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Old 30-01-2013, 21:56   #586
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Huh? It sounds like they are expecting to stay on the rhumb line? You cannot, using the CTS method, with anything but no current or a constant current from one direction only.
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Old 30-01-2013, 22:03   #587
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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
...... I have stressed several times in my posts in this thread that it is essential if you are finding a CTS and planning to use it, that once you have found your CTS, you immediately plot your expected ground track.
This is essential and I have not found this mentioned in the RYA text either.

I found this post of mine several hundred back at post #119 and then again in post #185, but I have repeated this several times:
This was the reply to the above:

Quote:
Originally Posted by jackdale View Post
Figure 80, page 93

Quote:
Finally, draw a straight line to the point where the arc intersects the intended track. The direction of this line represents the course to steer. It does not matter if it seems to be starting from the wrong place, because we are only interested in its direction: the boat won't actually be sailing along the line. Nor does it matter if any of the other lines drawn to calculate the course to steer pass over shallow patches, or hazards, or even cross the coastline: they are purely lines representing a mathematical construction. The only one that has any direct relationship to the movement of the boat across the surface of the real world is the one representing the intended track.

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.
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!
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Old 30-01-2013, 22:06   #588
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Huh? It sounds like they are expecting to stay on the rhumb line? You cannot, using the CTS method, with anything but no current or a constant current from one direction only.
Yes, I agree entirely, completely, 100%
See my post before this one, I was typing while you had posted
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Old 30-01-2013, 22:08   #589
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by DeepFrz View Post
Huh? It sounds like they are expecting to stay on the rhumb line? You cannot, using the CTS method, with anything but no current or a constant current from one direction only.
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.
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Old 30-01-2013, 22:24   #590
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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They are trying to stay close to the rhumb line.
WHAT???????

The whole point of finding a course to steer when you have variable current is that you ideally want to NOT keep to the rhumb line. Depending on the strength of the current, your computed CTS (using the RYA method or otherwise) could be deliberately taking you several nm from the rhumb line!

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That is why there are qualifiers like "expected" and "intended", not "predicted."
What is the difference between expected and predicted?

Quote:
Originally Posted by jackdale View Post
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.
You do NOT want to be on the rhumb line while steering a computed CTS with variable current.

The quickest way from A to B with variable current is NOT to follow the rhumb line.

If you wanted to follow the rhumb line for some reason (for example big ships) then you would simple follow the straight line track on your chartplotter, or in the absence of one, you would work out the compass heading for every hour of current (or half hour if you have the data) and change direction every hour .

I have repeated over and over that the problem with the RYA method of computing a single CTS is their fixation on the rhumb line.
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Old 30-01-2013, 22:33   #591
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(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
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Old 30-01-2013, 22:47   #592
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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(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
No, I haven't heard of this. I will be interested to read about about it later when I finally have some leisure time LOL.

I was a part time gliding instructor for many years, but with the quest for stong thermals taking you on a zig zag route on cross country flights (some of them several hundred km long), wind data was only assessed casually usually (it is probably very different now, particularly at a highly competitive rather than club level).

It was particularly relevant only when considering the best course to steer on the "final glide" home, the point at which you break off from a thermal thinking you will make it back to the airfield.
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Old 30-01-2013, 23:10   #593
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

I don't recall ever being taught that method of laying out a course line. Of course before we got VFR over cloud on a long distant VFR flight we didn't really need it except it would have been extremely useful for short fuel conditions. The way I was taught was to steer by the rhumb line, crabbing into the wind. It was a way to keep from getting lost in the great wide north and of getting found if you went down.
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Old 31-01-2013, 00:13   #594
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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...heyyy This is an irrelevant post, but I'm doing it anyway to show my interest.
That's a brute,jackdale. very cruel. My pencil is too dull to plot the wee tail and I gave up where the reward don't suit the punishment...

so for fun, I looked at messy paper and did as (cos235*3.7)+(cos295*.2)=-2.04miles Southing .( now,I know I'm going to get there in less than 4 because current helps in this southerly direction16-2=14..so can drop the .2 kn in 5th hour.)

then (sin235*3.7)+(sin295*.2)=-3.2121212..for displacement West

3.2/14=tan12.87 180-12.87=CTS =167.13T and is 1/cos167.13*14 in length =14.36 miles/4=3.6 hrs= 3hrs 36 minutes

CTS 167 for 3 hours and 36 minutes. Hope it's not foggy when I get CLOSE to B...I haven't actually plotted it hour by hour or even to the 1/2s because it'll be slightly wrong without being able to see the error well.
when you post, I'll be able to see gross mistakes anyways.
Hi HappySeagull
Your method here works well as the current in the final hour is close to zero, so the vectors arced from the end of the second and third hours are close to identical and you can therefore draw a line straight to B, rather than one line from each as in my method.

The answer you get is therefore more accurate than the one determined by the RYA method, in which you are not allowed to arc off directly to B unless the journey is exactly a whole number of hours.

As I keep repeating it is the fixation with the rhumb line (that is NOT the intended ground track unless the current is constant) that is the RYA method's downfall. In this case it is only a negligible error of a couple of degrees, but at times it can be 10 or 20 or 30 degrees off, depending on the current relative to the boat speed and course.

Note: if the current in the last hour of current data applicable is a significant amount and not parallel to the correct CTS, then it is impossible to use your method.

I have attached a diagram of what you are calculating in case anyone had trouble following your calculations:
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Old 31-01-2013, 00:42   #595
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by jackdale View Post
SWL - I have provided a real life example. I would like to see your solution please.
Quote:
Originally Posted by LJH View Post
Below is an OpenCPN solution using SWL's method. 168 degrees for 3 hr 34 minutes. (Blue line is k to B, Red line is ground track)

Also done by spreadsheet I got 167 degrees 3 hr 34 mins.
(https://docs.google.com/file/d/0Bz8m...dGeG80ZTA/edit)
My result was almost the same as LJH's (limited by the pencil thickess, I have been sharpening lots LOL).

CST = angle of line KB = 167 degrees true (before allowing for leeway)
Distance travelled through water= length of KB = 14.2 nm
Time taken = distance travelled through water / boat speed = 14.2 / 4 = 3.55 hrs = 3 hours 33 min
SMG = distance from A to B divided by the time taken = 16 / 3.55 = 4.5 knots

Note: The destination should be the safe spot you want to arrive at so that you can approach the final arrival point safely. This will NOT necessarily be on the rhumb line and it may be dangerous arriving along the rhumb line.
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Old 31-01-2013, 00:53   #596
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Working through AndrewTroup's example

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If you could please provide an answer to Andrew Troup's example in post #485, I will provide an answer to yours tomorrow when I am back on board after a day's work replacing the sacrificial strips on our headsails.
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I looked at your solution. How did you determine the position of "A", the departure point?
I foolishly skimmed through Andrew's instructions (it was such a long explanation LOL) and I misread the bit about the departure point being on another chart in the cockpit and that only the 'current displacement vectors' from the third hour onwards had been transferred to the chart down at the nav table. I read that the departure point was (3,4) and it isn't, so all my values for the RYA method are incorrect.

All the values using the SWL method are still correct.


Folks, I have put five solid hours into replying to all the posts this morning. I am off ashore to do work on our sails.

Hopefully I don't have to put armour on when I read any posts on this thread when I get back tonight .
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Old 31-01-2013, 01:12   #597
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Check figure 80 on page 93 of the RYA manual. Notice that you depart from A, you are never at C.

I now am really convinced that this method is flawed. It does not solve for Course Made Good or Speed Made Good.

There are three sides to the triangle for the SWL. A to K is the movement of the water. K to B is the movement of the water. If you leave A at 4 Kts, and add your movement through the water to the movement of the water you will get the movement over the ground.

In the RYA model if B=D than you are measuring from C to B. The difference is that that SWL applies a tide vector to the last part hour of your planning.
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Old 31-01-2013, 01:13   #598
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Check figure 80 on page 93 of the RYA manual. Notice that you depart from A, you are never at C.

I now am really convinced that this method is flawed. It does not solve for Course Made Good or Speed Made Good.
Where is your CMG computed?

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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.

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Old 31-01-2013, 01:57   #599
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Presentation of current data

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.
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Old 31-01-2013, 02:18   #600
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Figure 80, page 93

And



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.
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.
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