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Old 24-01-2013, 07:17   #211
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Re: Doctrine of the Imperative Triangle

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Originally Posted by Dockhead View Post
You are exactly solving a triangle
[... great explanation, see above ...]
The principle you have used to draw all of this is called the "Triangle Law of Vector Addition" -- see Vector Addition
Dockhead, that was well and correctly stated.
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Old 24-01-2013, 07:35   #212
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Re: Doctrine of the Imperative Triangle

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Originally Posted by Dockhead View Post
But you do use the distance, because that is what determines the position of B which allows you to measure an angle to it with your protractor.

You are exactly solving a triangle; you are just using an analogue method instead of crunching sines and square roots. I am sorry to be a pedant about it, but a lot of bombastic but erroneous things were said which really need to be cleared up.

This:

Attachment 53407

Is called a vector triangle. This is the "triangle in sight" in your method. It is nothing more and nothing less than that. It is a mathematical construct which is an analogue method of solving trigonometric (i.e. triangular) functions invented in the days when there were no digital means to easily crunch the numbers.

Your vector triangle consists of three lines, none of which bears any direct relationship to your passage. These three lines meet at three points. The lines are abstractions, every one. The course line defines the range and bearing between origin and destination. The tide vector line defines the sum of the individual tide vectors (themselves each a triangular function) -- the line defines the sum of all your work with the individual currents, and again it consists of range and bearing -- range is the net displacement, and bearing is the net direction. The water track line defines the distance you sail in the time frame of the triangle -- every leg corresponds to exactly the same time frame, which is THE essential relationship between them.

So when you read off the angle of the water line, you are receiving the output from these triangular functions and you get the pot at the end of that rainbow -- your CTS.

The principle you have used to draw all of this is called the "Triangle Law of Vector Addition" -- see Vector Addition
OK, OK I surrender LOL. You win .

But you don't need to draw them for this computation and you certainly don't need to draw a line between A and B.
Surely there can be no disagreement about that as I have done all my computations for all the examples without any line drawn between A and B?

I suggest one is not drawn as it just distracts people from thinking about where their ground track is (their intended course over ground in other words). It encourages them to plot the track, which I feel is essential, rather than assume their track over ground lies somewhere close to the line between A and B.
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Old 24-01-2013, 07:46   #213
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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
Example 3
The RYA instructor in the previous navigation thread requested a couple of times that I submit an example with oblique tides, varying in strength, so here goes .

Boat speed is constant at 4 knots throughout the journey.
You are motoring in flat water.
Destination is 11 nm due east.
Current:
1st hour: 8 knots 135 T
2nd hour: 6 knots 150 T
3rd hour: 2 knots 170 T
4th hour: 2 knots 10 T

What is the CTS?

Using the RYA method
Computations are plotted in the diagram below (note, these are usually made on a chart)

CTS = 46 degrees true

Perhaps Instructor Jackdale could confirm that I have plotted this correctly and that I come up with the same CTS that he does?
I will attempt this scenario, although I regard it as completely unrealistic. I must say that I would dissuade someone from venturing out into currents this strong in a vessel capable of 4 knots. The first part of the course will see the vessel being pushed back, which is incredibility poor seamanship.

Can you tell my where I would realistically find such currents? In the areas of the world with I am familiar, such strong currents only exist in passes in which we would find tidal rips, overalls and whirlpools. But what the heck.
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Old 24-01-2013, 07:52   #214
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Re: Doctrine of the Imperative Triangle

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(sigh) Deep Frz says the same thing about measuring A to B:

If SwL measures the distance, it's for interest only. It does not play any part in her method, nor does the line joining them.

In straightforward situations, measuring it can help with estimating how many hours of tide to allow before starting to check for being within 'striking distance', saving unnecessary premature checks.

In strong tides sweeping the boat far out to one side, it's not even helpful for that.

You can assert the contrary, but your assertion is based on supposition or assumption.

I suggest you ask SeaWorthy if she uses the rhumb distance for her method, and if so, what for.

It's remarkable that several individuals have suddenly developed more expertise in her method than she has herself.

How many times have you tried applying it using neither the rhumb line or triangles?

Unless you've tried, assiduously, and failed, you are in no position to claim they are indispensable.

As for the "tidal triangle", you are mistaking the representation for the thing.

Triangles are a tool to represent vectors.

One can also, if one prefers, use them to solve vector arithmetic. It's a fine method, one of several. (refer previous elaboration)

One could choose to draw a triangle as a representation of the tidal vectors in SeaWorthy's procedure as an extra and superfluous element, but it would not include the rhumb line. From the endpoint of those vectors, only one line is drawn. It does not form a side of a useful, let alone indispensable, triangle of which the rhumb line is the other side.

(re-sigh)

I have no interest in triangles, I only brought the subject up to refute DH's contention that the rhumb line must be an essential element of her method, because it was one leg of an essential triangle.

No, nope, nopity no. On both counts.

- - - - -

I don't see much merit in travelling further along this path because I can only reiterate the content of this and earlier posts.

It's was hoping to establish that the rhumb line is a key element in the RYA method and not the SwL method. It's not a controversial or difficult proposition. Surely if it was a key element SwL would draw it?

The reason I want to establish that proposition is that the dependence on the rhumb line is what (for purely geometric reasons) causes the RYA method to fail to provide a solution under some scenarios. The construction the method relies on is simply not possible.

I'm not bothered that SwL has got bogged down and is not yet addressing such scenarios.

Until a few more people have their 'penny drop' moment with the above proposition, there's not much chance of the issue being understood without another long-drawn-out, shambolic and energy sapping argument.

I agree with Andrew and would like to add that in general one needs to know
1. Where you are (A), and
2. Where you want to go (B).
Then you must decide how you will safely get there. Once you have used the RYA method, the SWL method or any other method for that mater, you must do a route study before you head out.

The departure will be similar with both the RYA method and the SWL method. From point A, apply the first tidal current hour to your chosen CTS. This will give you your track over the ground. for the first segment. It does not follow the imaginary line between A and B. You would then do this for the following segments. (I beleive that this is SWL's dashed red line.)

Simply put, the difference is at your destination. The SWL method the track will take you to point B. The RYA method gets you to point D and then you have to make a course correction to get to point B.

There is no need to have a line drawn from point A to B if you are not going to follow it. Knowing the distance from A-B is helpful for interest's sake, but it is not required to determine the CTS using the SWL method. You have plotted your desired course while doing your route study. For those who need to follow a line on your chart plotter then make hourly waypoints along your desired course that you have just plotted.
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Old 24-01-2013, 08:00   #215
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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I will attempt this scenario, although I regard it as completely unrealistic. I must say that I would dissuade someone from venturing out into currents this strong in a vessel capable of 4 knots. The first part of the course will see the vessel being pushed back, which is incredibility poor seamanship.

Can you tell my where I would realistically find such currents? In the areas of the world with I am familiar, such strong currents only exist in passes in which we would find tidal rips, overalls and whirlpools. But what the heck.
This trip is impossible. There is absolutely no way that it can be done. These currents are way too strong. If you method shows that the route is feasible, it is fatally flawed.
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Old 24-01-2013, 08:02   #216
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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You are confused about the purpose of a constant heading course. ....... Its purpose is to get you there faster by sailing a straight path through the water
No, not at all. I'm not sure why you would think that maintaining a single heading across a varying tidal steam would be fast. It's not.

Maintaining an optimal heading in relation to the tide being experienced is how you make the fast passage. This optimal heading obviously changes as a function of the tide strength and direction changing. If you are not addressing this change in tidal flow, you are not sailing your optimal course, so you will not be fast.

The single course to steer methods are simply taking the averages of the tidal flows to determine a single course that would give you a fairly good chance of arriving in the vicinity of your destination, assuming that the tide does roughly what the tide tables say it will. The particular methods in this thread don't address issues like wind impact, wave impact, boat design impact etc. but they could.

But I don't think either the RYA nor SWL have ever claimed their methods have anything to do with making a fast passage time.
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Old 24-01-2013, 08:37   #217
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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No, not at all. I'm not sure why you would think that maintaining a single heading across a varying tidal steam would be fast. It's not.
[...]
But I don't think either the RYA nor SWL have ever claimed their methods have anything to do with making a fast passage time.
Sorry, but between two points the proper constant heading will result in the least distance sailed, and thus the fastest passage (ignoring the wind, etc -- we're motoring in these examples). This applies regardless of current or current variations. The vector diagrams make this pretty obvious.

I believe that SWL has correctly claimed that her constant-heading gives the fastest passage.
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Old 24-01-2013, 08:44   #218
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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This trip is impossible. There is absolutely no way that it can be done. These currents are way too strong. If you method shows that the route is feasible, it is fatally flawed.
Are you a betting man Jackdale?
Would you like to put your money where your mouth is?

How does a bottle of fine single malt sound?
Nothing too peaty though please
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Old 24-01-2013, 08:44   #219
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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This trip is impossible. There is absolutely no way that it can be done. These currents are way too strong. If you method shows that the route is feasible, it is fatally flawed.
I am re-doing the numbers after a re-think. But there is a serious problem,

An important question.

How are you determining speed. Knotmeter, GPS, ?
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Old 24-01-2013, 08:45   #220
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Are you a betting man Jackdale?
Would you like to put your money where your mouth is?

How does a bottle of fine single malt sound?
Nothing too peaty though please
How are you determining boat speed? GPS, Knotmeter?
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Old 24-01-2013, 08:46   #221
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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No, not at all. I'm not sure why you would think that maintaining a single heading across a varying tidal steam would be fast. It's not.

But I don't think either the RYA nor SWL have ever claimed their methods have anything to do with making a fast passage time.
ROFL, why else do you want to sail a single CTS?
Because you want to make the FASTEST passage LOL. A couple of decades racing/cruising in Australia's tidal waters has taught me nothing if not that!
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Old 24-01-2013, 08:46   #222
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Hi again Bewitched
If you have one constant CTS and any cross current that is not constant for the entire journey, your "course over ground" if that is what you mean by your "intended course", has nothing to do with the line AB, so why draw it?
Ah.., OK all this may be terminology. The 'course line' in my mind is the line that is drawn on the chart to represent the 'intended course' that the boat should ideally take. I believe that the RYA call this the course line because that is exactly what it is intended to be. They do intend, in an ideal world, to sail along a straight line from A to B.

But, and it's quite a big but, this will be virtually impossible to do without positional information. So they opt for the next best thing, which is a method that gets them onto the course line at a defined point and defined time in the future.

Quote:
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Your "course over ground" (the track you will make on your chartplotter) is what you need to mark and consider once you have finished the computations .
This is your intended course for the journey.
OK, I didn't appreciate this aspect of your method. To make sure I understand properly, you are saying that the red dotted line on your diagrams is your intended course? How you intend to move the boat over the chart? The equivalent of what the RYA (and I) call a course line?

If so, I think you may have already addressed my one real comment about not having a course line. I think it could just have been lost in jargon.
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Old 24-01-2013, 08:47   #223
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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How are you determining boat speed? GPS, Knotmeter?
Knotmeter.
My speed through water the water is a constant 4 knots at a constant heading.
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Old 24-01-2013, 08:59   #224
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Ah.., OK all this may be terminology. The 'course line' in my mind is the line that is drawn on the chart to represent the 'intended course' that the boat should ideally take. I believe that the RYA call this the course line because that is exactly what it is intended to be. They do intend, in an ideal world, to sail along a straight line from A to B.

But, and it's quite a big but, this will be virtually impossible to do without positional information. So they opt for the next best thing, which is a method that gets them onto the course line at a defined point and defined time in the future.
The RYA are NOT suggesting in an ideal world that you want to sail along the course line with variable cross current.

And furthermore, the best thing is to get you to your destination LOL, not some point on the course line.

You would be prudent to nominate this spot a nm before the spot you want to anchor/moor etc, but whatever your nominated point, you are determining a CTS to get you there, NOT onto the course line.

Quote:
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OK, I didn't appreciate this aspect of your method. To make sure I understand properly, you are saying that the red dotted line on your diagrams is your intended course? How you intend to move the boat over the chart? The equivalent of what the RYA (and I) call a course line?
Yes

The red dotted line line I have been drawing is my intended course over ground to get me there in the SHORTEST time .
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Old 24-01-2013, 09:00   #225
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Jackdale, how about a wager of a bottle of fine single malt?
I am game if you are.

And by the way, do you agree that that a CTS of 46 degrees is the one you get using the RYA method?
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