Inaccurate RYA Teaching : CTS - Quest For a New Method

[jackdale]

Jim is quite right.

If you can see your destination. Simply line up some object behind it with the destination and keep the two of them aligned. You will steer your course made good. Your boat might be "crabbing " a lot, it this does work. I call this "eye ball" navigation.

[TeddyDiver]

Both methods are inaccurate but simple to use and close enough for most occasions IMHO. Agree with Jim above about the tide speeds is much greater source of inaccuracy..

[Seaworthy Lass]

Originally Posted by jackdale

The SWL method is faulty.

The rhumb line is crucial.

Example

Travelling from Cautionary buoy to Entrance Island
Current 000 T
Current 1st hour 2 knots
Current 2nd hour 1 knot
Current 3rd hour 0 knot

Givens
Distance = 14.69 miles
Boat speed = 5 knots
CMG = 239 T

To calculate
SMG = 4.4 knots
CTS = 229 T

Hi Jackdale
The journey will continue into the 4th hour.
For me to determine the CST and time taken could you please give me the value for the current in the 4th hour?

It is nearly 1am here and I bave just woken up having dozed off on the settee. I will work out my CTS tomorrow once I have the data for the current in the final hour of the journey.

Night all

[Seaworthy Lass]

Originally Posted by jackdale

.....If you can see your destination. Simply line up some object behind it with the destination and keep the two of them aligned. You will steer your course made good. Your boat might be "crabbing " a lot, it this does work. I call this "eye ball" navigation.

Jackdale, just one more question before I head off, what would you do if you encounter current along the way that is greater than or equal to your boat speed?

[conachair]

Originally Posted by Seaworthy Lass

The RYA instructor giving me grief in the other navigation thread certainly didn't know that!

And if it is so instinctive, why didn't one single person (and there were over 10,000 views of that thread) speak up to support me when for four days I insisted the method was an approximation and that the rhumb line would be hit at this point if the data was good? .

I did sort of point it out, was a bit noisy at the time though.

http://www.cruisersforum.com/forums/...ml#post1132478

[David_Old_Jersey]

Originally Posted by conachair

Anyway, I much prefer a CTS to a waypoint a few miles short ( and maybe a bit upstream), then do the rest as pilotage.

"Short and Wide" is part of the WAG Method© I use.

How far short and wide depends on where I am arriving, what is around to hit! and how familiar I am. But certainly always arriving upstream (nothing quite as dispiriting as punching the tide at the end of a journey - when you didn't have to .....plus always nice to know whether to turn left or right when the coast looks unexpectedly blank!)......obviously in the real world the GPS / Chartplotter nowadays takes a lot of the fun out of the latter bit ,

For the last few miles pretty much ignoring tidal drift (or compensating in conjunction with eyeball as part of pilotage) due to the short distances involved works fine. If you know approx where you now are starting from, (i.e. near your intended destination, whether that is where you intended to arrive or not don't matter), then easy enough to lay in a straight course at that time, at least as a guide........and if you don't know exactly where you are, then being pretty comfortable that you are short and wide to one specific side is nice to start from - rather than worrying about being long (too close to shore!) and no idea whether to turn left or right!.......especially in poor viz ). But autopilots nowadays take a lot of the fun out of that pondering as well ("what's the course been for the last hour or 2?"......."east" ).

In regard to "real world" tidal currents - in addition to the considerations (uncertainties) mentioned already, also gotta factor in when close to shore that currents can reverse, or simply speed up or lessen differently to what the Tidal Atlas says for that hour in that area (tide currents are not plotted for every square mile - usually pretty good, but never gonna be to the exact inch on every day. except by accident ).........all that means that the "short and wide" approach works well as requires the last mile or so to be a "live" calculation rather than simply a prediction from many hours past.

[jackdale]

Originally Posted by Seaworthy Lass

Hi Jackdale
The journey will continue into the 4th hour.
For me to determine the CST and time taken could you please give me the value for the current in the 4th hour?

It is nearly 1am here and I bave just woken up having dozed off on the settee. I will work out my CTS tomorrow once I have the data for the current in the final hour of the journey.

Night all

OK

current in the 4th hour is 0

The vector I presented is still accurate.

I have a squash match that I have to prepare for. But I will post some other examples when I get moment.


BTW - OpenCPN is a great way to set and drift.

[jackdale]

Originally Posted by Seaworthy Lass

Jackdale, just one more question before I head off, what would you do if you encounter current along the way that is greater than or equal to your boat speed?

OK I will attempt to vector those as well. But if the current is directly astern it will add to your bpat speed. If it is on the nose it will subtract. If the current speed exceeds boat speed when on the nose you will get pushed back. We get currents that run 16.5 knots; we avoid them at all times other than the turn (slack).

[jackdale]

BTW - when all of this is completed, the wind will blow from the direction in which we intend to sail.

[DeepFrz]

Seaworthy Lass, in order to assess the accuracy of either method you will have to plot your position at different time internals along the way. You will, if steering one heading, cut an arc through the water. By plotting your position for each hour you will be able to approximate the arc and determine the accuracy of the plot...given that all inputs are true for the problem in a classroom solution. In a classroom situation (where you can assume the data is correct with no underlying conditions) the only time you will be on the rhumb line is at your starting point and your destination.

The plots in the RYA problem give you CTS and Time to destination (the second being the parallel line to CTS). Because of unknowns they teach you to ignore the last section of the course so that you have time to make corrections. Plotting to the exact destination could put you on some reef in the Philippines (or other unknown obstacle).

[DeepFrz]

Jack, the CMG will only be on the rhumbline at the start and end of your calculation by using any of these techniques. (Sorry, I haven't checked yours yet).

Edit: add
Except in your example of using a constant current offset.

[Dockhead]

Originally Posted by mcarling

I just re-read my posts in this thread and I found nothing bombastic.

Your proposed experiment, as described, depends on precise input data where none are available and therefore cannot produce any meaningful results. If you have fun doing it, then it may be worthwhile for you. If you want to design an experiment that can produce meaningful results, then I suggest reading Karl Popper (heavy reading) or Nassim Nicolas Taleb (light reading).

Nonsense. We have high resolution tidal stream data from the UK Admiralty, dense matrix of diamonds, velocities expressed in hundredths of knots, and five minute intervals. We can (laboriously, but can) model a high precision CTS passage with this data, and compare it to the results of calculations made with different methods of approximation. It will be very interesting and shed a lot of light on this discussion.

This form of argument: "it's real complicated, therefore we can't figure it out", or "the data are approximations, therefore the answer is meaningless", or indeed "the input data have only two significant digits, therefore the output cannot have more than two significant figures" are all false. The relationship between precision of data and precision of information derived from data is very complex. A good example of this complexity is digital processing of optical imagery. These days it is even possible even to produce a focussed image from a blur - to alter the focal plane in an already captured image - producing, say, readable text out of a chaotic blur, apparently lacking sufficient information.

This a priori dismissal of CTS calculations based on certain specific approximations is wrong. It is a mere prejudice. It is a complicated question, the answer to which we do not know, and which will take a lot more than the wave a hand to figure out.

[Dockhead]

Originally Posted by Andrew Troup

Thanks for the apology, but you'd have to address the specific concerns I raised to convince me your change of heart is justified.



If so, that's a third problem I didn't consider.



Hmmm - I thought there were shoal patches in the Channel?

Given the prevalence of airborne moisture in them parts, I also assumed land was out of sight more often than it was out of mind...

(Talking about a part of the world I've only traversed on ships, or most recently by train - on my way, incidentally, to a very much unfruitful encounter with the RYA!)

You're still not understanding the RYA method. I'm very sympathetic because it took me an inordinately long time to figure it out myself.

You swing the end of the water track line onto the rhumb line to get the right angle of the water track, which is your CTS (after correcting for variation and leeway, of course). The clever trick here is you don't need to exactly match the LENGTH of the water track with corresponding evolution of the tidal vector line, which is complicated, to get the right angle. This allows you to do the calculation using only whole hours. The ONLY flaw in the RYA method is that it does not calculate a vector for the last partial hour, substituting the average of the passage. Seaworthy has brilliantly demonstrated that this can lead to large errors. What she didn't mention is that her problem is cleverly designed to maximize the error. The RYA method will produce zero error, if the last partial hour behaves like the average. It can do this if the tide is running in the middle of its range during the last partial hour, rather than at the top of the range, or bottom of the range, as in Seaworthy's example.

[jackdale]

Originally Posted by DeepFrz

Jack, the CMG will only be on the rhumbline at the start and end of your calculation by using any of these techniques. (Sorry, I haven't checked yours yet).

Edit: add
Except in your example of using a constant current offset.

I do not normally use the term rhumb line in coastal nav. I probably shoudl have stuck to Course made Good or Course to Make Good. Ideally they are then same.

[DeepFrz]

The rhumb line and CMG will only be the same at the beginning and end of the calculated distance or if the drift and all other variables remain the same. Otherwise by steering the CTS heading you will describe an arc (more likely an uneven curved track) or a S track if the current changes direction. Check this out the next time you cross by taking bearings or by locating your position with GPS. No fair correcting from GPS. <gr> Set the autopilot to steer by heading and see what happens. Or use the example you posted above and check your position at the end of one hr, two hrs, etc. You will describe an uneven curved line back to the waypoint.

If you calculate right to the destination you may have little room to manouver on the drift side of the rhumb line. That is why you set a waypoint short of your destination and you should be on rhumb line at that point (in a classroom environment).