Inaccurate RYA Teaching : CTS - Quest For a New Method

[jackdale]

If I was ever in a situation typified by example three I would be getting a lot of fixes. And as soon as you get a fix, you need to redo your set and drift.

[jackdale]

Originally Posted by goboatingnow

Computing CTS of course to this level of accuracy is of course ridiculous anyway.

[Dockhead]

Originally Posted by goboatingnow

SO heres a summary of SWL method,

Its worth noting that either by design or luck , this puts B the destination almost midway between the two tides.

Note the only difference between SWL methods and the RYA, is that teh RYA would take a 3 hour plot and "deflate" all tides by the same proportion of minutes, where as SWL applies this to the last tide only.

IN practice and in the classroom, I have pointed out the issue to students where B ends up almost in the middle of the two plots, and have suggested that the answer just be averaged out forexample , my dodgy protractor gives me a CTS from D2(S) as 53 and a CTS from D3(L) of 64, A quick visual inspection shows B in approx the middle, so add 5.5 to D2(s) gives us a CTS of 58.5 - Voila.

In real life none of this matters, as thats not what happens, as D(S or L) comes either reasonably close ( as a proportion of the last tide as hence can in effect be ignored, or is very far away and more tides have to be plotted.

Computing CTS of course to this level of accuracy is of course ridiculous anyway.

If you do two RYA calculations for the full hour before, and the full hour after, and calculate the linear proportion between them corresponding to the proportion of the last hour represented by the last partial hour, you will get precisely the same answer as you will get by using SWL's method. But SWL does it with a couple of swipes of the dividers, which is damned good.

In my opinion, this is eminently worthwhile. We will separately study the possible precision of these calculations given the level of precision of the input data.

[goboatingnow]

Step 6: Draw a line from the end of the 2 hour vector and mark off 8 nm and label that S

Step 7: Draw a line from the end of the 3 hour vector and mark off 12 nm and label that L

Despite SWL, saying she doesnt use the rhumb, Where do you mark off the S and L, SWL , you dont say. IN reality the "rhumb' line is irrelevant you are doing vector addition. hence the arc must close the triangle , which happens to be the rhumb line. ( from a vector addition perspective)

[goboatingnow]

Originally Posted by Dockhead

If you do two RYA calculations for the full hour before, and the full hour after, and calculate the linear proportion between them corresponding to the proportion of the last hour represented by the last partial hour, you will get precisely the same answer as you will get by using SWL's method. But SWL does it with a couple of swipes of the dividers, which is damned good.

I dont see that , she computes the ratio of time in the last tide, applies that ratio of time to tide vector and therefore correctly produces in effect a final 30 minutes ( approx in this case) tide plot.

This method assume that the final tide is constant over the hour. In practice of course it isnt. ( and since you interpolate from High Water,SWL is just as error prone as any other method)


In the few cases like this example, simple visual interpolation like I outlines two post ago , gives an answer within 0.5 degrees of Seaworthy. Given that the tidal data is visually interpolated to begin with , its as good as any,
"OMG Ive invented a new method"
Dave

[Dockhead]

Originally Posted by goboatingnow

I dont see that , she computes the ratio of time in the last tide, applies that ratio of time to tide vector and therefore correctly produces in effect a final 30 minutes ( approx in this case) tide plot.

This method assume that the final tide is constant over the hour. In practice of course it isnt. ( and since you interpolate from High Water,SWL is just as error prone as any other method)


In the few cases like this example, simple visual interpolation like I outlines two post ago , gives an answer within 0.5 degrees of Seaworthy. Given that the tidal data is visually interpolated to begin with , its as good as any,
"OMG Ive invented a new method"
Dave

The RYA method, by inflating the full hour vector triangle, applies the average tide of the whole passage to the last bit. sWL's made averages just the last hour. It is a big difference in precision, and not a worthless one, I think.

[goboatingnow]

Originally Posted by Dockhead

The RYA method, by inflating the full hour vector triangle, applies the average tide of the whole passage to the last bit. sWL's made averages just the last hour. It is a big difference in precision, and not a worthless one, I think.

Ive said this several times on the other thread,

The RYA method only assumes at the very worst a +- 30 minutes "inflation or deflation" Secondly all good instructors point out the case where you are right on or near the 30 minute boundary. The RYA method does not blindly "inflate " the triangle ad infinitum ,

As Ive demonstrated, simple visual interpolation is sufficient in the case of the close to 30 minute issue. Or replot using more accurate tides , say 30 minute tides where that is available,

Remember SWL claims better accuracy then the RYA method ,

(a) Only if you blindly apply it, which you are not taught in a class to do.

(b) ONLY , where the assumption applies that the hourly tide DOES rmain constant. In practice and from the way tides are lifted from atlas, or chart diamonds, there are far greater errors in the vectors, then any method corrects for. SWLs method "is not more precise", it is only "more precise" if one applies the axiom " The tide does not change over the hour applied" In reality that statement doesn't hold any validity.

Its merely pixies on the head of a pin stuff

IN practic in the application of multi hour complex tides, gross errors appear that dwarf teh theory. This causes either eyeball corrections or return to hourly plots.

Note that in sinusoidal reversing tides you get more precision then actually is computed as the tidal vector errors get cancelled out. ( its not a good one to use to examine any CTS method).


Theres a reason multi hour CTS are not generally taught outside the RYA, The method relies on underlying error prone data and is subject to massive real life errors.

In reality Ive never seen it used in complex situations, as the ground track ( as we've discovered) can be very hard to predict. ( leaving aside the poster child). Its too risky a method in typical areas where there are big tides and usually big obstructions!

[cal40john]

Originally Posted by Seaworthy Lass

Well I wouldn't waste my time in that sort of current either if I had to rely on the RYA method for a CTS .



I was motoring too. If you stayed home for two hours before departing the cross current would have been just as bad in the opposite direction and worst still would have been against you rather than with you!

So you would have literally gone backwards initially!

At the time I went although there was a fair amount of cross current, there was just as much pushing me to my destination.

And when you finally decided to go what method for determining CTS would you have selected?

I would miss the 8 knot and 6 knot currents and only have the 3rd and 4th hour 2 knot currents nearly perpendicular to my course one north one south. Are you now saying that there will be a west bound current the fifth hour?

I think I'll play around with your method, but for the sailing around here the current is wi' me or agin' me. I usually only have to worry about timing things for when the current will help me if severe or not go through choke points when it's running 7 knots.

You can do fun things like ebb is to the south in the San Juan Islands and to the north in Admiralty Inlet (Puget Sound). Caught the ebb out of the Islands and as it turned to a flood while I was in the Straits of Juan de Fuca caught the flood into Puget Sound, never saw less than 8 knots over the bottom.

NOAA Charts with animated currents. Silverlight Design and Development


John

[Seaworthy Lass]

Originally Posted by Dockhead

It exactly uses the analogue method which has existed for hundreds of years to overcome the lack of number crunching power in the pre-digital age . Brilliant simplicity. Fantastic; truly inspired. Unbelievable, really. I'm in awe (pending final confirmation of my understanding of it ).

What it does is simply finds the proportionate average set and drift of the full hour of which the final partial hour is a part, and uses that as the final jink of the vector sum leg.

It is - again, if I understood it correctly - mathematically perfect IF we assume that any part of a whole hour is average. This is definitely not true in reality, but it's as good as we can get without assuming something about the shape of the current development through time.

In any case, it is simple to apply, and the built-in error is approximately an order of magnitude smaller than what is produced by the RYA method. This is really good stuff

Bravo, Seaworthy

Yes Dockhead it is as easy as that. It is nothing really complicated.

And yes, it does assume the current is reasonably constant during that hour, but once you get good with the method you could tweek that proportion yourself according to whether the current was increasing or decreasing.

If you didn't want to measure the proportion, then just eyeball it. I have tried that for a few examples and I am never more than a degree off in the final result, compared to measuring the overshoot and undershoot with a ruler and calculating the proportion.

And the accuracy is always good (ignoring the limitations of the data) if you use a very sharp pencil and are very careful with the vector placement. With a bit of practice one to two degree accuracy could be expected.

And this works for ANY data you throw at it, not just limited conditions of low current coming from certain directions and not reversing at the end of the journey and ..........

Unlike the RYA method you will ALWAYS be within a couple of degrees and there will never be any figures that are wildly out .

[goboatingnow]

Originally Posted by Seaworthy Lass

Yes Dockhead it is as easy as that. It is nothing really complicated.

And yes, it does assume the current is reasonably constant during that hour, but once you get good with the method you could tweek that proportion yourself according to whether the current was increasing or decreasing.

If you didn't want to measure the proportion, then just eyeball it. I have tried that for a few examples and I am never more than a degree off in the final result, compared to measuring the overshoot and undershoot with a ruler and calculating the proportion.

And the accuracy is always good (ignoring the limitations of the data) if you use a very sharp pencil and are very careful with the vector placement. With a bit of practice one to two degree accuracy could be expected.

And this works for ANY data you throw at it, not just limited conditions of low current coming from certain directions and not reversing at the end of the journey and ..........

Unlike the RYA method you will ALWAYS be within a couple of degrees and there will never be any figures that are wildly out .

IU dont mind you arguing for your own method, I really dislike your characterisation of the RYA one. YOUR method is not real world precise, no more then any CTS is real world precise.

Ive demonstrated repeatedly how the RYA method is applied and in that situation as I said above, it could be simply visually interpolated, ( I got 58,5 degrees CTS as a result)

IN the real world , the vector addition results in far more obvious closer situations, in which case the RYA provides the same answer

The RYA method has stood the test of real navigators in the real world, within its limitations , like all limitations it is as accurate as any, apply with intelligence and as real world sailors it will be fine

[Dockhead]

Originally Posted by goboatingnow

Ive said this several times on the other thread,

The RYA method only assumes at the very worst a +- 30 minutes "inflation or deflation" Secondly all good instructors point out the case where you are right on or near the 30 minute boundary. The RYA method does not blindly "inflate " the triangle ad infinitum ,

As Ive demonstrated, simple visual interpolation is sufficient in the case of the close to 30 minute issue. Or replot using more accurate tides , say 30 minute tides where that is available,

Remember SWL claims better accuracy then the RYA method ,

(a) Only if you blindly apply it, which you are not taught in a class to do.

(b) ONLY , where the assumption applies that the hourly tide DOES rmain constant. In practice and from the way tides are lifted from atlas, or chart diamonds, there are far greater errors in the vectors, then any method corrects for. SWLs method "is not more precise", it is only "more precise" if one applies the axiom " The tide does not change over the hour applied" In reality that statement doesn't hold any validity.

Its merely pixies on the head of a pin stuff

IN practic in the application of multi hour complex tides, gross errors appear that dwarf teh theory. This causes either eyeball corrections or return to hourly plots.

Note that in sinusoidal reversing tides you get more precision then actually is computed as the tidal vector errors get cancelled out. ( its not a good one to use to examine any CTS method).


Theres a reason multi hour CTS are not generally taught outside the RYA, The method relies on underlying error prone data and is subject to massive real life errors.

In reality Ive never seen it used in complex situations, as the ground track ( as we've discovered) can be very hard to predict. ( leaving aside the poster child). Its too risky a method in typical areas where there are big tides and usually big obstructions!

Whether it's pixies on the head of a pin is an empirical question, which we will be dealing with anon.

It will always (but for the coincidences at the limits) be more accurate than RYA. It is hardly any trouble, and less trouble than judiciously applying RYA in the manner you suggest it should be. It is not a panacea, as currents don't develop linearly over any hour. But as a method - results gained for a given quantum of effort - it's tits. Brilliant.

[goboatingnow]

And this works for ANY data you throw at it, not just limited conditions of low current coming from certain directions and not reversing at the end of the journey and ..........

Unlike the RYA method you will ALWAYS be within a couple of degrees and there will never be any figures that are wildly out

Nope it relies on the tide remaining constant within the hour, In pratice it doesnt.

Its as error prone as any other method.

[Seaworthy Lass]

Originally Posted by Dockhead

If you do two RYA calculations for the full hour before, and the full hour after, and calculate the linear proportion between them corresponding to the proportion of the last hour represented by the last partial hour, you will get precisely the same answer as you will get by using SWL's method. But SWL does it with a couple of swipes of the dividers, which is damned good.

In my opinion, this is eminently worthwhile. We will separately study the possible precision of these calculations given the level of precision of the input data.

Ah, Dockhead, that is precisely my "method version one" that I got all excited about in the previous thread LOL.

The reason it was discarded was that it refers to the rhumb line. Check out example 3 in this thread. That original method of mine would have failed there as well as there never was a D before B, the first one was after .

[goboatingnow]

t will always (but for the coincidences at the limits) be more accurate than RYA. It is hardly any trouble, and less trouble than judiciously applying RYA as you suggest. It is not a panacea, as currents don't develop linearly over any hour. But as a method - results gained for a given quantum of effort - it's tits. Brilliant.

It is not DockHead. Its gives a more correct result on paper, and only so if one applies the axiom "The tide remains the same for the hour under consideration".

IN reality it doesnt and the values interpolated from the tide tables often have significant lack of precision in them. Thats makes SWL method no worse or better then any other

IN practice the real tides may introduce a chain or errors that could benefit SWL method, or the RYAs one or vice versa.

Its a fools gold to teach students increasing precise calculations on original imprecise data.

[goboatingnow]

Ah, Dockhead, that is precisely my "method version one" that I got all excited about in the previous thread LOL.

The reason it was discarded was that it refers to the rhumb line. Check out example 3 in this thread. That original method of mine would have failed there as well as there never was a D before B, the first one was after

SWL nones to my knowledge has actually seen a plotted example of method on a chart , The discussion about the rhumb line is entirely misplaced, its merely a vector addition