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Old 28-01-2013, 22:44   #541
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by Lodesman View Post
Now all I have to do is stay at dock or anchor for the first 1 hour 39 mins, get underway and steer 085 for the first 21 mins, and 078 for the rest to arrive in a total time of 2h21m. Yes I have to alter course, but it's supposed to be about efficiency and it's a small price to pay to knock 50 minutes of motoring off the trip. (see third image)

Can you do this with the SWL method?
Lodesman, I need to thank you profusely for looking at the whole problem of finding a better method than the one the RYA uses to determine CTS


You have made me realise some of the hidden depths in my method and I can't thank you enough for this (hidden depths in methods sometimes have more impact when revealed slowly, like hidden depths in lassies LOL).

I think my method applied another way may tell you the best time within a few minutes to leave on a journey (I haven't tested this out yet, so hope it works, but I am very excited about it). If I had enough time to sit and do all the calculations and I had all the tide data I could do this for years ahead!

First thing I realised when my eyes popped open this morning. Thank you, thank you, thank you! xxxxxx

I need to put a pot of soup on and before I head ashore for more work on the sails, but I will try and show how this is done for the same example you used (my data for example 3) once the soup is simmering.

Edited to add: leaving time would certainly be accurate to less than half an hour - it would be better if 5 minute tide data was used instead of hourly data
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Old 29-01-2013, 01:10   #542
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Soup and sails waits for no man (or lassie) so I will have to play with the above idea when I get back on board tonight.
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Old 29-01-2013, 03:05   #543
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Note that if D is not close to B then the instructions are to "redraw the plot using one more or less tide" although apparently "some instructors omit this bit".
I shudder to think how inaccurate those computations become if you don't find D at it's closest to B. The method is bad enough in some cases even when the closest possible D is selected!

Results are interpolated by "inflating or deflating the total tidal triangle" rather than by examining the actual current that will occur in the last portion of the journey.

I think in general there is little disagreement now as to how the RYA method is deployed and yes its up to the instructors to outline the limitations or to point out the underlying assumptions. The RYA course was designed as a classroom course. Hence the books are not complete in that regards. ( thats by design , the instructors have to make a few quid somehow!)

What I do NOT agree with is the characterisation of the degree of errors, The RYA case never inflates or deflates more the 30 minutes. Where the model falls outside of that you have to revert to first principles. That can be shown easily and you can demonstrate using the method where D doesnt lie on the rhumb line. ( The boat is somewhere on this arc approach).

Again the precision claimed in one method is based on an assumption of abrupt tide time quantum's, The RYA method assumes averages are accurate enough in real life.

The classroom examples, are all based on actual journeys across and actual chart using actual real data, The test RYA chart is a special Admiralty chart based around the south coast of the UK to the french coast. ( Theres also one based around Dublin). SO rather then pulling up meaningless tide vectors to demonstrate particular holes, these just use what people find in real life and the RYA method works fine in that regard. Postulating methods that take 2kn tides and then reverse them in direction to 0.5kn in an instant is in no way representative of real life

Ultimately the proof is comparison of actual journeys with real data. I have discussed this with Dockhead.

Until then it remains like many methods of CTS calculation a model. I don't dispute the maths, I merely dont accept that the RYA method generates any real issue in real life in the typical scenarios where it is applied.
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Old 29-01-2013, 07:48   #544
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by Seaworthy Lass View Post
Where do you get an "initial CTS - 009"
You need to determine a CTS before you can start drawing any ground tracks. You cant draw a ground track and then estimate CTS.
You are missing some steps between step 2 and step 3
Sorry I thought that was clear. The 'initial CTS' is determined before ground tracks are plotted. It's the line marked by the circled 3 - joining the end of the 3-hours' tidal vectors to B. I then measured the angle of the line to get the course 009. Against an adverse tidal set the optimum course (the one that gets you closest to destination) is the line from the end of the tidal vector toward the destination. This is essentially what I did, although I took the composite course with the third hour of tidal set, to give a more refined initial course. With this, I plotted the expected CMG against the adverse tidal sets. The end of these vectors (point C) is essentially the start point in the solvable tidal triangle and CB is the 'rhumbline'. Hope that answers your question.
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Old 29-01-2013, 10:16   #545
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

again about the model: Perhaps lodesman, goboatingnow, SWL, Jackdale(again), Dockhead...someone propose a reasonable model now, that can highlight the best method. "Reasonable" means quoted from a tide book, or using one of the three simple methods above, that all can follow and use to extrapolate values in more or less detail as it suits em for their proof.

..at least 1/2 hour increments are wanted in tabulated current and therefore are needed in the resultant track.More granularity is welcome, where it is important.
IF it helps illustrate the point in your method, the current max in the middle MAY be more than boat speed, but not at the beginning. Current needn't be perpendicular to A->B line, but humbly suggest a "clocking" current should be avoided unless it is an illustration of the method. Again, it need be quoted from a book or some proveable, shareable source.

Please do not plot on screen-grab charts. We do not all have the same charts or the ability to view them at the scale you can, where we want to dissect your method.

added: sorry, forgot this.
If you don't have a scanner for your sketches, you can describe your plot. This is harder, but perhaps scannerfolk will draw-and-post it for you,
Or draw it in an adequate computer program and post in a common format.jpeg,png,tiff etc.
Please no spreadsheets. I ,for one, can't do many proprietary formats.
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Old 29-01-2013, 12:50   #546
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Hi all
I am exhausted to the point of dropping here.
I want so much to run with my idea to see if it works, but i'm just not capable at the moment.

I now have a copy of the RYA Navigation Handbook by Tim Barlett 2003. Looking online to see if this is the latest version, interestingly I found heaps of free downloads as ebooks. 2008 seems to be the latest version.

Dockhead, you may want to upgrade your travelling companion, or do you carry around the old one for sentimental reasons?. You do not strike me as the sentimental type LOL.

I am just too tired to read and do it justice and I haven't downloaded the new version yet, but at a first glance the rhumb line (that EXTREMELY confusingly is termed the "intended track", which it says the vessel is unlikely to follow) is definitely used in the computation of CTS. It gives no other method.

Quote:
Originally Posted by LJH View Post
...it does not say anything more than the steps required to do the chart work. Nothing about adjusting for a partial hour! They seem to leave that for the instructor to talk about. If there is an instructor handbook, I hope it has some more guidance.
LJH, the 2003 version of the RYA Navigation handbook gives no way of computing SMG or time taken and not just because this is left to the individual instructor to do so - it specifically says it doesn't matter what scale is used for the vectors, it doesn't have to be the scale on the chart.

Signing out as I need to sleep. Off again ashore tomorrow. The sacrificial strip work went brilliantly today (the cruiser I am working with is just fabulous, if anyone in the Aegean needs work done on their sails I would highly recommend him, send me a PM if you would like to get in touch with him). Work needs to continue as access to the taverna is limited by the fact they need to shortly start preparing the premises for reopening in March.
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Old 29-01-2013, 14:09   #547
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Happy et al

These are real numbers.

Course 180 T 16.0 Miles
Depart at 1000
Boat speed (knotmeter) – 4 knots
Current
1000 1.1 knots 235T
1100 1.3 knots 235
1200 1.3 knots 235
1300 0.20 knots 295
1400 0.20 knots 080

For those interested. It is from the Botchie Ledge light off Victoria, BC to the Ediz Hook light off Port Angeles, WA across the eastern portion of The Straits of Juan de Fuca.

I used two current stations, first three hours are from 5.2 miles SSW of Trial Island, the next two hours are from 1.2 miles north of the Ediz Hook Light. The data is from OpenCPN. I used data from today.

I used a NOAA chart 18400 which is available for free as a download at Pacific Coast NOAA Nautical Charts
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Old 29-01-2013, 16:57   #548
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by Seaworthy Lass View Post
LJH, the 2003 version of the RYA Navigation handbook gives no way of computing SMG or time taken and not just because this is left to the individual instructor to do so - it specifically says it doesn't matter what scale is used for the vectors, it doesn't have to be the scale on the chart.

.
The "plotting" can be done on graph paper. The scale of the graph paper does not need to match the scale of the chart. The graph paper that I use looks like this http://static4.depositphotos.com/100...raph-paper.jpg
It has a 10x10 grid.

SMG is calculated the same way that it is for a single current, i.e. the length of the intended track from the point of departure to the the point at which the boat speed arc interests the intended track. On all sides of the triangle the speed is the length of the line.
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Old 30-01-2013, 00:39   #549
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by Seaworthy Lass View Post
......the 2003 version of the RYA Navigation handbook gives no way of computing SMG or time taken and not just because this is left to the individual instructor to do so - it specifically says it doesn't matter what scale is used for the vectors, it doesn't have to be the scale on the chart.
By the way, the 2003 version was reprinted in 2011.

Quote:
Originally Posted by jackdale View Post
The "plotting" can be done on graph paper. The scale of the graph paper does not need to match the scale of the chart. The graph paper that I use looks like this http://static4.depositphotos.com/100...raph-paper.jpg
It has a 10x10 grid.

SMG is calculated the same way that it is for a single current, i.e. the length of the intended track from the point of departure to the the point at which the boat speed arc interests the intended track. On all sides of the triangle the speed is the length of the line.
Stating "Using the chart itself as a worksheet" RYA navigation handbook actually describes this as the procedure:

"it does not matter what scale you use: most navigators prefer to stick to the scale of the chart. There is no reason, however, why you shouldn't use centimetres, quarter inches, or any other unit you like, so long as you are absolutely consistent throughout the calculation"

There is absolutely no way of checking then if you have selected the right number of tidal hours! This method is even worse than I thought when I first looked at an RYA video going through computations step by step!

Folks, the method described in the handbook (which is a general navigation text book, not just one to be used in conjunction with RYA courses) is seriously flawed
.
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Old 30-01-2013, 01:23   #550
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Hey SWL, jus wondering exactly what the RYA did to you to make you so piss3d with 'em.
:





(Not a serious question in case anyone is still wondering - just a way to boost my post count and thread count )
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Old 30-01-2013, 02:17   #551
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Hey SWL, jus wondering exactly what the RYA did to you to make you so piss3d with 'em.
:





(Not a serious question in case anyone is still wondering - just a way to boost my post count and thread count )
Indeed.

My dear, it is often a mistake in life to assume any given person (or institution ) is an idiot (it can be a mistake to assume anyone is not an idiot, too, but that's a different conversation ), especially when you're making a parallel assumption of being very clever yourself. The results can be embarrassing (I speak from experience!).

You shouldn't jump to conclusions like this. What they are saying is that you can plot on graph paper, in a bigger scale for more accuracy, as long as you keep all the proportions are the same. They are right, and this is what you should do if your vector triangle is going to be small at the actual chart scale. This is a separate step -- the final one of measuring the angle which will give you your course to steer.

Obviously you have to lay the vector triangle to scale on the chart to see whether you get there or not; and applying Dave's Missing Manual you would do the next full hour, too, and eyeball the difference and proportions to get a usable fudge. It's a perfectly usable system probably designed before good autopilots existed, giving results which are surely well within any possible accuracy of hand steering. With a decent eyeball doing the fudging, the results will be equal to the results of your method, and better in case the person using your method is not fudging where your method is nearing the limits of its capabilities.

The really excellent thing about your method, as I've said several times, is that it gives a much better result much less reliant on intelligent fudging with hardly any extra effort. That's why I'm going to see if we can't get it published for the broader good. But you are going too far if you assume that Tim Bartlett -- former Royal Navy officer and navigator, author of a whole bunch of respected books on navigation, including celestial navigation -- some of which I sleep with or at least carry with me everywhere I go -- is an idiot who would propagate idiotic methods with idiotic mistakes in them. In fact, he probably forgot more about navigation yesterday than you or I will ever know. The method he describes is a traditional one, and would not have survived generations of navigators if it were idiotic. It is designed for a quick and easy calculation of a CTS probably in the context of hand steering where 3 or 4 degrees is not going to make much difference. And in the context of very intelligent navigators. Nowadays we have the opposite context -- very good autopilots which can steer to a degree or so in reasonable conditions, and rather poor navigators spoiled by electronics and used to just "driving the dot" on a screen. So I think your method is really better suited for today's reality and should really replace the traditional one.

Actually, before sending your method to YM I think I'll try to discuss it with Tim Bartlett himself. I know someone who knows him well and can probably arrange to meet him if he's around. I'm flying to the UK this afternoon. Maybe I can get him to comment about it on here.
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Old 30-01-2013, 02:22   #552
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

P.S. -- And Lodesman, those guys still owe us some answers about an old argument we had about the Colregs -- I'll follow up on that too when I'm in Blighty.
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Old 30-01-2013, 07:24   #553
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by Seaworthy Lass View Post

"it does not matter what scale you use: most navigators prefer to stick to the scale of the chart. There is no reason, however, why you shouldn't use centimetres, quarter inches, or any other unit you like, so long as you are absolutely consistent throughout the calculation"

There is absolutely no way of checking then if you have selected the right number of tidal hours! This method is even worse than I thought when I first looked at an RYA video going through computations step by step!

Folks, the method described in the handbook (which is a general navigation text book, not just one to be used in conjunction with RYA courses) is seriously flawed
.
You proved that the scale does not matter yourself when you halved all of the numbers and got exactly the same answer in a previous example.

I have completed my "real situation" using the two version of the RYA method. I will post them after I have some breakfast. As per Happy's request I will be posting "sketches" that I completed on graph paper. I have also done the numbers on a raster chart. I would be interested in seeing an SWL method solution.
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Old 30-01-2013, 07:56   #554
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by Seaworthy Lass View Post
Stating "Using the chart itself as a worksheet" RYA navigation handbook actually describes this as the procedure:

"it does not matter what scale you use: most navigators prefer to stick to the scale of the chart. There is no reason, however, why you shouldn't use centimetres, quarter inches, or any other unit you like, so long as you are absolutely consistent throughout the calculation"

There is absolutely no way of checking then if you have selected the right number of tidal hours! This method is even worse than I thought when I first looked at an RYA video going through computations step by step!

Folks, the method described in the handbook (which is a general navigation text book, not just one to be used in conjunction with RYA courses) is seriously flawed .
For a sailor underway, standing at the chart table, the handiest piece of paper is the chart and most of the time using the course lines drawn on it with everything done at chart scale is the easiest way to go. Sometimes, especially on an extremely small scale or large scale chart, the triangle would be too small or too large to be useful, so it's easier to pick a useful scale and draw the triangle on the untravelled part of the chart. My diagrams on plain paper were scaled at 1 cm = 1 NM.
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Old 30-01-2013, 07:58   #555
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by Dockhead View Post
P.S. -- And Lodesman, those guys still owe us some answers about an old argument we had about the Colregs -- I'll follow up on that too when I'm in Blighty.
I haven't forgotten. It will be good to put that thread to bed after all this time.
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