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Old 17-02-2013, 18:02   #736
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Start->Click image for larger version

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trying to finish!->Click image for larger version

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aiming at Destination start to finish. Drawing's not perfect but close enough. This is what I "see" as a rhumb line argument versus a single CTS.
Faster corrections will keep it closer to the rhumb,but every increment of "pause,think, correct,alter" will take it further from it. Oversteering(damping) is going to be yet another issue that has to apply to any steering whatever, so it might be cancelled out for any method.
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Old 18-02-2013, 08:10   #737
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
(I have not bothered calculating the time taken using the RYA method as this method does not actually have you arriving at B unless you change course along the way.)

A minute difference in this case if you simply crab! Given the inaccuracy induced in the computed CTS by using average tidal stream data, I think you would be better simply crabbing in this example.

I had a look at a few other examples given by Jackdale and given low amount of average current relative to boat speed for reasonably short journeys, the time difference is also very small for these if you elect to crab.
SWL IN all cases, based on the underlying tide assumptions you get to B using the RYA, ( or more correctly with all methods you arrive as close as the underlying method and data assumptions allow), ie there is an error circle centered around B for SWL and RYA methods and you arrive inside that , in theory of course, in practice the real error circle is far larger.

IN the RYAs case the first upto mins of the next tide is assumed to be similar to the previous tide so you remain on the CTS and you arrive at B. thats the assumption

As you pointed out yourself, in typical tides and typical boat speeds, crabbing along the rhumb is better general strategy and the differences between this and single CTA is really small

Dave
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Old 18-02-2013, 09:52   #738
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Yes
All
these methods,RYA,Bowditch, whatever) are to instruct and avoid the scenario of NOT taking drift into account....

...where a clueless, hapless boat heading just "rhumb", finds herself miles downstream and lost.

Yes,goboatingnow -incremental "improvements" is what it comes down to comparing the methods.

SWL's method works. I just think it need be simpler explained. A simpler refinement of averages for instance won't lose any notable accuracy.

Determine this proportion accurately by measuring the distance from S to B (=SB) and also from B to L (=BL) and calculating:
SB / SB+BL
= 1.6 / 1.6+2.1 = 0.43

Multiply this by the strength of the current for this last hour and you now know the boat displacement due to current for this period.
= 0.8 x 0.43 = 0.3 nm

Mark 0.3 nm on the final current vector and label it K


I'd like the SWL method evolve a simpler method of "averaging "just using a ruler and /or a compass. I haven't considered what it would be, but I have an idea percolating that there is one.
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Old 18-02-2013, 10:20   #739
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

I'd like the SWL method evolve a simpler method of "averaging "just using a ruler and /or a compass....

but simply eyeballing a rough proportion along the displacement vector would work pretty well!
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Old 18-02-2013, 10:20   #740
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by HappySeagull View Post
Yes
All
these methods,RYA,Bowditch, whatever) are to instruct and avoid the scenario of NOT taking drift into account....

...where a clueless, hapless boat heading just "rhumb", finds herself miles downstream and lost.

Yes,goboatingnow -incremental "improvements" is what it comes down to comparing the methods.

SWL's method works. I just think it need be simpler explained. A simpler refinement of averages for instance won't lose any notable accuracy.

Determine this proportion accurately by measuring the distance from S to B (=SB) and also from B to L (=BL) and calculating:
SB / SB+BL
= 1.6 / 1.6+2.1 = 0.43

Multiply this by the strength of the current for this last hour and you now know the boat displacement due to current for this period.
= 0.8 x 0.43 = 0.3 nm

Mark 0.3 nm on the final current vector and label it K


I'd like the SWL method evolve a simpler method of "averaging "just using a ruler and /or a compass. I haven't considered what it would be, but I have an idea percolating that there is one.
Hi Hapless
Any new ideas would be fabulous. The RYA method definitely needs improving.

I think the only time it is really worthwhile finding a single CTS with variable cross current is when the average current is relatively high compared to boat speed. Unfortunately, this is where the RYA method can fail badly if D does not end up very close to B. (Note that for long journeys over one or more tidal cycles where it can also be advantageous, neither method is probably really required, just drawing a line between the end of the displacement due to current and the destination gives a close enough result).

One way of simplifying my method is simply to eyeball the proportion. It is a crude technique but with practice seems to produce a result within a couple of degrees.

Looking forward to hearing your thoughts .
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Old 18-02-2013, 10:21   #741
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by HappySeagull View Post
I'd like the SWL method evolve a simpler method of "averaging "just using a ruler and /or a compass....

but simply eyeballing a rough proportion along the displacement vector would work pretty well!
LOL, did you see we posted simultaneously .
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Old 18-02-2013, 10:47   #742
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

haha, that's funny alright. But good.I flapped off and did one. I got cts 355 and if this is good enough, then it makes SWL a lot easier for seagull brains and keeps closer to your original inspiration...pencil,paper protractor.


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added but there might be something slightly more elaborate wanted sometime ? as an option, but still simpler than measuring with rulers and computing.
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Old 18-02-2013, 11:00   #743
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by HappySeagull View Post
haha, that's funny alright. But good.I flapped off and did one. I got cts 355 and if this is good enough, then it makes SWL a lot easier for seagull brains and keeps closer to your original inspiration...pencil,paper protractor.
Yes, I like the idea of that .
I suggested this a while back, but was shot down for proposing a reduction of accuracy LOL.

Eyeballing the proportion works well in my opinion (I have done it first each time in all the examples I have worked through). Given the limitations of the tidal data, it gives me all the accuracy I need. It avoids the large errors produced by the RYA method with a negligible amount of extra work (determining the tidal stream data and plotting the displacements due to current is really the majority of the work).
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Old 18-02-2013, 11:36   #744
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

...and not to forget that following the rhumb is also a "method". just how easy is it to pop out exactly (?) every hour (added: or half hour?)and change the autopilot's heading? That is an assumption of that method unless a computer program is connected.

Yes, a single CTS is going to be further from the rhumb part of the time.But these examples are assuming we are tooling along in one increasing or decreasing current cycle...not an ebb changing to flood part way along etc. so they are still extreme in a sense, in the effort to illustrate the principles.

The next interesting bit is the constant argument about "data". I am working on that, but would be curious as to how the RYA explains drift. It may be perfectly explained in a chapter of an RYA book I don't have. But so far, I haven't been moved from my own peeve by the pages that were offered.
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Old 18-02-2013, 11:58   #745
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

oh, and Unfortunately, this is where the RYA method can fail badly if D does not end up very close to B.
I'm repeating myself and maybe it was mentioned before that, but what if I just "improved" the RYA method by using a CTS drawn as a 1/2hour increment, "where D does not end up close to B".

ie: instead of "estimate boat goes" xmiles in yhours, draw arc" maybe "xmiles in y hours+1/2hr, draw arc in certain situations" will "cure" the problem. This steals from your method but ...
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Old 18-02-2013, 11:58   #746
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by HappySeagull View Post
...and not to forget that following the rhumb is also a "method". just how easy is it to pop out exactly (?) every hour (added: or half hour?)and change the autopilot's heading? That is an assumption of that method unless a computer program is connected.
No need to do that at all. Just set the autopilot to head to a way point and it will keep you on the rhumb line.
Regardless, it is best to be out in the cockpit permanently to keep a good lookout for other seagulls and fish .

Quote:
Originally Posted by HappySeagull View Post
Yes, a single CTS is going to be further from the rhumb part of the time.But these examples are assuming we are tooling along in one increasing or decreasing current cycle...not an ebb changing to flood part way along etc. so they are still extreme in a sense, in the effort to illustrate the principles.
No need to assume that. The method also works well with changing tides. Over long journeys such as the English channel it can probably slash an hour or so during spring tides (it was a bit under half an hour shorter for the example Jackdale posted).

Quote:
Originally Posted by HappySeagull View Post
The next interesting bit is the constant argument about "data". I am working on that, but would be curious as to how the RYA explains drift. It may be perfectly explained in a chapter of an RYA book I don't have. But so far, I haven't been moved from my own peeve by the pages that were offered.
I will check out the RYA text I have when I have some free time and report back.
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Old 18-02-2013, 12:13   #747
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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oh, and Unfortunately, this is where the RYA method can fail badly if D does not end up very close to B.
I'm repeating myself and maybe it was mentioned before that, but what if I just "improved" the RYA method by using a CTS drawn as a 1/2hour increment, "where D does not end up close to B".

ie: instead of "estimate boat goes" xmiles in yhours, draw arc" maybe "xmiles in y hours+1/2hr, draw arc in certain situations" will "cure" the problem. This steals from your method but ...
Yep, this would certainly improve the RYA method!

But even more improvements could then be made if they considered the actual tidal stream for the last half hour, something they are loathe to do, preferring for some odd reason to apply the average of the tidal stream data that has already occurred.

Finally if they arced towards B rather than the rhumb line (I still don't get the obsession with the rhumb line when navigating a single CTS), then yes, the RYA method would start to work really well LOL.

I offer all my ideas without any strings attached LOL. I just hope they do something to improve the technique. It seems to take up a chunk of their navigation teaching (and it is examined) and it is included in their general text, yet the technique is truly flawed.
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Old 18-02-2013, 15:50   #748
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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
Yep, this would certainly improve the RYA method!

But even more improvements could then be made if they considered the actual tidal stream for the last half hour, something they are loathe to do, preferring for some odd reason to apply the average of the tidal stream data that has already occurred.

Finally if they arced towards B rather than the rhumb line (I still don't get the obsession with the rhumb line when navigating a single CTS), then yes, the RYA method would start to work really well LOL.
At the risk of sounding like a stuck record, this is what I did with my method. I've been calling it "my method" just to distinguish the way I approached the problem, but I consider it an extension of the basic tidal solution.
This 'obsession' as you call it, is necessary only as far as the rhumb line provides a reference for completing the tidal triangle solution; without it, there are too many "unknowns" to complete the equation. With your solution, you're basically comparing the deltas of the bracketed whole-hour boat vectors to estimate the length of the final part hour tidal vector. You can do the very same thing with the RYA method - for instance if the 2-hour triangle intercepts AB at C, short of B, and the 3-hour triangle intercepts the rhumbline past B at point D, then the ratio of CB and BD can be used the same way you use your KB (iirc) ratios to determine the final partial hour of tidal vector. As C B and D are on the same line, you can measure the last partial hour mathematically as CB/CD.

As I've said before I can't speak for the RYA, but it looks to me that they assume the more likely situation where tidal set goes from (for example)1.3 kts in the final whole calculated hour, to 1.1 kts in the final partial hour, so averaging the last 0-30 minutes won't be grossly inaccurate - certainly given all the other errors that "real life" will inevitably throw into the mix.
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Old 18-02-2013, 16:15   #749
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

"..Just set the autopilot to head to a way point and it will keep you on the rhumb line.""
...maybe yours does this, but mine just steers a course. It is very old, I suppose.
It doesn't even have a remote.

lodesman, I will try this:
You can do the very same thing with the RYA method - for instance if the 2-hour triangle intercepts AB at C, short of B, and the 3-hour triangle intercepts the rhumbline past B at point D,
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Old 18-02-2013, 16:32   #750
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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CTS is 356
and again, an eyeball would work along current vector too.
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