Inaccurate RYA Teaching : CTS - Quest For a New Method

[Andrew Troup]

Deep Frz

If you need to paddle (or worse, swim) across a stream whose midstream current is as fast as you can travel, you will never get there if you maintain station on a perpendicular rhumb line. No matter if you have unlimited endurance at that speed.

On the other hand, if you paddle at a constant heading, you can cross a river flowing twice as fast as you can paddle (if you can keep it up long enough) and still end up at the same point on the opposite bank as following the perpendicular rhumb line.

Of course, the quickest way across is the "Third Way": to point the bow on a perpendicular heading. You will reach the other side, further downstream (assuming laminar flow, no rocks, eddies or bends) in the same time as if the river was not flowing.


But this, too, is a constant heading strategy, not a rhumb line strategy.

Neither strategy is a "single answer to all problems", and anyone promoting one to the exclusion of the other should think hard about the quality of their advice.

ON EDIT: I entirely agree that it's crucial to know where you plan to be, and where you actually are, at all times, in challenging waters.

So it's essential (and not difficult) to be able to plot the ground track for your proposed Constant Heading passage (if that's the appropriate strategy for the situation). Then check the clearance on all dangers en route, and work out escape plans for downtide dangers which are uncomfortably close, should the plan not measure up to reality.

The other part - keeping track of where you are (and monitoring whether it's where you planned to be) is very easy, if you have functioning GPS.

Even if you don't, it's generally not difficult, in most tricky sailing waters, but I guess that will change eventually if lights and marks continue being phased out.

[Andrew Troup]

Here's my light-hearted but serious attempt to disprove Dockhead's proposition that Seaworthy's method relies on the rhumb line. I promise there won't be a single ant.

I also hope my story will explain how to apply her rather elegant method, and possibly even help some people deeply understand and (if they wish) remember it.

I'm going to separate her process into two stages, and relate those to a thought experiment, in a modified real world situation.

In this thought experiment, the two stages provide a way of simplifying (and, I hope, clarifying) the process.

In the real passage, the boat is swept successively in various directions by current.

At the same time, we're complicating things horribly, by motoring on a constant
(ooops – non-varying!) heading.

In the thought experiment, we arrange for the whole "current thing" to happen first while we drift, sunbathing optional.

Then when Stage I is over, we're on duty. We turn the current off, and do the "motoring thing" in a nice straight line, through water and over land. (Stage II)

Provided we motor for an equal duration to the timespan we'd already spent being set in various directions by currents, we'll end up at the same (real world) destination if we point the boat on the same heading as in the real world. And each timespan (I & II) will equate to the single real world timespan.

Conceptually, I hope you'll agree the model is a whole lot simpler than reality.

Those who are sitting at the front will however note that we will not arrive at the drill rig from the same direction as in the real scenario.

This is because our model represents the purest form of the "ignore the ground track" school of thought, for which Dockhead's former enthusiasm was notable, and heart-warmingly infectious ;-)

The entirety of Seaworthy's method consists of running Stage II of this model three times. The three runs are almost identical in nature.

The first time, we stop the current machine after a whole number of hours x, when we determine that we would overshoot if we carried of for another whole hour before starting the "motoring thing".

We motor straight towards the drill rig for x hours. Then we get out a tape-measure and see how far short of the rig we are.

Then we rewind to where we started motoring, turn the tidal current back on for a whole 'nother hour. When we arrive at this new patch of ocean, we turn the current machine off, re-apply sunblock, and once again motor straight towards the rig.

This time we pass between the legs and carry on past it, on the same heading, until we've clocked up x+1 hours under motor on this run.

Ditto with the tape measure.

(Note that we can do these two passes in the reverse order, say if we overshoot before realising)

The third time is the lucky last: we now know, from comparing the distances we measured, what fraction of an hour y/z we should have continued running the current machine, so that on this third run we will arrive exactly at the destination after motoring for x + y/z hours.

Provided we topped up the stupid little tank on the frigging military surplus Seagull outboard with the frayed starting rope ... curse those confounded imaginary budget cuts (no disrespect to other Seagulls of the Happier persuasion)

NOTE FROM HER IMAGINARY ROYAL HIGHNESS: "We are not amused. Our loyal subjects will note that the Exchequer was considerably depleted pursuant to the requisition for the Current Machine. Our pockets are deep, but there are limits."

Here endeth the gospel about "the Immaculate Conception by SeaWorthy, founding member and first President of the Republican Boating Association". Not a bad sheila, and not at all stuck-up.
　
Now, Dockhead: can you Spot The Rhumbline in any of these three runs of Stage II?

For the life of me, I can't.

In fact, once we've been swept to the position after x hours (which commences Stage II), if we fell overboard trying to start the outboard (nasty things!), the only thing our surviving crew would need to know to arrive at the destination using the three runs of stage 2 of the model, would be WHEN we started, but not WHERE.

'Ah yes', you might say, 'but how about Stage I?'

OK: Imagine doing Stage I in a fog. We can't see either our departure point, or our destination.

No problemo. No sunbathing ... but there wasn't anything else to do, if you think back.

All we have to hope is that the fog will clear before x hours have elapsed. Provided we can see the drill rig by then, we're home free. (Or, more accurately, into Stage II)

Rhumb line? Schmum line !

We can't very well draw one, if we can never see both ends at the same time.

What's more: no triangles were involved at any point, whatsoever. No trig.

No sextants, even (d'OH !!!)

Now... who wants to have a go at editing the story to reflect the (simpler) RYA method?

[Dockhead]

Originally Posted by Andrew Troup

Here's my light-hearted but serious attempt to disprove Dockhead's proposition that Seaworthy's method relies on the rhumb line. I promise there won't be a single ant.

I also hope my story will explain how to apply her rather elegant method, and possibly even help some people deeply understand and (if they wish) remember it.

I'm going to separate her process into two stages, and relate those to a thought experiment, in a modified real world situation.

In this thought experiment, the two stages provide a way of simplifying (and, I hope, clarifying) the process.

In the real passage, the boat is swept successively in various directions by current.

At the same time, we're complicating things horribly, by motoring on a constant
(ooops – non-varying!) heading.

In the thought experiment, we arrange for the whole "current thing" to happen first while we drift, sunbathing optional.

Then when Stage I is over, we're on duty. We turn the current off, and do the "motoring thing" in a nice straight line, through water and over land. (Stage II)

Provided we motor for an equal duration to the timespan we'd already spent being set in various directions by currents, we'll end up at the same (real world) destination if we point the boat on the same heading as in the real world. And each timespan (I & II) will equate to the single real world timespan.

Conceptually, I hope you'll agree the model is a whole lot simpler than reality.

Those who are sitting at the front will however note that we will not arrive at the drill rig from the same direction as in the real scenario.

This is because our model represents the purest form of the "ignore the ground track" school of thought, for which Dockhead's former enthusiasm was notable, and heart-warmingly infectious ;-)

The entirety of Seaworthy's method consists of running Stage II of this model three times. The three runs are almost identical in nature.

The first time, we stop the current machine after a whole number of hours x, when we determine that we would overshoot if we carried of for another whole hour before starting the "motoring thing".

We motor straight towards the drill rig for x hours. Then we get out a tape-measure and see how far short of the rig we are.

Then we rewind to where we started motoring, turn the tidal current back on for a whole 'nother hour. When we arrive at this new patch of ocean, we turn the current machine off, re-apply sunblock, and once again motor straight towards the rig.

This time we pass between the legs and carry on past it, on the same heading, until we've clocked up x+1 hours under motor on this run.

Ditto with the tape measure.

(Note that we can do these two passes in the reverse order, say if we overshoot before realising)

The third time is the lucky last: we now know, from comparing the distances we measured, what fraction of an hour y/z we should have continued running the current machine, so that on this third run we will arrive exactly at the destination after motoring for x + y/z hours.

Provided we topped up the stupid little tank on the frigging military surplus Seagull outboard with the frayed starting rope ... curse those confounded imaginary budget cuts (no disrespect to other Seagulls of the Happier persuasion)

NOTE FROM HER IMAGINARY ROYAL HIGHNESS: "We are not amused. Our loyal subjects will note that the Exchequer was considerably depleted pursuant to the requisition for the Current Machine. Our pockets are deep, but there are limits."

Here endeth the gospel about "the Immaculate Conception by SeaWorthy, founding member and first President of the Republican Boating Association". Not a bad sheila, and not at all stuck-up.
　
Now, Dockhead: can you Spot The Rhumbline in any of these three runs of Stage II?

For the life of me, I can't.

In fact, once we've been swept to the position after x hours (which commences Stage II), if we fell overboard trying to start the outboard (nasty things!), the only thing our surviving crew would need to know to arrive at the destination using the three runs of stage 2 of the model, would be WHEN we started, but not WHERE.

'Ah yes', you might say, 'but how about Stage I?'

OK: Imagine doing Stage I in a fog. We can't see either our departure point, or our destination.

No problemo. No sunbathing ... but there wasn't anything else to do, if you think back.

All we have to hope is that the fog will clear before x hours have elapsed. Provided we can see the drill rig by then, we're home free. (Or, more accurately, into Stage II)

Rhumb line? Schmum line !

We can't very well draw one, if we can never see both ends at the same time.

What's more: no triangles were involved at any point, whatsoever. No trig.

No sextants, even (d'OH !!!)

Now... who wants to have a go at editing the story to reflect the (simpler) RYA method?

LOL. An imaginative story. A bit like the fairytale of Andrei Strelok, whom the tsar orders to "Go hence, I know not where, bring back something, I know not what ("Poidi tyuda, ne znayu kuda, prenesi chto, to ne znayu" ). But it's not applicable. It's not a navigation problem at all, if we don't know where we're going, but rather, have to motor around with a magic tape measure figuring it out.

I'll give a different example which is applicable:

You are on the shore of an island where there is a perfectly circular rotary current. You want to get to the shore of another island which is on the same perfectly circular rotary current at the same radius from the center of movement.

How do you get there most efficiently?

There is an added kink in this we have not been considering -- what is the optimum speed? And different speeds will give different navigation solutions.

Since we have a net gain in VMG from the current, we can actually drift, or for a better navigation solution, let's say sail at bare steerage, 2 knots.

And so we can work out a solution. And yes, Andrew, the course line is part of it. The course line is nothing other than one possible representation of the bearing and distance to the destination, which you must know to solve any normal navigation problem. You don't need to draw it as a line on a chart, but you need to have these two data -- otherwise you don't even know where you're going! We can think of many cases where efficient navigation takes us over a ground track that has nothing to do with the rhumb line at all, of course. But that doesn't mean we don't need bearing and distance to destination.

In my example, the water track line might be only one cable long, as we burble along maintaining bare steerageway, being swept right to the destination by the current, but the navigation solution is still a triangle, a weird one, but a triangle, and the hypotenuse is still the course line, even thought that has Sweet Fanny Apple to do with our ground track, never intersecting it, and only touching it at the very ends, and our heading is still the angle of the tiny little water track, in that solution.

Bearing and distance to destination are part of every normal navigation problem. Otherwise we might just as well be Christopher Columbus -- sail around until you run aground on India. That's not navigation, if we don't know where we're going.

[bewitched]

I've been following this thread with interest. These are my comments and observations. I hope they are objective and not too critical.

The RYA method attempts to give a single course to steer that would in x hours put you on the course line and close to your destination or waypoint (B). It aims to get you moving over the chart as close to your intended course line as is possible if restricted to only steering a single course for some reason.

Therefore, the course line is not simply a construct from which to draw tidal and CTS vectors as has been suggested in this thread, it is fundamentally the course that you intend to take, over the ground, that keeps you away from danger.

The SWL method, as far as I can make out, and please correct me if I'm wrong, attempts to provide a single CTS that takes you directly to the destination (B) and therefore it is accounting for that little bit of distance at the end that the RYA method ignores. So naturally the CTS will be slightly different for each method because they are aiming at different end points.

So the CTS in the RYA method aims to get you to a point just short of B with a single CTS, whereas the SWL method aims to get you directly to B with a single CTS.

They are both planning methods which must be verified where possible by observation during the passage. However, as a planning method, the SWL approach does have the serious flaw that there is no intended course plotted onto the chart. So how do you know if things are going to plan or not?

[Dockhead]

Originally Posted by bewitched

I've been following this thread with interest. These are my comments and observations. I hope they are objective and not too critical.

The RYA method attempts to give a single course to steer that would in x hours put you on the course line and close to your destination or waypoint (B). It aims to get you moving over the chart as close to your intended course line as is possible if restricted to only steering a single course for some reason.

Therefore, the course line is not simply a construct from which to draw tidal and CTS vectors as has been suggested in this thread, it is fundamentally the course that you intend to take, over the ground, that keeps you away from danger.

The SWL method, as far as I can make out, and please correct me if I'm wrong, attempts to provide a single CTS that takes you directly to the destination (B) and therefore it is accounting for that little bit of distance at the end that the RYA method ignores. So naturally the CTS will be slightly different for each method because they are aiming at different end points.

So the CTS in the RYA method aims to get you to a point just short of B with a single CTS, whereas the SWL method aims to get you directly to B with a single CTS.

They are both planning methods which must be verified where possible by observation during the passage. However, as a planning method, the SWL approach does have the serious flaw that there is no intended course plotted onto the chart. So how do you know if things are going to plan or not?

ALL constant heading passages over moving water take you off the rhumb line, including the RYA method.

It is most definitely not part of the RYA method, any more than it is part of Seaworthy's, to try to keep you on "the course that you intend to take, over the ground, that keeps you away from danger." None of the constant heading approaches even considers the ground track. In a typical Channel crossing, you will deviate 12 miles or more from the rhumb line, whether you are navigating by RYA method, Seaworthy's, or mine. To "keep away from danger" you have to figure out separately where you think you're going to be over ground at any given time -- it's a different problem (and of course, an important one).

[Lodesman]

Originally Posted by LJH

As you said in your post (#710) of Re: Distinct Activities: Shackled by a Common Name? your method is optimum from a big ship perspective. If I am trying to track a rumbline in a situation where the tidal flow and leeway are not in my favour then my VMG along the rumb line almost comes to a stand still. If I use the CTS method then I will be up current and will maintain a significant VMG. Much more efficient overall. If I am a 5-6 knot speed range then I want to have as much as possible working in my favour.

Right thread, wrong post. Look at post #699. Or in this thread, post #24. In 710 I was explaining that big ships don't worry so much about such efficiencies, and gave a reason why following the rhumb line is not 'dumb'; it had nothing to do with the method I described to achieve the OP's aim.

[Wotname]

Originally Posted by DeepFrz

Yes, I agree that the wind triangle is essentially the same as a current triangle.

I am working on a real world example of why the rhumb line is important.

As a pilot I was taught to create a wind/course triangle and CTS. Then when in the air to use features along the rhumb line to correct the CTS to bring me back to the rhumb line. It was always stressed that it was very important to stay on/close to the rhumb line as this was the track that had been studied for hazards. Of course we checked for hazards that were within a certain distance of the rhumb line, I believe it was on an angle from A of 15 deg. on both sides of the rhumb line.

Staying on the rhumb line was also stressed in my boating navigation courses and the importance of always knowing where your were, as near as could be calculated by using different methods, was always stressed. If a sailor gets used to drifting off the rhumb line on a constant CTS, unless he plots his track points (as has SWL) he will not know how far off a hazard he may be. She will also not be sure of her position should she have to start a DR plot in the case of sudden fog. I think it is very important, if not for speed reasons, but for safety reasons to stay as close as possible to the rhumb line while sailing in any unfamiliar water. Maybe this sense of importance comes from learning to sail in Jacks waters, which can be extremely hazardous, as can so many other areas of the world.

As well as learning to sail on the Wet Coast of Canada I used to do a lot of backwoods Canoeing. Knowing where you were at all times was extremely important in the wilderness.

Originally Posted by Andrew Troup

Deep Frz

If you need to paddle (or worse, swim) across a stream whose midstream current is as fast as you can travel, you will never get there if you maintain station on a perpendicular rhumb line. No matter if you have unlimited endurance at that speed.

On the other hand, if you paddle at a constant heading, you can cross a river flowing twice as fast as you can paddle (if you can keep it up long enough) and still end up at the same point on the opposite bank as following the perpendicular rhumb line.

Of course, the quickest way across is the "Third Way": to point the bow on a perpendicular heading. You will reach the other side, further downstream (assuming laminar flow, no rocks, eddies or bends) in the same time as if the river was not flowing.


But this, too, is a constant heading strategy, not a rhumb line strategy.

Neither strategy is a "single answer to all problems", and anyone promoting one to the exclusion of the other should think hard about the quality of their advice.

ON EDIT: I entirely agree that it's crucial to know where you plan to be, and where you actually are, at all times, in challenging waters.

So it's essential (and not difficult) to be able to plot the ground track for your proposed Constant Heading passage (if that's the appropriate strategy for the situation). Then check the clearance on all dangers en route, and work out escape plans for downtide dangers which are uncomfortably close, should the plan not measure up to reality.

The other part - keeping track of where you are (and monitoring whether it's where you planned to be) is very easy, if you have functioning GPS.

Even if you don't, it's generally not difficult, in most tricky sailing waters, but I guess that will change eventually if lights and marks continue being phased out.

After re-reading many posts, these two stand out to me and I think Andrew is on the money.
Sometimes one's best interest is served by maintaining a rhumb line and at other times, by maintaining a single CTS regardless of ground track. The prudent navigator should know how to do both and which to use when.

I suspect the RYA method doesn't do either that well but I could be wrong!

[Andrew Troup]

Dockhead, I'm surprised, stunned actually, at your cavalier dismissal.

I didn't realise you would have zero appetite for metaphor, or parable.

I wasn't describing a trip.

I feel almost embarassed on your behalf, that I have to spell this out, but now I've started I supposed I have to continue.

I was setting out a close and careful analogy for the SwL method of planning a trip.

It was fanciful in the hope that it might keep the reader's interest, and perhaps for a few twisted individuals, even be memorable.

The unrealistic omissions you complain about (eg not knowing whence or whereto) are the parts I omitted for the sole and specific purpose of demonstrating that SeaWorthy's method does not (as you incorrectly believe) make any call on the rhumb line.

Maybe someone else will see what is (to my surprise) completely opaque to you.

Even better, maybe you will summon up the goodwill to repay the compliment I paid you by thinking you would get my drift, and the effort, by having another crack at it, in the light of what I've now told you?

[Seaworthy Lass]

'Morning all .
I am awake and alert .

Will take me a while to go and read all the posts carefully. I will reply to all the points raised, just give me time (I am a slow typist).

Watch this space for the next dramatic example later today

[Lodesman]

Originally Posted by Andrew Troup

If you consider that Seaworthy's method relies on the rhumb line to deliver a CTS and distance through water to the true destination, you need to remember that this is harder to prove than disprove. (A bit like the proposition that 'all rabbits are brown').

The best you can do by way of proof, it seems to me, is to try to disprove it, and discover that you cannot.

I, on the other hand, think I can. Fairly simply.

Of course SWL's method still has the 'rhumb line'. She starts at point A and goes to point B - and measures the distance between them. The only thing she doesn't do is draw the pencil line between them. It is still part of the tidal triangle that is the basis of her method. I really can't believe this argument.

[Seaworthy Lass]

Originally Posted by Lodesman

The method I proposed uses what you call the rhumb line, doesn't require interpolation/extrapolation, and easily portions the final vector without guesstimation - yet you haven't commented on it

Hi Lodesman
Apologies for not replying earlier. I have been just swamped with writing and working out examples and plotting them and in between life at anchor must continue (in the middle of sail repairs as well).

As soon as you said your method uses the rhumb line, I tuned out. As I will show in the example I will post today, it is the fixation on the rhumb line that is the source of the worst problems with the RYA method, and when combined with extrapolating the data to give the effect of the current in the last part of the journey it gives results that are spectacularly out!

[Seaworthy Lass]

Originally Posted by Lodesman

Of course SWL's method still has the 'rhumb line'. She starts at point A and goes to point B - and measures the distance between them. The only thing she doesn't do is draw the pencil line between them. It is still part of the tidal triangle that is the basis of her method. I really can't believe this argument.

No need to draw the pencil line at all in my method. I measure the distance between A and B, but after that I do not refer to that line AT ALL.

There is no triangle in my method if the tides are not all perpendicular to the course line.

[Seaworthy Lass]

Originally Posted by Paul Elliott

Not exactly true, it depends on the relative magnitudes as well. Here's a stupidly simple example:

Sailing at exactly 9 kts, with a 1 kt (+/- 10%) favorable current, how long to travel one mile? With a 1kt current, 6 minutes. With a 1.1 kt current, 5.941 minutes. A 10% change in current results in approximately a 1% change in the result. You can obviously change the proportions up and down, and get different results.

I have no idea how this plays into the competing calculations.

Thanks for reminding me Paul.

I will drop any reference to % error in future.

In my first example, the RYA method gave a CTS that was 5 degrees out.
In my second example the RYA method gave a CTS that was 8 degrees out.

The example I will give today will be dramatically more .

[Seaworthy Lass]

Originally Posted by LJH

The RYA method will get you nicely to D, which may be a significant distance from B. Your method would probably be easier to illustrate if you end with a significant crossing current so that the vector through S is well separated from the vector through L.

Yes, good idea. I will draw several step by step diagrams for a simple example, so that the SWL method is easier to understand.

I really want to present my next example first though, so I will leave explaining my method better until later when people are more convinced it is the method to use instead of the RYA method.

[Lodesman]

Originally Posted by Seaworthy Lass

No need to draw the pencil line at all in my method. I measure the distance between A and B, but after that I do not refer to that line AT ALL.

There is no triangle in my method if the tides are not all perpendicular to the course line.

The combined set vectors = a single vector that is one side of the triangle. The line between start point and destination, whether drawn or not, is a side of the triangle. And your CTS is the third side of the triangle. Your method is not dramatically different from mine, other than your calculation of the final part hour is rather ad hoc. Tidal set does not need to be perpendicular to the course line to work out a tidal triangle.

Out now and off to bed; will return in the AM. Cheerio.