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Old 22-01-2013, 11:35   #46
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by mcarling View Post
There is a good reason why experience leads one to over-estimate the precision of the method, which I will try to explain. There are multiple sources of imprecision. Each of them is independent from the others and each can be wrong in either of two (or more) directions. Most of the time, the errors will not all be in the same direction, so the errors will usually tend to cancel each other, to some extent. However, occasionally, all the errors can be in the same or similar directions and compound each other. In a hundred ocean crossings, most people would never see all the errors line up in the same direction but, on rare occasions, it will happen.
This is very true, and that is why, instead of making bombastic pronuncimentioes based on some WAG, I am going to test my hunch with an experiment. My hunch may not be right, but it at least derives from multiple data points.
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Old 22-01-2013, 11:37   #47
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by Dockhead View Post
This is very true, and that is why, instead of making bombastic pronuncimentioes based on some WAG, I am going to test my hunch with an experiment. My hunch may not be right, but it at least derives from multiple data points.
Go Dockhead!
Looking forward to seeing results .
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Old 22-01-2013, 11:47   #48
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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
.

Several times I called for help from members to chime in and support me that the RYA method was simply an approximation and would take you to D not B and there was DEAD SILENCE from everyone. Now suddenly everyone is saying "of course it is imprecise" .
Where were all these comments when I was struggling on my own?


:.

Speaking for myself - because I only figured out the RYA method today . Unlike some people, I refrain from commenting on things I know I don't understand

I still don't understand your method, but I can confidently confirm that it gives precisely the right answer for this particular problem

To understand fully your method, I will need protractor and chart and a non-perpendicular tide problem.
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Old 22-01-2013, 11:51   #49
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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
Several times I called for help from members to chime in and support me that the RYA method was simply an approximation and would take you to D not B and there was DEAD SILENCE from everyone. Now suddenly everyone is saying "of course it is imprecise" .

Where were all these comments when I was struggling on my own?
In my defence, I lost the will to live on that thread a while back , so missed the start of this thread. In any event, my maths simply not good enough to get into an argument with anyone! If not for those minor details I would have jumped in .

But given that my method of course adjustment for tides does very intentionally involve the WAG factor! the fact that one mathematical method may be superior to another of no great matter to me.

The WAG Method© used both because having known error works in practice (if only for being clear that the position is not precise, especially at the destination) but also simply because my maths in the warm and dry not up to much on paper, let alone in my head when cold, wet and tired! - and IMO of fundamental importance for any method to be easy to use in practice, and IMO worth sacrificing accuracy to acheive that, especially when that accuracy is largely theoretical.....

....I will try and put something on paper at some point, if only for the entertainment of others .
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Old 22-01-2013, 11:57   #50
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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In my defence, I lost the will to live on that thread a while back
You and forty-eleven others, I suspect....

PS : This is a nice thread, though, so far ... it's like waiting in the trenches on a sunny day, waiting waiting waiting for the shelling to start again, no?
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Old 22-01-2013, 12:35   #51
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

I agree that the essential problem with the RYA method is that they use the rhumb line as a geometric construction aid.

This isn't a big problem in the usual case, but in unusual cases, their construction technique, to interpolate the final hour of tide, is simply not possible, using the rhumb line. In such cases, it needs to be swung through 90 degrees.

To my mind, they compound the above problem by dumbing down the objective, shifting the destination to some arbitrary place on the rhumb line.

Presumably this is to pander to those for whom interpolation might be confusing, and I have considerable sympathy for the proposition that "It's better to be approximately right than exactly wrong" (which is possibly where mcarling is coming from)

Once again, this facet of the RYA method is not a big problem in the usual case, but like most dumbed-down rules, it misleads the user by diverting them away from an understanding based on first principles.


- - - - -

I'm personally not inclined to learn a prescriptive method (whether RYA or Seaworthy Lass's considerable improvement) for problems like this which I'm hardly ever going to encounter. It's too much for my brain to carry a recollection of so much detail, with so little prospect of return on investment.

I'd feel quite differently if my home port was on one side of the English Channel (as the Brits rather self-referentially refer to "La Manche" ).
Then I think Seaworthy's method would be 'top of brain' for me.


Getting back to my actual situation: I personally prefer to take the time, if and when such rare occasions arise, to sit down and derive a procedure from first principles.

It probably won't need to be particularly accurate, but it can be designed to suit the requirements, and the available data, of the actual situation.


The following works for me, but everyone's brain is wired differently:

All I expect to have to remember for such a procedure to take shape with minimal mental contortions is three things: I'm indebted to whoever introduced the ant on the treadmill (unless the tides are unidirectional, I discard the treadmill), to Seaworthy for nudging me towards a checkered tablecloth to represent a multidirectionally mobile ocean, and finally: the crampons!

The checkers remind me to break each hour's tidal set into Northing and Easting. These can then easily be totted up for the aggregate; no plotting required. (although I would always plot from the raw tidal data, as a cross check)
Perhaps the last hour's tide can be expressed as Northing per hour, and Easting per hour...

The crampons are to remind me, pointedly, to :

PLOT THE FRIGGEN GROUND TRACK before setting off.
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Old 22-01-2013, 12:39   #52
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by Andrew Troup View Post
You and forty-eleven others, I suspect....

PS : This is a nice thread, though, so far ... it's like waiting in the trenches on a sunny day, waiting waiting waiting for the shelling to start again, no?
LOL, I think I will be the only one shelling this time LOL.

In the last example the RYA method was 9% out.

The next example is about to be posted.
The RYA method is out 14% in this next simple example .

Wonder how long before CF members start to say "well, maybe this is an unacceptable amount of error".

Then how much longer before they say "this is pretty pathetic".

The RYA method may give a great approximation in lots of case, but it fails badly in others. This needs to be recognised and not ignored .
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Old 22-01-2013, 12:58   #53
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

I have read the thread quickly and have not done the examples. I did watch the RYA video.

As a Coastal Navigation Instructor for three different standards (Canadian Yachting Association, International Sail and Power Association and International Yacht Training), the RYA (and sailtrain) method is the one I teach and use.

It is also the method used in Bowditch - page 103s-104 (http://msi.nga.mil/MSISiteContent/St...N/Chapt-07.pdf)

As well as other navigation manuals, such as Chapman Piloting, that I use as reference.

For very long distances in tidal waters in which the tidal stream changes velocity and direction, additional calculations are needed.

I will go back through the thread and see if I can understand the issues.
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Old 22-01-2013, 13:00   #54
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by Andrew Troup View Post
I agree that the essential problem with the RYA method is that they use the rhumb line as a geometric construction aid.

. . .

The crampons are to remind me, pointedly, to :

PLOT THE FRIGGEN GROUND TRACK before setting off.
Andy, the use of the rhumb line turns out not to be the problem with the RYA method. I apologize for any confusion created by my earlier mistake in thinking so. The problem is the failure to calculate any tidal vector for a partial hour at the end of the passage, instead "inflating" or "deflating" the vector triangle, in effect fudging the uncalculated partial hour using the average of the rest of the passage.

The ground track is not usually a problem when you're out of sight of land
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Old 22-01-2013, 13:08   #55
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by jackdale View Post
I have read the thread quickly and have not done the examples. I did watch the RYA video.

As a Coastal Navigation Instructor for three different standards (Canadian Yachting Association, International Sail and Power Association and International Yacht Training), the RYA (and sailtrain) method is the one I teach and use.

It is also the method used in Bowditch - page 103s-104 (http://msi.nga.mil/MSISiteContent/St...N/Chapt-07.pdf)

As well as other navigation manuals, such as Chapman Piloting, that I use as reference.

For very long distances in tidal waters in which the tidal stream changes velocity and direction, additional calculations are needed.

I will go back through the thread and see if I can understand the issues.
Hi Jackdale: Your link to Bowditch is for a simple one-hour single vector. How to deal with that is uncontroversial. We have been arguing about something orders of magnitude more complex - how to calculate a single optimal CTS for a multi-hour passage across tidal waters with varying currents.
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Old 22-01-2013, 13:20   #56
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Andy, the use of the rhumb line turns out not to be the problem with the RYA method. I apologize for any confusion created by my earlier mistake in thinking so.
Thanks for the apology, but you'd have to address the specific concerns I raised to convince me your change of heart is justified.

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The problem is the failure to calculate any tidal vector for a partial hour at the end of the passage, instead "inflating" or "deflating" the vector triangle, in effect fudging the uncalculated partial hour using the average of the rest of the passage.
If so, that's a third problem I didn't consider.

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The ground track is not usually a problem when you're out of sight of land
Hmmm - I thought there were shoal patches in the Channel?

Given the prevalence of airborne moisture in them parts, I also assumed land was out of sight more often than it was out of mind...

(Talking about a part of the world I've only traversed on ships, or most recently by train - on my way, incidentally, to a very much unfruitful encounter with the RYA!)
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Old 22-01-2013, 13:22   #57
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

EXAMPLE TWO
MY METHOD


Boat speed is constant at 3 knots throughout the journey.
Destination is 6 nm due east.
Current is always from the north:
1st hour 3 knots
2nd hour: 1 knots
3rd hour: zero

Extend the distance travelled at the end of 2 hours (6 nm) from the end of the 2 hr vector. It falls short of B.
Try for 3 hours (falls past B).
So the journey takes between 2 and 3 hours and the vector for the third hour is under consideration.

No need for any calculations of what proportion we need as it is zero.
So simply label the tip of the sum of the vectors at the 2 hour mark (= same spot as the 3 hour mark), label it K and extend a line from K to B.

The CTS is the direction of the line KB = 56 degrees
Time taken = length of the line KB divided by boat speed = 7 / 3 =
2.3 hours

PS I wrote the wrong time on the diagram (divided by 4 instead of 3, too may examples lately with 4 knots LOL). So ignore the time on the diagram. The rest is OK.
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Old 22-01-2013, 13:25   #58
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

EXAMPLE TWO
RYA METHOD


Same conditions as above.
Boat speed is constant at 3 knots throughout the journey.
Destination is 6 nm due east.
Current is always from the north:
1st hour 3 knots
2nd hour: 1 knots
3rd hour: zero

CTS = direction of the line CD = 48 degrees
Estimated time taken = extrapolated length to B divided by 3
= 8 / 3 = 2.7 hours (
PS Please ignore the time on the diagram - I divided by 4 instead of 3, as forgot speed had changed in this example. Even Seaworthy Lasses are not infallible

The RYA result is 14% out !
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Old 22-01-2013, 13:31   #59
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Hi Jackdale: Your link to Bowditch is for a simple one-hour single vector. How to deal with that is uncontroversial. We have been arguing about something orders of magnitude more complex - how to calculate a single optimal CTS for a multi-hour passage across tidal waters with varying currents.
The RYA method is a good approximation. Even over an hour current can change substantially. If you want a more accurate CTS then you need to account for leeway which can be done in advance only if you know the type of vessel you are sailing, the wind speed and the point of sail.

The problem with current velocity "predictions" is that they are subject to all sorts of meteorological conditions. Barometric pressure that is not normal (1013 mb) will lower or raise tides which will influence currents. Wind has a huge impact on current. Some of the early current prediction software had wind as a factor that could be input.

To be honest when I cross Georgia Strait which has changing currents especially when you get near the Fraser River, which can have strong outflow and requires a separate calculation, I use my tide tables to estimate what the current will be about half way across. Crossing the Strait to get into the Gulf Islands is problematic because one of the passes has not anchorages in which you can wait.

I do not think that any method can make an accurate prediction.

Like I tell my students about plotting. A three bearing fix (or better a couple or more transits) is the best position, an estimated position is good so is a running fix, a DR is the worst - but it beats doing nothing.
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Old 22-01-2013, 13:52   #60
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Sure, but how much is that? Hundredths of degrees? Tenths? Tens of degrees? You have no idea. This, without more, is meaningless, and you have made statements unsupported by knowledge which you actually possess. It's an empirical, quantitative question - the degree of imprecision of the data, and to what degree of imprecision that translates to in the result.
The speed of tidal flows is, at best, known to 2 significant figures. Therefore, any calculation with the speed of the tidal flow as an input can never have more precision than 2 significant figures.
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