .
The SWL method uses far more steps. Occam's razor applies.
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Old 15-02-2013, 10:46   #716
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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I redid the diagram and a CTS 354.
See post #707.
Happy has now modified the example, as a full hour of the first lot of current data actually applies. So your CTS was OK initially, just the ground track needed replotting .
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Old 15-02-2013, 10:47   #717
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

I have to say I dont know whats been attempted here, other then CTS computations, You cant compare the methods using the same input data as was used to determine the CTS in the first place.

This is because the assumptions about how the underlying tide data applies in real life is different in each method.


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Old 15-02-2013, 11:15   #718
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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The SWL method uses far more steps. Occam's razor applies.
Jackdale, a simpler technique is fine as long as its limitations are recognised. Students need to be warned that if D does not lie very close to B, the computed CTS may be significantly in error using the RYA method. As I understand it, this is is not currently taught (one instructor has even commented that checking to see if D is within half an hour of B is not taught by many instructors, and examples with this are not given during the courses or exams).

What I have objected to from the start is this RYA instructor's constant insistance of how "mathematically precise" the RYA method is and how following it will not have you arriving at D (a point on the rhumbline), only at B (the destination). The first time I looked at the method it was instantly apparent this is not the case.

Digging deeper and speaking to people who have undergone RYA courses (and been examined on CTS computations) the RYA method seens to be touted as being very accurate if the data input is correct. This is simply wrong.

Wth a little extra work, accuracy can be improved. The result is still limited by assumptions regarding the tidal stream data, but in some cases a significantly better result is obtained. This is what the SWL method offers .
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Old 15-02-2013, 11:30   #719
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

SORRY! my fault,Jackdale. See?(above) I did another plot that is YOU. Your own ep method was just fine in the event.


NOW I am doing an swl...
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I got a CTS 356 so there's no point in doing another plot.

SWL discover CTS.gpx
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Old 15-02-2013, 11:30   #720
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Jackdale, a simpler technique is fine as long as its limitations are recognised. Students need to be warned that if D does not lie very close to B, the computed CTS may be significantly in error using the RYA method. As I understand it, this is is not currently taught (one instructor has even commented that checking to see if D is within half an hour of B is not taught by many instructors, and examples with this are not given during the courses or exams).

What I have objected to from the start is this RYA instructor's constant insistance of how "mathematically precise" the RYA method is and how following it will not have you arriving at D (a point on the rhumbline), only at B (the destination). The first time I looked at the method it was instantly apparent this is not the case.

Digging deeper and speaking to people who have undergone RYA courses (and been examined on CTS computations) the RYA method seens to be touted as being very accurate if the data input is correct. This is simply wrong.

Wth a little extra work, accuracy can be improved. The result is still limited by assumptions regarding the tidal stream data, but in some cases a significantly better result is obtained. This is what the SWL method offers .

Again the RYA is mathematically precise, its vector addition is correct. The RYA method does get you to your destination as it simply says that , based on its assumptions of how the underlying tide data apply you continue on , on your original CTS either for a + or - % of 30 minutes , hence bringing you to your destination .

This "accuracy" or " inaccuracy" is based on the assumptions that the RYA method makes about the underlying tide , in excess of 1 hour. Its also worth noting that this is also the method used in Bowditch and others , even if they only postulate one hour. Is they simply say continue on the one hour CTS, and you arrive ( in say 10 minutes later, etc).

The RYA simply is based on the fact that precise computations of CTS, simply service no useful purpose and that the tide in the first 10 minutes is not substantially different to the last 10 minutes of the previous tide , Hence the WL method of applying rigid time quantum's has no actual applicability in real tides.

As I constantly referenced in you posts, I would appreciate if you actually asked me what goes on in a class room.

Firstly multi hour CTS is only really covered for a very short period, often a few hours on the final night of chart work. Secondly any instructor worth his or hers salt points out the limitations of multi-hour CTS. ( irrespective of the method deployed in calculating them)

Secondly the actual YM practical exam, sometime requires you to demonstrate that you can compute such CTS, it doesnt require you to sail a single CTS to get to the destination. The Examiner is quite happy to allow you to do one hour plots, or any other method that is justified and safe. The theory course is not a requirement to do a RYA YM exam.

To summarise

* In real life Multi hour single CTS has little to offer in a complex tide situation, it may serve as a "pointer" ,

* In practice nearly everyone uses a hybrid between single CTS, recomputing CTS or sailing either hourly CTS, or just crabbing along the rhumb line.

* Accuracy of any method can only be demonstrated ( and hence claimed) by demonstration with real life , ( or near real life) experiments. Wide claims, not backed by any "physical " evidence of RYA inaccuracies do not in fact invalidate the RYA method. ( Nor has the RYA method been subject to such real life tests, other then the feed back over the years that the RYA receives).

* As has been demonstrated when you take some real tide data, the difference in the methods is very small often a degree or two , in real life this is way way smaller then the errors experienced from other sources as to render any further "attempt" at accuracy , by increasing precision , to be worthless

Dave
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