Inaccurate RYA Teaching : CTS - Quest For a New Method

[jackdale]

Originally Posted by Seaworthy Lass

If you could please provide an answer to Andrew Troup's example in post #485, I will provide an answer to yours tomorrow when I am back on board after a day's work replacing the sacrificial strips on our headsails.

I looked at your solution. How did you determine the position of "A", the departure point?

[jackdale]

Originally Posted by LJH

Below is an OpenCPN solution using SWL's method. 168 degrees for 3 hr 34 minutes. (Blue line is k to B, Red line is ground track)

Also done by spreadsheet I got 167 degrees 3 hr 34 mins.
(https://docs.google.com/file/d/0Bz8m...dGeG80ZTA/edit)

Thanks for that.

How did you determine SMG?

[LJH]

Originally Posted by jackdale

Thanks for that.

How did you determine SMG?

The distance K to B is 14.3 miles distance through water @ 4 Kts = 3.57 hrs.

[jackdale]

Originally Posted by HappySeagull

jackdale, not to whine, but I was hoping for a grosser model.

Happy?

Gross enough?

Ediz Hook to Entrance Victoria Harbour. - current data from tomorrow.

Depart 1000
CMG 000
Speed 4.0
Current
1000 – 1.0 295
1100 – 1.0 295
1200 – 1.4 235
1300 – 1.5 235
1400 – 1.2 235
1500 – 0.5 235

[jackdale]

Originally Posted by LJH

The distance K to B is 14.3 miles distance through water @ 4 Kts = 3.57 hrs.

You are using boat from the knotmeter.

You need to use SMG to determine ETA. You are never at K, unless you turn off your engine and drift there. You are departing from A and your destination is B.

I do not think that SWL's method allows to calculation of SMG, which is an estimate of the speed that you are actually travelling over the bottom as a result of a boost from the current. In an ideal situation your SMG and the SOG from your GPS would be identical.

[LJH]

Originally Posted by jackdale

You are using boat from from the knotmeter.

You need to use SMG to determine ETA. You are never at K, unless you turn off your engine and drift there. You are departing from A and your destination is B.

I do not think that SWL's method allows to calculation of SMG, which is an estimate of the speed that you are actually travelling over the bottom as a result of a boost from the current.

You gave us a boat speed of 4 Kts. That should be the speed showing on your speed log!

No you are not at K just like in the RYA method you are not a C!

You sail 14.3 miles through the water to move 16 miles across the ground. That means your water mass moves 1.7 miles. Your SOG would be 16 miles/3.57 hrs = 4.48 Kts. A 1.7/3.57=.48 Kt boost from the current. Your ETA is still 3 hrs 34 mins after you depart as I had answered before.

[LJH]

CTS 015 degrees 4 hrs 35 mins arriving at 14:35.
18.37 miles through water to cover the 16.5 miles to entrance of Victoria Harbour will be about 3.6 Kts SOG.

Originally Posted by jackdale

Happy?

Gross enough?

Ediz Hook to Entrance Victoria Harbour. - current data from tomorrow.

Depart 1000
CMG 000
Speed 4.0
Current
1000 – 1.0 295
1100 – 1.0 295
1200 – 1.4 235
1300 – 1.5 235
1400 – 1.2 235
1500 – 0.5 235

[jackdale]

Originally Posted by LJH

No you are not at K just like in the RYA method you are not a C!

Check figure 80 on page 93 of the RYA manual. Notice that you depart from A, you are never at C.

I now am really convinced that this method is flawed. It does not solve for Course Made Good or Speed Made Good.

[jackdale]

Originally Posted by LJH

CTS 015 degrees 4 hrs 35 mins arriving at 14:35.
18.37 miles through water to cover the 16.5 miles to entrance of Victoria Harbour will be about 3.6 Kts SOG.

Check your numbers with D = 16.0 I should have included that. Duh?

SMG 3.4 (both)
CTS
4 hour – 020T
5 hour – 017T
ETA 1442 (both)

I would use the 5 hour vector.

[jackdale]

We teach our students to use SMG and the Distance along to CMG to determine ETA.

[Seaworthy Lass]

Originally Posted by Dockhead

My dear, it is often a mistake in life to assume any given person (or institution ) is an idiot (it can be a mistake to assume anyone is not an idiot, too, but that's a different conversation ), especially when you're making a parallel assumption of being very clever yourself. The results can be embarrassing (I speak from experience!).

You shouldn't jump to conclusions like this. What they are saying is that you can plot on graph paper, in a bigger scale for more accuracy, as long as you keep all the proportions are the same. They are right, and this is what you should do if your vector triangle is going to be small at the actual chart scale. This is a separate step -- the final one of measuring the angle which will give you your course to steer.

Obviously you have to lay the vector triangle to scale on the chart to see whether you get there or not; and applying Dave's Missing Manual you would do the next full hour, too, and eyeball the difference and proportions to get a usable fudge. It's a perfectly usable system probably designed before good autopilots existed, giving results which are surely well within any possible accuracy of hand steering. With a decent eyeball doing the fudging, the results will be equal to the results of your method, and better in case the person using your method is not fudging where your method is nearing the limits of its capabilities.

The really excellent thing about your method, as I've said several times, is that it gives a much better result much less reliant on intelligent fudging with hardly any extra effort. That's why I'm going to see if we can't get it published for the broader good. But you are going too far if you assume that Tim Bartlett -- former Royal Navy officer and navigator, author of a whole bunch of respected books on navigation, including celestial navigation -- some of which I sleep with or at least carry with me everywhere I go -- is an idiot who would propagate idiotic methods with idiotic mistakes in them. In fact, he probably forgot more about navigation yesterday than you or I will ever know. The method he describes is a traditional one, and would not have survived generations of navigators if it were idiotic. It is designed for a quick and easy calculation of a CTS probably in the context of hand steering where 3 or 4 degrees is not going to make much difference. And in the context of very intelligent navigators. Nowadays we have the opposite context -- very good autopilots which can steer to a degree or so in reasonable conditions, and rather poor navigators spoiled by electronics and used to just "driving the dot" on a screen. So I think your method is really better suited for today's reality and should really replace the traditional one.

Actually, before sending your method to YM I think I'll try to discuss it with Tim Bartlett himself. I know someone who knows him well and can probably arrange to meet him if he's around. I'm flying to the UK this afternoon. Maybe I can get him to comment about it on here.

Ouch!
That was a painful barb!

Dockhead, I am absolutely mystified by your change of heart. You and I have been exchanging views and ideas and friendly banter on this thread, and up to now you have provided me with such wonderful challenges and support (and ultimately agreed with most of my conclusions), but suddenly a post like this is being directed at me.

I can only think something I said has inadvertently upset you. I just can't think what it could be, but if this is the case, I apologise deeply. Absolutely no offence has ever been even vaguely intended in anything I have written to you at any stage.

I am accustomed to an academic environment where ideas (wild or otherwise) are freely tossed about and hotly debated, holes are poked in theories without insults and inspiration is gained all around. Everyone ultimately benefits. If no one worries about making mistakes and looking foolish, great progress can be made.

I have made LOTS of errors in these last couple of threads on navigation, but I have admitted each of these as soon as I realised my stupidity. For every step I have taken back, I think most people wading through all these posts have seen me progress a couple forward. I have learned a tremendous amount, in fact I think we have all benefited.

I have inadvertently stumbled on something important and I am not deterred in pursuing the issue regardless of how long certain navigational methods have been taught.

Although I have joked that I am picking examples to put the RYA on the spot, that is simply my Aussie sense of humour coming out. They are a wonderful, highly respected organisation and perhaps I would gain more by being extremely deferential.

Since I have found there are flaws with the method the RYA are using to determine the CTS, my aim is, however, to give examples where this occurs. it would show nothing if I gave examples where the RYA gets an answer close to mine, so yes, I am deliberately picking ones where there are problems (and I could churn out thousands). I agree the RYA method is a good approximation sometimes, but I am pointing out that it can also give very flawed results and people using the method are not warned of this by the RYA.

When I came back on board upset yesterday, my very wise hubbie said I was silly to take any barbs directed at me to heart on CF. If I wanted to play with the boys, I needed to take it on the chin.

So have put bandaids on all my wounds and I am back in the fray

[DeepFrz]

So have put bandaids on all my wounds and I am back in the fray

Hang in there SWL. Remember the powers that were sanctioned Galileo to House Arrest for the rest of his life. You only have to face a few unearned literary barbs.

[Seaworthy Lass]

Originally Posted by HappySeagull

well, SWL, I'd like to nail down my speed versus drift for the best CTS...this text don't do it for me, I'd agree. But, "assess the tidal stream that the boat will experience at each hour of the intended passage" is pretty similar to your first instruction here:

Step 4: You know the passage will take longer than that if there is total current against you (and shorter if it it with you), so make an educated guess how much longer by looking at the current along the way (this may actually be difficult, but that is a whole different topic). Which made me fall back on my own ignorant charms and incantations to ward off the squalls in the thread....

I expect you have a revision for this? It seems to be the crux of your method?

No, I am sorry I have no revision for this. I was just pointing out that the method used by the RYA of initially gathering the current data was very similar to mine - complicated. I make no attempt to suggest how to decide on the best data, simply what to do with it once you have it.

I am not focussed on how the RYA gather the data, just what they do with it once they have it.

Originally Posted by HappySeagull

I apologise if I missed it but there's a ton of stuff here and you will know best.
I did get " the averaging bit" by the way but got distracted ...

(There is nothing to suggest you look at the arc relative to the destination, just to what they call the "intended track" (a VERY misleading term anyway, as it is actually the rhumb line, not a track you intend being on).Not the track you will be on, that's for sure. ....

My Peeve:Is it mentioned as a caution to plot where I will be, hour to hour (or half-hour) , if I blithely follow this CTS? That bugs me. You could easily sail into danger or a shipping lane , because in these diagrams you are only known to be at Start + Finish. It could be elsewhere in this book- I -don't- have.

Yes, it bugs me too, so that is why I have stressed several times in my posts in this thread that it is essential if you are finding a CTS and planning to use it, that once you have found your CTS, you immediately plot your expected ground track.
This is essential and I have not found this mentioned in the RYA text either.

I found this post of mine several hundred back at post #119 and then again in post #185, but I have repeated this several times:

Originally Posted by Seaworthy Lass

I fully agree, but your ground track does not have to be a straight line for you to focus on the dangers.
Determine your CTS to give you the most efficient time (it may be necessary for lots of reasons even when leisurely cruising), then see where the plot of your ground track is and check there are no dangers and if all is OK, follow the CTS .

Originally Posted by Seaworthy Lass

If you are ever using a CTS it is ESSENTIAL to plot your ground track. These are the dotted red lines on most of my examples.

If the RYA is not teaching this when they teach CTS , then they definitely should. How else do you know what hazards you will encounter?

[Seaworthy Lass]

Originally Posted by DeepFrz

Hang in there SWL. Remember the powers that were sanctioned Galileo to House Arrest for the rest of his life. You only have to face a few unearned literary barbs.

Thanks DeepFrz .
You made me smile .

I am sitting here very early trying to answer a few posts before I head ashore this morning. It's just gone 6 am and I have been at it nearly an hour, so don't know how much I will achieve today, but with coffee cup in hand I will do my best.

After work on the sails I have been arriving back on board truly exhausted and it is a bad time to be trying to think clearly and answer concisely and as accurately as I can, so I got up extra early today .

[jackdale]

Originally Posted by Seaworthy Lass

Yes, it bugs me too, so that is why I have stressed several times in my posts in this thread that it is essential if you are finding a CTS and planning to use it, that once you have found your CTS, you immediately plot your expected ground track.
This is essential and I have not found this mentioned in the RYA text either.

Figure 80, page 93

And

Finally, draw a straight line to the point where the arc intersects the intended track. The
direction of this line represents the course to steer. It does not matter if it seems to be
starting from the wrong place, because we are only interested in its direction: the boat
won't actually be sailing along the line. Nor does it matter if any of the other lines drawn to
calculate the course to steer pass over shallow patches, or hazards, or even cross the
coastline: they are purely lines representing a mathematical construction. The only one that
has any direct relationship to the movement of the boat across the surface of the real
world is the one representing the intended track.

The CTS will carry along the expected ground track. The CTS is not the expected ground track. That ground track is the CMG between A and B. It is the first line that you draw. (page 92)

The multi current vector is simply an extension of a single current vector.