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Old 26-01-2013, 08:09   #406
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Lodesman and others with rhumbline obsession..

Actually there's no need to measure the 'rhumb line' nor to use it anyway in the process. Thou it's a fact that it's still there it's totally meaningles for the outcome. For your convenient, if you want to write the distance in the log, measure it but you don't really need it..

BR Teddy
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Old 26-01-2013, 08:26   #407
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by TeddyDiver View Post
Lodesman and others with rhumbline obsession..

Actually there's no need to measure the 'rhumb line' nor to use it anyway in the process. Thou it's a fact that it's still there it's totally meaningles for the outcome. For your convenient, if you want to write the distance in the log, measure it but you don't really need it..

BR Teddy
If it's easier for you to understand, then think of it like this:

It is essential to know the relative positions of "A" and "B" (that is, range and bearing from A to B) in order to solve the angle which will be your course correction.

Mathematically, the relative positions of "A" and "B" are a line. But you don't need to think about that, if it bothers you.

The range and bearing from A to B -- if you prefer to think of it like that -- are not "meaningless for the outcome" -- you cannot calculate the angle without this. If you are working the problem in analogue (that is, on paper with a protractor), you must place B on your chart, graph, or sheet of paper in the correct position relative to A. Otherwise you can't draw the line to B from the end of your tide vectors line and can't solve the problem.
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Old 26-01-2013, 08:56   #408
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by TeddyDiver View Post
Lodesman and others with rhumbline obsession..

Actually there's no need to measure the 'rhumb line' nor to use it anyway in the process. Thou it's a fact that it's still there it's totally meaningles for the outcome. For your convenient, if you want to write the distance in the log, measure it but you don't really need it..

BR Teddy
Agreed.

The tides set vectors are used from point A to find an offset point.
The distance from the offset point to point B is required.
Nowhere in the flowchart do you need to measure or apply the rhumbline A-B.
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Old 26-01-2013, 09:05   #409
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by Dockhead View Post
If it's easier for you to understand, then think of it like this:

It is essential to know the relative positions of "A" and "B" (that is, range and bearing from A to B) in order to solve the angle which will be your course correction.

Mathematically, the relative positions of "A" and "B" are a line. But you don't need to think about that, if it bothers you.

The range and bearing from A to B -- if you prefer to think of it like that -- are not "meaningless for the outcome" -- you cannot calculate the angle without this. If you are working the problem in analogue (that is, on paper with a protractor), you must place B on your chart, graph, or sheet of paper in the correct position relative to A. Otherwise you can't draw the line to B from the end of your tide vectors line and can't solve the problem.
In the SWL method we do not calculate a course correction and apply it to the rhumbline course. We calculate the course and distance between the two points K and B. With the two points the you use the difference in the lat and long to find the course and distance.
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Old 26-01-2013, 10:00   #410
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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In the SWL method we do not calculate a course correction and apply it to the rhumbline course. We calculate the course and distance between the two points K and B. With the two points the you use the difference in the lat and long to find the course and distance.
As I said before, you are welcome to think of points with coordinates rather than lines, if it works better for you. It's all the same. You don't need to draw any lines at all if you're doing it analogue, with a protractor. You can just mark off the three points A, B and K -- and there's your vector triangle.

All the sides must be equal in time -- that's your constant. Once you have worked out where K is, then you have enough information to do the complete trigonometric solution and get whatever angle you want, in this case, you need a CTS, expressed either as a compass direction (if you've laid it out and plotted it on a chart) or as a course correction (in case you're doing it on graph paper, back of a napkin, or whatever.

However you do it, you're solving a vector triangle. "Trigonometry" means "triangle-measuring" in Greek (I wrote that above). You are literally measuring a triangle when you put your protractor down and measure off the last angle.

All the same operations can be done with math and with no drawing or plotting, and you will get exactly the same answers. It's harder to do with math when you have currents which run in directions other than perpendicular to the course line, because you don't know any angle of the triangle. Therefore you have to know the length of every line (sorry; if you don't like lines, then -- you have to know the position of all three points on some kind of piece of paper). In other words, you can't just add up the tides like on the English Channel example. So this is easier done by plotting (although a computer makes the math solution reasonable as well, if you prefer it).

The English Channel example is dead simple because the tides can be assumed to all run perpendicular to the course line. It's simpler because you know one of the angles of the vector triangle -- the angle between the course line and the tidal vector line is 90 degrees. So you really don't need to plot this kind of situation; you just add up the tides to learn the length of the tidal vector line, and you easily get the water track line using the Pythagorean Theorem. Then you can solve the angle between the course line and the water track line with the excellent rule of thumb of one degree per mile at 60 miles and Bob's your uncle.

But absolutely all of these methods without exception are vector triangle solutions. The three sides of the triangle are: course line, tidal vector line, water track line, and they must be equal in time for the math to work (or the plot to work, if you're doing it with a protractor). If you have a phobia of lines, you can think of it as knowing three points: Origin, Destination, and the point at the end of the tide vector line. It's all the same.
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Old 26-01-2013, 10:12   #411
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Welcome to club of all of the rest of us who have taken off in a wrong direction, and after much argument ended up learning a lot from it! What's cool is that we all learned as much from the argument as you did.
+1

Quote:
Consequently a vector diagram must form a closed loop, because leaving it open can only be done by bringing in positional information from the "outside world" about the two endpoints of the open diagram.

Another way of saying this:
The only information we can put into (or extract from) a vector diagram is vectors.
Smuggling fixed or isolated points in, or out, is not valid.
Ta Da!
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Old 26-01-2013, 10:13   #412
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by Paul Elliott View Post
This morning I re-worked the numbers using vector addition (instead of directly solving the triangle) and verified my results. Of course this makes sense now, since SWL has found her error.
It was an embarrassing omission forgetting to change the amount of current in the second hour and confusing everyone (including myself LOL). I just could not work out why our computations varied until I realised what had occured .
Thanks for working through the example and pointing it out .

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This strong-current stuff reminds me of the San Francisco "Three Bridge Fiasco" race, where we often have strong currents and light winds. One year I crept ahead of many competitors by casually slipping my anchor overboard and relaxing in the cockpit eating lunch. The other guys were fighting their way upstream, all the while drifting slowly backwards.

Here are my "3BF" blog posts: Three Bridge Fiasco Report VALIS, Three Bridge Fiasco, 2010 VALIS, Three Bridge Fiasco 2012 VALIS. We did absolutely no course plotting, but we did look at the current predictions to see which was the favored route for the day. Of course the wind usually determines the route, and all my current projections go out the window.
Beautiful boat Paul!

Picking the best route according to current can make a huge difference sailing.

I remember years ago racing around Phillip Island in Victoria Australia. The skipper was a passionate windsurfer and had explored every inch of the terrain around there and studied the flow of current and consistently won races simply by picking the best areas according to whether current needed to be avoided or taken advantage of.

His aim was to make sure that for every race we were sitting in the pen with sail covers on having beer/bubbly before the next boat crossed the line
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Old 26-01-2013, 10:21   #413
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by LJH View Post
Nice when the 1:60 rule is easy to apply.
offset 1.1 miles west = 181.1 course
time taken will be [60/cos(1.1)]/6=10 hr 6 mins
monitor your XTE diverging out to 6.5 miles west of the rhumb line before you start converging..
I think you would have diverged 7 nm west
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Old 26-01-2013, 10:34   #414
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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As to currents off Cherbourg -- at springs at HW Dover +3 the nearest tidal stream diamond reads 6.3. That's an average over an hour and over a certain area. The peak local rate will be around 8 knots.

That's why you're really stuffed if you arrive downtide at springs and peak rate in a small boat. It's actually very dangerous, because there is no refuge to the West -- the widow-making Alderney Race awaits you around Cap Hague. Standard procedure for this case is to get an anchor down if you can -- if the sea is not too rough -- and wait for the rate to subside to less than your cruising speed. If it's too rough to anchor, you motor against the stream to reduce lost miles until you can start to make headway. Or you turn around and go back to England -- not such an unusual case.
I had a quick look at Cherbourg. Are you able to use either entrance to the Outer Rade, giving you some reserve with stronger currents or do you select an initial destination an extra mile or two up current in these cases?
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Old 26-01-2013, 10:46   #415
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Lodesman and others with rhumbline obsession..

Actually there's no need to measure the 'rhumb line' nor to use it anyway in the process. Thou it's a fact that it's still there it's totally meaningles for the outcome. For your convenient, if you want to write the distance in the log, measure it but you don't really need it..

BR Teddy

This is the great thing about CF - you can always count on someone who hasn't participated in (or really read) the thread to come along and toss in an insulting and otherwise worthless comment.

If you can point out anywhere in the thread where I said it was necessary to write or measure the rhumb line, I will gladly eat my hat. My point has been all along that SWL's method is still based on a vector triangle - and it is useful to understand the underlying principle of any method you use in navigation. Otherwise you have to chalk it up to PFM (pure f-ing magic) - which is fine if you depend on voodoo, fate or whatever to get you from point A to B; for me, I like to know what's going on.
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Old 26-01-2013, 10:48   #416
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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SWL has come up with an ingenious method of doing this calculation which with practically zero extra effort will give you a much more precise result than RYA. It is really worth learning -- it is really totally superior to RYA.

The other ingenious thing about SWL method is that it greatly simplifies getting the passage time. It's really worth learning!
Thanks for the VERY generous praise Dockhead .

When I posted the method initially I did not include the easy way of determining time taken. It just occurred to me when I was going through examples. It is so very neat I thought .

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If SWL agrees, I'm going to send it to Dick Durham at Yaching Monthly and see if he will write an article about it. It's that good.
Oh, that would be wonderful. I am "keen to spread the word" LOL.

The reliability of the RYA method for determining CTS really bothers me. It can be great in lots of examples, but can let you down badly at other times (even without looking at extreme cases where it can give a result that is 30+ degrees out).

It bothers me too that I would not have gone back to the 'drawing board' had not a CF member sent me a private message to point out just why the original method I came up with in the other thread could fail at times too. He wants no credit for making me rethink things .
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Old 26-01-2013, 11:07   #417
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

an
Quote:
Originally Posted by Seaworthy Lass View Post
I had a quick look at Cherbourg. Are you able to use either entrance to the Outer Rade, giving you some reserve with stronger currents or do you select an initial destination an extra mile or two up current in these cases?
Yes, you can use either entrance. If you looked in Google Earth, you saw the extraordinary length of the sea walls.

Cherbourg is an incredible harbor with extraordinarily massive walls and quays and locks. In terms of cubic meters of stone and concrete, much bigger than any of the English or American harbors I've seen.

The area enclosed by the outer seawalls is like a huge bay. You can anchor out there if you want, but I have always preferred the pleasures of the Port d' Plaisance with the cosy Cherbourg Yacht Club.

The seawall has three fabulous Napoleonic forts, one in ruins after WWII. The town itself is typical Normandy, with ancient crooked streets and fabulous brasseries, little holes in the wall with fabulous wine cellars, open air markets, and all the joys of French coastal towns.

It's definitely worth 8 to 10 hours of sailing to get there!
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Old 26-01-2013, 11:12   #418
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

I am so thrilled the tide is finally turning.

It has been such an uphill battle fighting against the current belief that the RYA method to determine the CTS (that has been taught and used for decades and is at times a good approximation), is in some cases seriously flawed and that there is another reasonably simple, very reliable alternative method.

I must thank the very stubborn RYA instructor in the previous thread who asserted the RYA method was a mathematically perfect way of getting to B (the destination) if the data used is correct. Rather than simply continuing to argue with him, he inspired me to find a better technique. So I extend thanks .

I may crack open a bottle of champagne tomorrow using the SWL technique
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Old 26-01-2013, 11:21   #419
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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 must thank the very stubborn RYA instructor in the previous thread who asserted the RYA method was a mathematically perfect way of getting to B (the destination) if the data used is correct. Rather than simply continuing to argue with him, he inspired me to find a better technique. So I extend thanks .
It must be said that a crucially valuable role is played by people who stubbornly insist on a wrong idea, especially if they are able to argue their position well. They force the rest of us to blow off the cobwebs and relearn all our stuff in order to prove to them the error of their thinking. Without Capt Force in the other thread, I would not know half as much as I know about CTS passages as I know now. Most of us have, at one time or another, played both roles
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Old 26-01-2013, 11:52   #420
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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This is the great thing about CF - you can always count on someone who hasn't participated in (or really read) the thread to come along and toss in an insulting and otherwise worthless comment.

If you can point out anywhere in the thread where I said it was necessary to write or measure the rhumb line, I will gladly eat my hat. My point has been all along that SWL's method is still based on a vector triangle - and it is useful to understand the underlying principle of any method you use in navigation. Otherwise you have to chalk it up to PFM (pure f-ing magic) - which is fine if you depend on voodoo, fate or whatever to get you from point A to B; for me, I like to know what's going on.
Sorry Lodesman, my bad . Had a couple of quotes and delated the other one and left wrong name..
I have read and participated in the thread and I still stand behind my comment 'there's no need to triangulate anything' in SWL method nor in my method.
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