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Old 31-01-2013, 02:49   #601
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by cal40john View Post
I think you're wrong here about CTS is an attempt to keep you on the straight line ground track between A and B with variable currents.

The guys in England gave examples where you essentially had 6 hours of current perpendicular to your course in one direction, then 6 hours in the opposite, taking about 12 hours to get across the channel. Resultant CTS by RYA is going to be the compass course from A to B. Your track over the bottom is going up channel for awhile then down channel. A method to keep you on or near the A to B ground track would not allow that.

A single constant current for the entire path, not a multi current problem, will keep you on the A to B ground track.
Yes exactly correct

And just to reiterate the point, when using a single CTS, the variable current doesn't have to change direction, just it's rate for the ground track to be significantly different to the rhumb line. The single CTS remains the most efficient way of dealing with variable currents.
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Old 31-01-2013, 03:03   #602
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by Seaworthy Lass View Post
This was the reply to the above:



The RYA definition of the intended track is "a straight line from the point of departure to the intended destination and beyond", so unless the current is constant (zero or otherwise) for the entire journey, then the only line "that has any direct relationship to the movement of the boat across the surface of the real world" is NOT the one representing the intended track. It could be anywhere as the current varies from hour to hour and pushes you all over.

On pages 97-99 of the RYA handbook an explanation is actually given on how to estimate position. This is not stressed as being an essential step which is done before commencing a journey, the very last paragraph on pg 99 vaguely states "it can even be done in advance".

On the other hand I (and by the sound of it HappySeagull too) feel that is is absolutley essential to plot your ground track when you are following a single computed CTS and possibly zigzagging all over the map! Particularly since discussions on the other recent navigation thread clearly showed that the ground track you instictively feel you will be following may be wildly out!
The choice of words "intended track" in the book is unfortunate -- he certainly doesn't mean (and it's obvious from the rest of the text) that you expect to follow the rhumb line. None of this has anything to do with plotting a ground track -- it is setting up the relationship between "A" and "B" which is necessary to build your vector triangle.

The "course line" is abstract, too, just like all the other lines in the vector triangle. The only reason why it "bears relationship" to what you're really doing is because you do physically start at "A" and end at "B". The course line abstractly represents aggregated CMG and VMG of your passage.
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Old 31-01-2013, 03:10   #603
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by jackdale View Post
Check your numbers with D = 16.0 I should have included that. Duh?

SMG 3.4 (both)
CTS
4 hour 020T
5 hour 017T
ETA 1442 (both)

I would use the 5 hour vector.


I will redo this in Open CPN and describe my steps below:
image at https://docs.google.com/file/d/0Bz8m...9adGczMUk/edit

First use the SWL method to find the CTS
  • Plot A and B
  • Plot the tide vectors (blue route at bottom.)
  • The next steps are the thin black dashed lines)
  • find point S by measuring 16 milesfrom end of 4th tide in the direction of B
  • find point L by measuring 20 miles from the end of the 5th tide to past point B
  • create a rout L-B-S view route properties. Divide dist B-S (1.28) by route length (3.03) to get proportion of 5th tide to use (0.42).
  • Plot point K by applying the SB/LBS ration to the 5th tide (235/1.2*0.42=235/0.51)
  • join K and B view route properties - bearing K to B is CTS. enter your boat speed and you will get the duration of your trip.

CTS=016
Duration 4 hrs 25 Mins
(this will be 3.62 Kts SOG)

Now to check the course. This will also cross check that the method above was done correctly.
  • Now go to A and plot your ground track:
  • Hr 1 = from A 016/4 plus 295/1
  • Hr 2 = Hr 1 plus 016/4 plus 295/1
  • Hr 3 = Hr 2 plus 016/4 plus 235/1.4
  • Hr 4 = Hr 3 plus 016/4 plus 235/1.5
  • Dest = Hr 4 plus .42*016/4 plus .42*235/1.2
***If this destination is not B then the solution was inaccurate.***
Connect the points A-Hr 1 - Hr 2 - Hr 3 - Hr 4 - B and you have your ground track - check for hazards to navigation etc.

Now you can:
transfer this route to your chart plotter; and/or,
monitor you progress in OpenCPN
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Old 31-01-2013, 03:11   #604
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by jackdale View Post
They are trying to stay close to the rhumb line.

That is why there are qualifiers like "expected" and "intended", not "predicted."

I have already listed numerous factors that work against the textbook current "prediction" accuracy.

What methods other than staring at a chartplotter or establishing a transit will keep you "on the rhumb line" while handsteering. A autopilot steering for a waypoint might get you close.
He has badly expressed himself. If you follow constant CTS in a changing current your actual ground track will obviously not follow the rhumb line.

If the current is slight, you might stay close, but it is not the purpose of the exercise to try to "stay close", but rather, to sail the most efficient path.

Whether or not it is "essential to plot your ground path" depends on where you are and what hazards there are. In many cases you don't need to do this at all -- certainly I never do it. Starting out from Needles for Cherbourg, for example, you have the entire Channel from Land's End to the Dover Straits to sweep back and forth, if you need it, so it's pointless to worry about it. You watch it on the plotter.

In a rock-strewn place like N Brittany the situation is very different -- there of course you need to understand exactly where you will be at all times and keep a sharp eye on the chart plotter as well. Still you don't "plot your ground path" -- it's usually too complicated to be practical. If you're doing the simple mathematical right triangle method, you will know your XTE at every hour so you can do a very rough plot based on that.
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Old 31-01-2013, 03:18   #605
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by jackdale View Post
The CTS will carry along the expected ground track. The CTS is not the expected ground track. That ground track is the CMG between A and B. It is the first line that you draw. (page 92)

The multi current vector is simply an extension of a single current vector.
With a multi current vector a CTS will not be the rhumb line from A to B. If you want to travel along the rhumb line you will change the course you are steering everytime the current changes.
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Old 31-01-2013, 03:31   #606
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by LJH View Post
With a multi current vector a CTS will not be the rhumb line from A to B. If you want to travel along the rhumb line you will change the course you are steering everytime the current changes.
Exactly. Or you can change course for every period you have current data. So if you have hourly data, you can calculate a mini-constant-CTS passage for every hour which will take you on a constant CTS from one point on the rhumb line to another point on the rhumb line an hour later. You still stray off the rhumb line in between but less far than if you sail a constant CTS for the whole passage.

Or you can put your pilot on "track mode", and it will steer to keep COG equal to BTW. If the pilot sees you have exceeded a certain maximum allowed XTE (due to the current's changing), it will alter course to bring you back on. It will then go back to holding a heading to keep COG and BTW equal until the next time you stray off the rhumbline beyond your set maximum XTE. It works very well.

Or you can do it that way yourself, using any means you have to calculate XTE, and whatever frequency of calculation which is convenient.

There are good reasons to do this -- for example, you are buffeted by different currents but you need to sail a narrow safe path between hazards.

But all of these modes are less efficient -- will take your further through water and take longer to arrive -- than sailing a single CTS for the whole passage.
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Old 31-01-2013, 03:35   #607
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by daddle View Post
(Parachuting in here) Have you wizards heard of "pressure pattern navigation" in aircraft. They choose a CTS based on the pressure (tide) at A and B only without concern for what (current) is between.

I know it does not apply well here. But it is interesting. Google it.

Sorry for the interruption ... Carry on
Yes, essentially you are trying to make use of the circular pattern of the wind around the pressure systems. It requires you to use isobaric heights along your track. ie if you want to fly at 18,000 you would use the 500mb chart you would find the isobaric heights at selected points along your track to calculate the drift at those points. It is best used for long range navigation.
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Old 31-01-2013, 05:22   #608
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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
Bewitched, are you convinced yet that the quickest way to sail a leg with variable cross current is by starting with a single computed CTS (if water depth is constant, so advantage can't be taken of less or more current in certain areas and you don't need to tack or gybe) and then thinking about leeway, wind in certain areas, etc to determine the best route?

This initial CTS determination will be the quickest way of getting from A to B if you are motoring in calm water with variable cross current. The quickest way is NOT following a straight track on the GPS!
Sorry, not convinced. Quite the opposite I'm afraid. I may start a new thread to explain the reasons why I prefer to follow the course line rather than taking your thread off subject.

Quote:
Originally Posted by Seaworthy Lass View Post
Next time you are racing, could you please ask your tactician (when he has some free time) to input several currents for a long leg of a race in deep water (eg greater than two hours) and see what the program comes up with? Also ask him if the program tells you what tack to be on if the wind is on the nose? One of the two tacks will be advantageous. I would be very interested in the results, as I am sure you would be
There are some examples on the website, but basically it looks very much like the Vende Glode tracks. Yes it tells you where to tack and gybe. Paul gave a pretty good run down of how the software crunches the numbers.
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Old 31-01-2013, 06:26   #609
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by bewitched View Post
Sorry, not convinced. Quite the opposite I'm afraid. I may start a new thread to explain the reasons why I prefer to follow the course line rather than taking your thread off subject.



There are some examples on the website, but basically it looks very much like the Vende Glode tracks. Yes it tells you where to tack and gybe. Paul gave a pretty good run down of how the software crunches the numbers.
Seaworthy Lass is correct. Unfortunately it's not helpful to mix up the question of efficient steering through moving water, with the question of how to tack and gybe -- different questions. A good program like Neptune will figure both and aggregate the information to give you an overall strategy with courses to steer, optimum timing of tacks and gybes, optimum point of sail, etc.

Every English Channel sailor knows that the fastest way across the Channel is to calculate a single course to steer, and steer that single heading all the way across. Your ground track will make a big arc (or a big "S" curve) taking you far away from the direct path over ground (the rhumb line). But it's faster because a single heading takes you in a straight line through the water.

See the Neptune site for examples:

PASSAGE PLANNER PLUS

You can download a trial version and play with it.

Here is a comparison we did in another thread between a constant heading strategy versus sailing right down the rhumb line:

captforcescenario.xls

It's interesting -- and another proof of the idea of constant heading sailing -- that the more often you change course to stay on the rhumb line, that is, the tighter you stick to the rhumb line, the slower you go. You will see that we analyzed the rhumb line course hourly, then five minutely, then minutely.
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Old 31-01-2013, 06:55   #610
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

If you ask Neptune for an optimal route for motoring from Needles to Cherbourg, for example, here is what you get:

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Note your big "S" curve over ground.

Nevertheless, at 5 knots through the water, you get there in just under 12 hours. Magic? Not at all -- you were sailing straight through the water and you were not paying attention to the fact that the tide was sweeping you back and forth. If you had sailed straight down the rhumb line, it would have taken you 14 hours or more.
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Old 31-01-2013, 09:27   #611
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

I like a pre-plotted track so my gps plotter doesn't spook me when I am steering CTS....also, dodging traffic and the rest of real life, may allow me to return to my predicted path...
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Old 31-01-2013, 10:16   #612
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Re: Presentation of current data

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Originally Posted by Seaworthy Lass View Post
PS Important note with my result.

I have taken the current data as everone else has to be the average for the hour.

If it is not and you want to leave at 10, you need to allow for half an hour of the 10 oclock data and repeat calculations.

Part of the limitations of ANY method are how the current data is presented. We still need to have the most accurate method for what data we have otherwise other errors are being introduced.

Even where the SWL method shows a great gain versus another method, it may never be required as increments unless it is shown that the gain is due to the gross calculation or the fine calculation. To guard against this, it behoves to include closer calculations, but may stress the peanut gallery's attention span where not strictly required.

I think Strong currents should have a closer calculation... say 1/2 or 15 minute plots: this is why it's nice to have MAX and Slack times, so any current@ time you need can be calculated.
Where currents are weak, the calculation is neglibly different.

It's my feeling that your method vs RYA (or whatever others may whimsically appear from the Peanut Gallery) is best expressed where currents are stronger at the END of the trip.
added: and perhaps the best place too for incremental (1/2 hour, 15 minute data) As to presenting this in the SWL instructions, hmmm. hopefully, only in the proofs, I'd say, because simple is good.
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Old 31-01-2013, 10:18   #613
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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I like a pre-plotted track so my gps plotter doesn't spook me when I am steering CTS....also, dodging traffic and the rest of real life, may allow me to return to my predicted path...
Sure. It all depends on where you sail, of course.

Our biggest problem is avoiding getting run down by ships. You are crossing the world's busiest shipping lane, often described as being like a squirrel running across a busy motorway . There are no rocks or other hazards on this route, so ships are what we concentrate on. YMMV in other waters.

Using a program like Neptune, your predicted ground track is spit out automatically. Otherwise you'll need to do it by hand.

If you dodge traffic there's usually no point in getting back on your predicted track. Usually more efficient just to go back on your heading, and deal with any variance in arrival caused by your dodge together with all the other variances (speed always being the biggest one), in your final approach.
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Old 31-01-2013, 10:26   #614
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Re: Presentation of current data

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Originally Posted by HappySeagull View Post
Even where the SWL method shows a great gain versus another method, it may never be required as increments unless it is shown that the gain is due to the gross calculation or the fine calculation. To guard against this, it behoves to include closer calculations, but may stress the peanut gallery's attention span where not strictly required.

I think Strong currents should have a closer calculation... say 1/2 or 15 minute plots: this is why it's nice to have MAX and Slack times, so any current@ time you need can be calculated.
Where currents are weak, the calculation is neglibly different.

It's my feeling that your method vs RYA (or whatever others may whimsically appear from the Peanut Gallery) is best expressed where currents are stronger at the END of the trip.
added: and perhaps the best place too for incremental (1/2 hour, 15 minute data) As to presenting this in the SWL instructions, hmmm. hopefully, only in the proofs, I'd say, because simple is good.
An insightful comment. Tim Bartlett mentions this. Not only strong currents at the end, but current speed relative to boat speed. If current speed is small relative to boat speed, then this less demanding of the methods, and at some point CTS itself becomes superfluous. Strong currents demand careful rate calculations; with weak ones a guess will do. That's in the RYA navi handbook.
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Old 31-01-2013, 12:47   #615
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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SWL, you will need a very sharp pencil!
I keep resharpening it every few lines drawn .
I have been keeping shavings and sharpener in a container to stop them scattering and so that the sharpener is always easily at hand. Very fine lines and points are important for accuracy in any of the chart computed methods!

The end is thankfully in sight for work on the sacrificial strips of the head sails . When I have some free time I will go through my method (as per instructions in Post # 2 of this thread) step by step with diagrams so that it is easy for everyone to follow.

It really is not very onerous and provides a consistently good result each any every time, whether the situation decribed is one frequently encountered or not.

I will be presenting this method to the RYA and will hopefully be able to discuss it personally with Tim Bartlett at some stage.
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