Inaccurate RYA Teaching : CTS - Quest For a New Method

[jackdale]

Originally Posted by Seaworthy Lass

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?

It is a heuristic and propaedeutic device.

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?

I gave a realistic scenario of a passage based on the area in which I sail. The currents in the Strait are mild, but we must account for them.

[Andrew Troup]

Originally Posted by jackdale

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....

I will stay in one spot if the weather is poor.

Well, I applaud and share those sentiments, but good luck with 'staying in one spot' if the weather is 'poor' in the Western Approaches.

Even oil rigs would be pushed to do that, so I'm not sure what you'd tie up to; supertankers are notorious for failing to maintain station in that location.

And if you can cross the Atlantic in under seven days, and/or predict the weather on the far side before leaving, you're in the wrong profession.

I ask again, how much experience do you have in dealing with really challenging tidal conditions: at the far end of a passage taking some weeks?

[Seaworthy Lass]

Originally Posted by Seaworthy Lass

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).

Originally Posted by jackdale

OK I am calling BS on that one. The largest tidal ranges in the world are in Canada. No where near my cruising grounds.

Sorry, you are right I was working from memory. Even Seaworthy Lasses make mistakes when they are tired. I have just Googled it and Derby has the highest tidal range in Australia with 11.8 metres (39 feet).

[Lodesman]

Originally Posted by Andrew Troup

Navigators are taught to think of vectors as triangles.

Presumably this is because triangles are a useful way to represent vectors. One of several useful ways.

No no no noppity nope!

The only one here who has said that vectors are triangles, is you. The sides of the triangles in a vector equation are vectors - nothing more. While it may be possible to solve vectors in other manners, to the sailor with nothing more than a chart and a pencil, using the triangular method has got to be the simplest. If you have two vectors, you can solve for the third - that is the crux of this argument. In both the RYA's and SWL's methods this vector equation is being solved. In the so-called multi-hour passage plan, you actually have to solve for two vectors - SWL's method essentially draws two final vector triangles then interpolates between the two resultant vectors to estimate the actual tidal set vector, from which we determine the CTS vector.

PS. Pilots solve 2D triangles too - for the same reasons; wind is like set.

[LJH]

Originally Posted by Seaworthy Lass

Sorry, you are right I was working from memory. Even Seaworthy Lasses make mistakes when they are tired. I have just Googled it and Derby has the highest tidal range in Australia with 11.8 metres (39 feet).

Glad you caught yourself on that one. After living near the Fundy shore for 10 years, I was about to jump in before Jack posted the article about the tides.

[Lodesman]

A number of peeps have touched on this, but none have come out and stated the obvious. As I've said, I have never needed to calculate a single CTS accounting for varying set. I can see the utility in it, especially in situations like crossing the English Channel. Now a question about how it is actually used:

With the standard "plot the course line, steer to maintain the course line" method, it is very easy to see if what you're doing is working and adjusting as necessary. We all know that currents give us max and slack times, so the space in between is something in between. All of the plans (RYA, SWL, mine) assume constant set during the hour, then instantaneous change and constant set for another hour, etc. We all know that's not what happens. So you make your calculations, use SWL's method, get your CTS and head off on it. Assume also that being a prudent nav-god you plot out your planned COG. After awhile you take a fix and find yourself off your planned path - what do you do?

Do you calculate actual set and adjust to maintain your planned track, or do you put faith in your calcs and hope that it will all average you back in the end???

Quite frankly, I think this was Dave's point that planning to the nearest half hour of arrival (as per RYA method) is perfectly useful, as at some point you are going to need to account for inherent fudge factor and amend your CTS.

[goboatingnow]

I think this thread has now become a circular argument and its purpose has been lost.


Dave

[Seaworthy Lass]

Originally Posted by Seaworthy Lass

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?

Originally Posted by jackdale

It is a heuristic and propaedeutic device.

You leave students to discover for themselves that the RYA use an unreliable method for determining CTS?

The fact is that the RYA method will not work in all circumstances as I have shown in a few examples. I could churn these out by the dozen.

You may say that the examples I have presented are unrealistic, but here are places in the world with large tidal ranges and surely logic dictates that any method taught to deal with cross current should produce an accurate result regardless of what data is thrown at it.

Yes, tidal data may be hard to determine accurately, but when a method can give results that are 10 or 20 or 30 degrees out and there is an alternative method that produces an accurate result why not consider adopting it? How else do we progress?

There is nothing wrong with a "rough and ready" method, but it is extremely important to recognise it as such and not try to teach that its accuracy is limited only by the data input

When some RYA instructors insist that the method they teach is mathematically correct, I consider this a problem .

[Andrew Troup]

Originally Posted by Lodesman

The only one here who has said that vectors are triangles, is you.

You're getting me mixed up with Dockhead.

I have occasionally been called a d*ckhead, maybe that's why.

I've lost count of how many times I've said vectors are NOT triangles.

My recent post started by a verbatim quote, where Dockhead said they were, which I carefully refuted.

I eagerly await enlightenment on any weak links or structural problems with my refutation.

[goboatingnow]

Originally Posted by Lodesman

A number of peeps have touched on this, but none have come out and stated the obvious. As I've said, I have never needed to calculate a single CTS accounting for varying set. I can see the utility in it, especially in situations like crossing the English Channel. Now a question about how it is actually used:

With the standard "plot the course line, steer to maintain the course line" method, it is very easy to see if what you're doing is working and adjusting as necessary. We all know that currents give us max and slack times, so the space in between is something in between. All of the plans (RYA, SWL, mine) assume constant set during the hour, then instantaneous change and constant set for another hour, etc. We all know that's not what happens. So you make your calculations, use SWL's method, get your CTS and head off on it. Assume also that being a prudent nav-god you plot out your planned COG. After awhile you take a fix and find yourself off your planned path - what do you do?

Do you calculate actual set and adjust to maintain your planned track, or do you put faith in your calcs and hope that it will all average you back in the end???

Quite frankly, I think this was Dave's point that planning to the nearest half hour of arrival (as per RYA method) is perfectly useful, as at some point you are going to need to account for inherent fudge factor and amend your CTS.

A sensible question. In practice when faced with a multi hour CTS computation and facing complex tides I question the usefulness of the multi hour CTS. I personally , plot one to two hour tidesl plots and drop that EP onto the chart and into the chartplotter. ( im a fan of the drop waypoints ahead )

I will have done a course review of any strong tides that are likely to affect me. If they are significantly adverse compared to my speed. I will seek to plan my passage to either avoid them or just wait.

Other then that I will work up new CTS ( often using just the 1in 60 rule ) and give that to the helm. This allows me to stay reasonably close to the rhumb, but at least account for the tide immediately ahead of me. I will revise my CTS. Based on actual tide data.

Within sight of land I will almost always use transists to guide me home especially in strong tides and or back eddies.

Dave

[Lodesman]

Originally Posted by Seaworthy Lass

Well I would not have overshot it LOL, I would have stopped when I got there. If your calculations are correct, being 4 minutes out with my time estimate is insignificant. I have rounded figures off to the closest 0.1 nm as this was a limitation of my measurements, so that is how the small discrepancy occured.

I thought the whole point of your method over RYA's was that you wouldn't need to adjust your CTS

Stopping short wouldn't help as you're approaching from the South, you pass 2C East of the destination.

Originally Posted by Seaworthy Lass

Had the 'prudent' sailor waited two hours I think he would not have made any progress against the current (in fact he would have gone backwards).

He would have made great progress during hours 3 and 4. You never stated the current in the 5th hour.

[Seaworthy Lass]

Originally Posted by LJH

I for one like the SWL method as it plans to get me to my destination. It only took a few mouse clicks, and less than a minute, to come up with a solution to her last example in OpenCPN. Not appreciably longer than the RYA method. takes. So since the SWL method as a planning tool gets me a plan to where I want to go, is a much better than a planning tool that doesn't?

Great that my method was resonably easy to use!

Thanks for reporting back LJH.
'Night all, getting late here. Thanks for all the debate. Exchange of ideas is a healthy thing

[goboatingnow]

Originally Posted by Seaworthy Lass

You leave students to discover for themselves that the RYA use an unreliable method for determining CTS?

The fact is that the RYA method will not work in all circumstances as I have shown in a few examples. I could churn these out by the dozen.

You may say that the examples I have presented are unrealistic, but here are places in the world with large tidal ranges and surely logic dictates that any method taught to deal with cross current should produce an accurate result regardless of what data is thrown at it.

Yes, tidal data may be hard to determine accurately, but when a method can give results that are 10 or 20 or 30 degrees out and there is an alternative method that produces an accurate result why not consider adopting it? How else do we progress?

There is nothing wrong with a "rough and ready" method, but it is extremely important to recognise it as such and not try to teach that its accuracy is limited only by the data input

When some RYA instructors insist that the method they teach is mathematically correct, I consider this a problem .

Your method depends on assuming the tide behaves as you write it down. It doesn't

The RYA , certainly , I do , clearly explains to its students the limitations in multi hour CTS computations. Multi hour is only really taught at YM level. Good students realise and are taught that all these " models" have errors.

Again seaworthy you prescribe accuracy to data that isn't accurate. Pulling numbers out of the air makes it look right but unless you work through it from the extraction of the tide data yout can't really see your chasing chimeras.


The RYA method is mathematically correct. Are you arguing that my triangles arnt correct. All you are arguing is that YOUR method on inflating the last triangle is better then the RYAs method of inflating the average. You have advanced no information to suggest that tides have abrupt time boundaries nor compared real life examples of both methods to clearly illustrate why yours is superior. You have persisted in dreaming up mathematical numbers to illustrate the point forgetting how those numbers are extracted from the data and how the data is derived in the first place.

And you persist in the face of two experienced people constantly trying to point out not your errors of mathematical execution, but your under lying assumptions.

The idea that you compute some sort of single CTS that let's you drive into the Marina berth say 10 hours in advance is just laughable in real life ( please DH don't mention crossing the channel !!)

Dave

[goboatingnow]

Originally Posted by Seaworthy Lass

You leave students to discover for themselves that the RYA use an unreliable method for determining CTS?

The fact is that the RYA method will not work in all circumstances as I have shown in a few examples. I could churn these out by the dozen.

You may say that the examples I have presented are unrealistic, but here are places in the world with large tidal ranges and surely logic dictates that any method taught to deal with cross current should produce an accurate result regardless of what data is thrown at it.

Yes, tidal data may be hard to determine accurately, but when a method can give results that are 10 or 20 or 30 degrees out and there is an alternative method that produces an accurate result why not consider adopting it? How else do we progress?

There is nothing wrong with a "rough and ready" method, but it is extremely important to recognise it as such and not try to teach that its accuracy is limited only by the data input

When some RYA instructors insist that the method they teach is mathematically correct, I consider this a problem .

Just to hone in on your specific issues.

( 1) you haven't churned out any examples. You have plucked tides from the air. you have applied the maxim that time boundaries are immutable , does a 2kn tide go to 0.5 in 0 seconds of course not. Take sone real tidal data you will see that he averaging method is as good as any other


(2) tidal range has nothing to do with tidal set or drift. They are related by the relationship is often complex. There are many places that have significant set and drift but little range for example.

(3) no evidence been advanced to suggest the RYA method properly applied ( and you have never allowed anyone to show you how it is properly applied you persist on using a You tube video ) generates anything like 30 degrees out, that's simply true.

I'm really annoyed at this nonsense. You've turned a theoretical argument into a vendetta against the RYA. It doesn't add one iota to the advancement of anything.

You asked me to outline the RYA method I did so, I also put in all the provisos that apply , I provided a you tube link to give you a run at the basics its not the exhaustive run through of the whole tide and CTS module of the Yacht master theory course . In class the limitations of the extraction of tides is looked at as are warnings as to what level of precision should be ascribed to them.

Your argument boils down to an argument over the validity of tidal data. But no one had advanced any arguments to or against this issue ( OK Dockhead and I started a little session for a bit )

Dave

[Lodesman]

Originally Posted by Andrew Troup

You're getting me mixed up with Dockhead.

I have occasionally been called a d*ckhead, maybe that's why.

I've lost count of how many times I've said vectors are NOT triangles.

My recent post started by a verbatim quote, where Dockhead said they were, which I carefully refuted.

I eagerly await enlightenment on any weak links or structural problems with my refutation.

You've said it twice:

Originally Posted by Andrew Troup

A triangle is just one way of representing a vector,

Originally Posted by Andrew Troup

Navigators are taught to think of vectors as triangles.

Presumably this is because triangles are a useful way to represent vectors. One of several useful ways.

Near as I can tell Dockhead mentioned "vector triangle" which is clear from the context means "triangle using 3 vectors as sides".

Is that a structural problem or a weak link?