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Old 24-01-2013, 14:05   #286
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by jackdale View Post
This is part of the passage planning process. During the execution of the plan you had better be getting a fix and then recalculating everything. I want to be able to do that quickly. Any who followed either method for 4 hours without monitoring progress is a fool.
Yes, but if you start with an error of 10 or 20 or 30 degrees in your compass heading (which the RYA method in some circumstances can give you, as I have illustrated) it does not bode well for the voyage .
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Old 24-01-2013, 14:06   #287
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by jackdale View Post
This is part of the passage planning process. During the execution of the plan you had better be getting a fix and then recalculating everything. I want to be able to do that quickly. Any who followed either method for 4 hours without monitoring progress is a fool.

For coastal cruising I want a fix every 30 minutes, for near coastal every hour. For water every watch change.
I agree with all of that, but it's not really relevant to the discussion.

But speaking from the real experiences of a good number of English Channel crossings, the navigator is doing a terrible job if you have to correct your heading every hour. We don't use any of the methods discussed here; we have our own traditional Channel ones I have described elsewhere. We end up with cumulative set relative to the rhumb line, and you don't need to take any "fixes" to check your progress - you just check XTE output by your plotter against cumulative set you predicted every hour. Simples. You should be bang on if your average speed is working out according to plan. We have very accurate tide data and often manage a complete crossing, 60 miles on the rhumb line and can be 70+ miles through the water, without any course correction before the final hour. It's usually a beam reach in steady F5 or F6 in a W or SW wind, and we all know quite accurately what speed we make in that, so we achieve quite a high degree of precision, usually.
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Old 24-01-2013, 14:09   #288
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Just to demonstrate the foolishness of a "more precise" methodology.

(a) The tides are not constant over any particular hour
(b) The tide you plot is based on an arrival time at a particular point in time,( errors of time) you may not arrive there when you say you might, invalidating the figure in the first place.
(c) A multi hour CTS can result in significant deviation from the planned rhumb line, resulting in different tides actually affecting the boat.( errors of position)

All these means that the actual tide data you extract has often very significant errors in it, Hence the method is ascribing more "precision" then is there to begin with.

All these means that the assumptions built into the RYA method and SWL method all result in errors, It doesnt mean seaworthys method is more precise, its means it merely applies the assumption of data consistency rather more accurately in theory. A theory that in practice may not apply at all.
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Old 24-01-2013, 14:11   #289
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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But speaking from the real experiences of a good number of English Channel crossings, the navigator is doing a terrible job if you have to correct your heading every hour. We don't use any of the methods discussed here; we have our own traditional Channel ones I have described elsewhere. We end up with cumulative set relative to the rhumb line, and you don't need to take any "fixes" to check your progress - you just check XTE output by your plotter against cumulative set you predicted every hour. Simples. You should be bang on if your average speed is working out according to plan. We have very accurate tide data and often manage a complete crossing, 60 miles on the rhumb line and can be 70+ miles through the water, without any course correction before the final hour. It's usually a beam reach in steady F5 or F6 in a W or SW wind, and we all know quite accurately what speed we make in that, so we achieve quite a high degree of precision, usually
Dockhead, you really have to stop going on about teh channel, its a poster child for multi CTS and about the only situation you can apply it safety. please look at typical complex multi hour non symetrical tides.

Thats the common situation , hourly or maybe two hourly plots are the best you can do.
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Old 24-01-2013, 14:31   #290
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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
Yes, but if you start with an error of 10 or 20 or 30 degrees in your compass heading (which the RYA method in some circumstances can give you, as I have illustrated) it does not bode well for the voyage .

1) The example 3 is extreme and unrealistic. As many sailors especially those with expereince in big tides and currents have pointed out, we would not go out in those conditions, which woulf only exist in narrow channels and be subject to numerous hazards.

2) If, in reality, I ever go into that situation, I would use eye ball navigation in the final phase of that passage.

In reality, I would construct a new vector for every 30 to 60 minutes after monitoring the progress.

Get real.

Just how much experience do you have in tidal conditions?
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Old 24-01-2013, 14:34   #291
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by goboatingnow View Post
Dockhead, you really have to stop going on about teh channel, its a poster child for multi CTS and about the only situation you can apply it safety. please look at typical complex multi hour non symetrical tides.

Thats the common situation , hourly or maybe two hourly plots are the best you can do.
Dockhead is also using a chartplotter. I am not sure about RYA, but all of my coastal navigation courses are based on traditional (non GPS) navigation methodologies. We start with the basics before getting into electronic navigation.
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Old 24-01-2013, 14:35   #292
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Just to demonstrate the foolishness of a "more precise" methodology.

(a) The tides are not constant over any particular hour
(b) The tide you plot is based on an arrival time at a particular point in time,( errors of time) you may not arrive there when you say you might, invalidating the figure in the first place.
(c) A multi hour CTS can result in significant deviation from the planned rhumb line, resulting in different tides actually affecting the boat.( errors of position)

All these means that the actual tide data you extract has often very significant errors in it, Hence the method is ascribing more "precision" then is there to begin with.

All these means that the assumptions built into the RYA method and SWL method all result in errors, It doesnt mean seaworthys method is more precise, its means it merely applies the assumption of data consistency rather more accurately in theory. A theory that in practice may not apply at all.
Yep.
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Old 24-01-2013, 14:43   #293
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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.
Dockhead

The position of B is not determined by distance.

It's determined by being a point we are trying to reach.

Once again, you're substituting one representation ('this is how far it is from us') for the thing itself.

B can be specified in numerous ways which make NO reference to where we are when we decide to go to B.

Lat/Long is one.

This fixation with where we have departed from is presumably what Seaworthy battled with, and dropping that fixation was (she tells us) crucial to developing the method you have hailed as brilliant.

I wish my whimsical parable had worked for you, because it spelled out the chartwork of the Seaworthy method in a way I hoped would help others make the same mental leap she has evidently made.

Once the first hour's tidal vector is put on the chart, the point of departure is of NO FURTHER INTEREST in the solution.

(We're NOT solving for ground track, which DOES rely on the point of departure; that's a separate procedure her method does not address)

The remainder of the procedure is carried out with no reference to it, and if that part of the chart got masticated by the ship's baby, it wouldn't matter a damn.


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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.
......

The principle you have used to draw all of this is called the "Triangle Law of Vector Addition" -- see Vector Addition
Dockhead

I'm sorry that you wore Seaworthy down on this point, because it seems to me she was correct.

The link you posted says this:



Common methods adding coplanar vectors (vectors acting in the same plane) are
  • the parallelogram law
  • the triangle rule
  • trigonometric calculation
------
They don't say "only methods", they say "common methods"

Another common method, available only to hominids with opposable thumbs and pencils, is a graphical method based on the purer concept of a vector spelled out in my last post.

The triangles you see in the graphical method are imported to it by your mindset.

In your terms, the triangles your mind's eye conjures are a construct.

They are sufficient to solve vectors, but they are NOT necessary.


You are welcome to carry on thinking in triangles; but for those with opposable thumbs there is the simpler option of thinking in straight lines.
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Old 24-01-2013, 14:54   #294
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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1) The example 3 is extreme and unrealistic. As many sailors especially those with expereince in big tides and currents have pointed out, we would not go out in those conditions, which woulf only exist in narrow channels and be subject to numerous hazards.

2) If, in reality, I ever go into that situation, I would use eye ball navigation in the final phase of that passage.

In reality, I would construct a new vector for every 30 to 60 minutes after monitoring the progress.

Get real.

Just how much experience do you have in tidal conditions?
Everyone talks as if we somehow get to pick our location and departure times, as if we were sitting in the yacht club bar waiting for the right phase of the tide.

What if we're caught up in a severe multi-day blow coming up to the Western Approaches to the English Channel, en route from the Americas?

So severe that we're forced to run off, with only perhaps a 20 degree sector of discretion over our track, if we're lucky.

I refer people to Dockhead's lyrical description in an earlier post, of the various and considerable tidal current challenges which await.

We don't always get the luxury of 'waiting' (hovering?) for the right moment, and the right start point, to negotiate them.

If we do end up in the wrong place at the wrong time, the best we can hope is that conditions moderate to the point where we can actually take charge of our heading and speed again, and do some fancy planning -- because otherwise we're in a spot of bother, chaps.

Just how much experience do you have in really challenging tidal conditions, at the far end of a major passage?
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Old 24-01-2013, 14:57   #295
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Im not sure what is being discussed, But I beleive the key issue is that a chart is not neccessary to determine CTS, the rhumb line is not needed.

AND its not needed in the RYA method either. As has been pointed out all we are doing is using ONE way to solve vector calculations.

Neither the RYA or seaworthy method ( as they are in essence the same) need a rhumb line, You merely construct a graphical solution to a vector addition issue.

It often confuses students doing vector addition on a chart that somehow, they represent actual paths over which the boats goes etc.

Dave
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Old 24-01-2013, 15:09   #296
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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If we do end up in the wrong place at the wrong time, the best we can hope is that conditions moderate to the point where we can actually take charge of our heading and speed again, and do some fancy planning -- because otherwise we're in a spot of bother, chaps.

Just how much experience do you have in really challenging tidal conditions, at the far end of a major passage?

Again the nature of the extraction of tide data means there are significant errors in using it to predict the future, then you add the effect of the boat itself.

hence performing long multi hour CTS calculations in complex tides is a somewhat futuile process. One that students often undertake and then realise that in real life the errors make the method moot to a large degree. Its particularly salient in regards sailing vessels, where the tide can be a large percentage of the vessels speed. Hence inaccuracies in the underlying tidal data and the boats progress mean they are particularly prone to being wrong.

Hence any prudent sailor does exactly as Jackdale does, You WAIT, either at sea or in harbour. You wait to clear bars and marinas cills, you WAIT to time your arrival at tidal gates etc. We can generally always go slowly.


It would be the height of folly to compute a large multi CTS in real life ( leaving aside Dockheads poster child situation) and then sail off with it tucked under your arm.

(a) Firstly it may be difficult to determine ground track and hence ensure clearance

(b) (a) may be compounded by deviations from the theoretical progress rendering much of the pre-calulation moot.

(c) The rhumb line , is often picked to facilitate arrival at a particular point often at navigation markers etc to verify position. This is hard to do with a combined passage CTS.

in practice , on such journeys you can only reliably look ahead for a couple of hours. I prefer to remain "reasonably close" to my intended track, by only computing CTS for 1-2 hours. It means that if im wrong , I not terribly wrong. I except that I am not sailing the theoretically most efficient course or method , if I wanted that Id use a 747
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Old 24-01-2013, 15:22   #297
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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The example 3 is extreme and unrealistic. As many sailors especially those with expereince in big tides and currents have pointed out, we would not go out in those conditions, which woulf only exist in narrow channels and be subject to numerous hazards.
So you admit the RYA method only works for certain conditions? If so, do you warn students that your technique will fail terribly in scenarios like this?

In the last example I presented you would get exactly the same error in the CTS (out by a whopping 37 degrees) if you halved the current and the boat speed. Boat speed may be low as perhaps the prop is damaged.

I note that in the last example you asked me to compute so that you could illustrate just how accurate the RYA technique could be, you presented a situation with just one knot and then half a knot over only half of a 16nm journey. What on earth was that supposed to prove? That the method worked if you didn't have much current?

Quote:
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If, in reality, I ever go into that situation, I would use eye ball navigation in the final phase of that passage.
Eyeballing doesn't help if you have put yourself in a situation where you can't make any progress in the last portion of the journey due to adverse tide

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Get real.
Just how much experience do you have in tidal conditions?
That comment is a bit below the belt. There is no need to be rude. The instructor on the previous thread gave me a lot of grief and it is just totally unnecessary.
Let's work together to result in a better technique to be put under consideration given the inadequacies of the RYA method to determine the CTS have come to light.

By the way, I raced and cruised Australia's tidal waters for over tweny years.
While many of the tides significantly less, some places have an extreme tidal range of 16 m (over 50 feet, significantly more than the 18 foot tidal range with 16.5 knot current you quoted below).

I certainly would not want to be using the RYA method in these conditions!!!!

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I live with very large tides (18 foot range) and currents (16.5 knot maximums).
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Old 24-01-2013, 15:30   #298
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Seaworthy , please stop crusading against the RYA, it ill becomes you. You have taken a theory and have not tested it against real world conditions, Your theory works ONLY because of your assumptions about tide data and you ascribe a precision to them that simply isnt there.

Think of it like this

what you are doing is computing an Estimated position into the future. All such techniques of working up EPs , suggest that the navigator draw a circle of error. Such an error circle increases as one predicts more and more hours into the future.

Hence at the end you have quite a large circle. Any method that puts you inside that circle is valid. The argument about who is closer cannot be had as the data isnt there.
Quote:
The instructor on the previous thread gave me a lot of grief and it is just totally unnecessary.
IM that person and I have a name BTW. I didnt give you grief, I maintained what you were doing was fundementally wrong as you are ascribing precision to data that simply isnt that precise. We have no way of knowing what you have done is in real life is any better. We do know that the RYA method has been tested extensively by its adherents.

Its a fundamental mistake to "invent" precision where it doesnt exist. If you wish to engage on a multi year programme to show that your method corresponds in reality to what is really out there , then I applaud your commitment. Until such time its an unproven method, The RYAs isnt.

Quote:
I certainly would not want to be using the RYA method in these conditions!!!!
You do know that the RYA is the official provider of Yachting Australia certification programmes.
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Old 24-01-2013, 15:34   #299
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Andrew - before I venture out into any of the situations, I go through a lengthy planning process. Involving up to a 7 day weather forecast, tide and current situations (some of our passes can only be transited with great care. (An example North to the Broughtons - SailNet Community). We have to wait for the turns in this passage. I have one chance a day to cross the Nahawitti Bar at the end of Vancouver Island.

I will stay in one spot if the weather is poor. The most dangerous item on a boat is a calendar. I have abandoned plans to go around Vancouver island because of the gale / storm force winds. And I have waited out hurricane force winds.

I wait to pick my times to leave Lahaina for Vancouver.

The planning model I like used to be here http://www.portsmouthport.
co.uk/pmsc/pm21001.htm
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Old 24-01-2013, 15:40   #300
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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
While many of the tides significantly less, some places have an extreme tidal range of 16 m (over 50 feet, significantly more than the 18 foot tidal range with 16.5 knot current you quoted below).

I certainly would not want to be using the RYA method in these conditions!!!!


OK I am calling BS on that one. The largest tidal ranges in the world are in Canada. No where near my cruising grounds. http://docs.informatics.management.d...6-2004-153.pdf

I was curious about your experience. We get folks from Ontario where there are no tides, who have no comprehension of their power.

BTW - halfing currents and speeds will produce the same results, it is called mathematics.
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