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Old 22-01-2013, 23:59   #106
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

COURSE OVER GROUND TRACK
= red dotted line

MY METHOD
Example 1

Note that I arrive at the destination B on one single heading at a constant travelling at a speed of 4 knots.
I arrive in the shortest possible time given I cannot alter my speed .
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Old 23-01-2013, 00:01   #107
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

COURSE OVER GROUND TRACK
= red dotted line

RYA METHOD
Example 1
Note that if you are following the RYA method at some stage you need to alter your heading if you want to arrive at B.
The RYA method does not get you to the point you want to be (B).
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Old 23-01-2013, 00:04   #108
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

This fixation with the rhumb line (course line) is truly the source of the problems with the RYA method.
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Old 23-01-2013, 00:07   #109
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by Dockhead View Post
That's what I thought too, until others here proved to me that it's actually the other way around -- an arc for a symmetrically changing direction current, a sinoid S curve for a single direction current. It's weird, but true.
That could be true depending on the data set. If you have a low offset current at the beginning followed by a high current in the same direction followed by a low current again in the same direction (or at least the offset will be to the same side) you could end up with an S curve (or reverse S). However for the examples given so far the track will approximate a single curve on the same side of the rhumb line. That is why I think it is best to calculate the CTS on at least an hourly basis, more often in tight circumstances.

As to my reference to rhumb line for coastal nav. that may be an influence of my flight training.
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Old 23-01-2013, 00:11   #110
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by mcarling View Post
That's excellent if one has an opportunity to take bearings (and may be fairly good with even just one bearing taken) at the waypoint.
Huh? Bearings?

If the data was correct (and I was motoring on a calm day in calm water) and you blindfolded me and just told me to look at the compass and watch I would arrive exactly at B.
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Old 23-01-2013, 00:16   #111
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

SeaWorthy, what you are doing is exactly the same thing, same method. The only thing is that you don't draw in the rhumb line, but it is inferred by the points. The error was not in the RYA method but in the teaching, as others have said. Again, I agree that if you shift the destination point past your wanted destination, or waypoint, you will not arrive at your waypoint.
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Old 23-01-2013, 00:17   #112
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by DeepFrz View Post
That could be true depending on the data set. If you have a low offset current at the beginning followed by a high current in the same direction followed by a low current again in the same direction (or at least the offset will be to the same side) you could end up with an S curve (or reverse S). However for the examples given so far the track will approximate a single curve on the same side of the rhumb line. That is why I think it is best to calculate the CTS on at least an hourly basis, more often in tight circumstances.

As to my reference to rhumb line for coastal nav. that may be an influence of my flight training.
If you calculated your course to steer on an hourly basis you would have roughly a straight ground track. You lave LOTS of courses to steer then.

What if you hit current along the way that you could make no headway against for an hour or two or three?

The whole point of determining ONE CTS for the entire journey is to get you there most efficiently, coping with adverse conditions along the way (as long as the average current for the WHOLE journey was not against you and equal to or greater than your boat speed).
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Old 23-01-2013, 00:20   #113
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by DeepFrz View Post
SeaWorthy, what you are doing is exactly the same thing, same method. The only thing is that you don't draw in the rhumb line, but it is inferred by the points. The error was not in the RYA method but in the teaching, as others have said. Again, I agree that if you shift the destination point past your wanted destination, or waypoint, you will not arrive at your waypoint, using these examples.
I hate to disagree with you DeepFrz, but the RYA focus on the course line and this is why the method is inherently wrong.

I don't focus on the course line AT ALL (why do you want to?). I focus on getting to B.
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Old 23-01-2013, 00:22   #114
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by Seaworthy Lass View Post
If you calculated your course to steer on an hourly basis you would have roughly a straight ground track. You lave LOTS of courses to steer then.

What if you hit current along the way that you could make no headway against for an hour or two or three?

The whole point of determining ONE CTS for the entire journey is to get you there most efficiently, coping with adverse conditions along the way (as long as the average current for the WHOLE journey was not against you and equal to or greater than your boat speed).
A straight ground track is the most efficient way to get from point A to point B in these examples.
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Old 23-01-2013, 00:23   #115
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote by me:
"or at least give a better approximation than the RYA method does, as it has me heading to the destination every time, not some arbitrary point."

Quote:
Originally Posted by DeepFrz View Post
How could this possibly be true if you are steering one heading and the current is changing speed?
What do you think now that you have had a look at my two diagrams DeepFrz?

I arrive at the destination despite one compass heading and a constant speed the WHOLE way.

The RYA method does not, as it for some reason it wants to focus on the course line, not B.

PS "Heading to my destination" in the original statement means compass heading. My compass heading is steady for the journey and this heading will have me arriving at the destination every time, unlike the RYA method.
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Old 23-01-2013, 00:31   #116
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Focusing on your ground track will keep you away from dangers. If the navigator of the USS Guardian had focused on his ground track and allowed a decent amount of room to go around a world heritage site he would not have hit the reef.
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Old 23-01-2013, 00:33   #117
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by Seaworthy Lass View Post
PS "Heading to my destination" in the original statement means compass heading. My compass heading is steady for the journey and this heading will have me arriving at the destination every time, unlike the RYA method.
I get it now. I also think that you are basically using the same method of triangulation and that the instructor was saying something I don't get. Maybe I should go back to the other thread, as I put it on ignore...
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Old 23-01-2013, 00:35   #118
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by DeepFrz View Post
A straight ground track is the most efficient way to get from point A to point B in these examples.
Hours and hours were spent on the thread
Distinct Activities: Shackled by a Common Name?
by several people trying to convince another CF member that this was not the case. They eventually succeeded, much to the joy of those trying to convince him (including me although I threw in the towel long before a couple of other people did), as this is a truly important point to understand when sailing.
His post acknowledging his faulty thinking was one of the best posts I have ever read on CF!

In summary of what was proven, a single CTS is the most efficient was of getting from A to B NOT a straight ground track.
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Old 23-01-2013, 00:39   #119
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by DeepFrz View Post
Focusing on your ground track will keep you away from dangers. If the navigator of the USS Guardian had focused on his ground track and allowed a decent amount of room to go around a world heritage site he would not have hit the reef.
I fully agree, but your ground track does not have to be a straight line for you to focus on the dangers.
Determine your CTS to give you the most efficient time (it may be necessary for lots of reasons even when leisurely cruising), then see where the plot of your ground track is and check there are no dangers and if all is OK, follow the CTS .
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Old 23-01-2013, 00:41   #120
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by Dockhead View Post
Andrew, to elaborate a little on this -- I'm not sure that it is helpful to think of the destination as being shifted. It's not really. Imagine this -- if the intersection point is half an hour before you reach the destination, and the last half hour of tide behaves in the way favorable to the RYA method, you just keep sailing on your same heading you exactly reach the destination.

It is not so much destination shifting as a mathematical proxy for the other leg of the of the vector triangle. It does not steer you to a shifted destination necessarily, it steers you towards the real destination but using an averaged last partial hour. Using a line other than the course line will not work.
Dockhead

Once we've come to a common understanding on this point, I hope you will address any residual validity in my points a) and b)

OK?

I think your explanation and terminology seeks to explain the behaviour of the RYA method, whereas mine seeks to understand the underlying concepts.

(I don't think our explanations are at odds, FWIW)

It's as if you're describing what happens to the revs when I move the 'throttle' lever on a diesel engine.

whereas I'm saying "This is why it happens".

I think the latter description is more useful when the former description does not happen as expected.

- - - - -

Here's my attempt to explain the underlying process in conceptual terms, leading up to the phase you are describing:

(for simplicity, I'm assuming no errors in data or execution, a vessel motoring, and current data expressed as mean values for each hour):

The vectors for x whole hours of current are added, from the start point.

When enough vectors have been stacked that the last vector's endpoint (lets call it "C after x hrs", or "C...") is close enough that we can get within an hour of the true destination in x hours of steaming, a position "D" is marked on the rhumb line (a line passing through both the departure and the true destination.)

This mark is made at a distance from "C..." which corresponds to x hours of steaming.

(Sometimes this will not be possible, for geometric reasons, but that's for another discussion)

A line is now drawn from C... through D, and this is taken as the Course To Steer.

The length of "C...D" corresponds to the distance through the water from the departure point to D, after a period of x hours

My explanation pauses at this point, because this is the end of the conceptually simplest phase of the process, and the information we've now got is potentially useful. (eg "That's near enough; we can eyeball it from here")

A shortened version of what we've done so far, sounds like this to me:

"Work out the point D on the rhumb line which is nearest to the true destination, reachable after x whole hours. Work out the CTS for that destination"

(optional: Work out the distance through the water to that adjusted destination)

The next step is to get us to the true destination.

The distance through the water to the true destination is arrived at by what you call 'inflation' of the optional distance mentioned above. I don't think I need to go into that in any detail, because there is only one thing about the remainder of the process which seems relevant to my point, and that is this:

If we follow the same CTS we worked out for the adjusted destination, we will only pass through the true destination under the special circumstances you describe.

Under those circumstances, both the course to steer and the distance through the water will be correct.

Under any other circumstances, neither will be correct

But ...... IN ALL INSTANCES we will first pass through the 'adjusted' destination, at the time (and distance) we expect to.

So I think it's helpful to treat it as an interim destination. Sometimes it will be useful to use it as such, but conceptually I think it is always valid to think of it as such.

If we can't remember the fiddly bit at the end, we've still got something solid and comprehensible (and correct), and it's back on the rhumb line.

Which (as long as we KNOW that's what to expect) is generally going to be a good thing.



Please tell me which parts of this make sense to you, and which do not.
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