Inaccurate RYA Teaching : CTS - Quest For a New Method

[HappySeagull]

less arguing, more plotting like I say, I don't have a dog in this fight. but I'm happy to compare any and all ideas.
Treats!
seagulls like treats! a revelation a day keeps dementia at bay! (or is a symptom..?)

[goboatingnow]

less arguing, more plotting like I say, I don't have a dog in this fight. but I'm happy to compare any and all ideas.

you cant compare "ideas" with each other , only with real life. ie one compares "models" with the action of the real life activity. then you use those results to compare the "models". We have not done that here at all.

I have not argued against SWLs method, I have said on several occasions that based on the assumptions of the tide data that she assumes the method makes better use of that assumption. Equally the RYA assumes a different relationship with the underlying tide data.

Its apples and oranges.

dave

[jackdale]

Originally Posted by Seaworthy Lass

See post #707.
Happy has now modified the example, as a full hour of the first lot of current data actually applies. So your CTS was OK initially, just the ground track needed replotting .

The DR track is out 2 degrees . The current vectors are correct, the first .5 hours is .4 knots.

Regardless - I would tell the helm 355 as a course asssuming no leeway, variation or deviation.

[jackdale]

Originally Posted by Seaworthy Lass

Jackdale, a simpler technique is fine as long as its limitations are recognised. Students need to be warned that if D does not lie very close to B, the computed CTS may be significantly in error using the RYA method. As I understand it, this is is not currently taught (one instructor has even commented that checking to see if D is within half an hour of B is not taught by many instructors, and examples with this are not given during the courses or exams).

What I have objected to from the start is this RYA instructor's constant insistance of how "mathematically precise" the RYA method is and how following it will not have you arriving at D (a point on the rhumbline), only at B (the destination). The first time I looked at the method it was instantly apparent this is not the case.

Digging deeper and speaking to people who have undergone RYA courses (and been examined on CTS computations) the RYA method seens to be touted as being very accurate if the data input is correct. This is simply wrong.

Wth a little extra work, accuracy can be improved. The result is still limited by assumptions regarding the tidal stream data, but in some cases a significantly better result is obtained. This is what the SWL method offers .

If you want real accuracy we should should not be using Euclidian geometry, on a Mercator projection, with a Cartesian graph and a Newtonian concept of motion.

I am sticking with what works in the world of sailing in realistic conditions.

Many other "disciplines" use rules of thumb that work for their parameters. Demographers work with a rule of 70 for determining the doubling times of populations; an accountant would not use that rule to determine the doubling time of a bank account.

[HappySeagull]

ah, yes. Real life.
Speaking of THAT, here's a quick sketch of the autopilot tooling along the rhumb line using the same assumptions the others used.

Takes about 2 hours 40 minutes .
autopilot.png Views: 98 Size: 21.4 KB ID: 54945" style="margin: 2px" />

[HappySeagull]

RYA 2 hours 45 minutes (based on closest approach) CTS=256
SWL 2 hours 40 minutes (per "2.7 hrs") not plotted here CTS=255
Autopilot 2 hours 41 minutes (but is not a perfect plot) 3 hourly CTSs 348,003,006 plotted at 15 minute intervals.

RYA+Autopilot.gpx

my quick take-away is not more than: using half-hour increments of drift will improve plots.

and of course, garbage in, garbage out, so the real "sum of drift" needs close consideration for a real trip to track very near these plots. ...apparently, the Autopilot can't lose by much until the drift over-matches the boat.

[Seaworthy Lass]

Originally Posted by HappySeagull

ah, yes. Real life.
Speaking of THAT, here's a quick sketch of the autopilot tooling along the rhumb line using the same assumptions the others used.

Takes about 2 hours 40 minutes .

Hi Happy
Gotta love enthusiastic seagulls .

Examples are not useless, they occasionally trigger all sorts of tangent thoughts.

In this case, your approximation of the time taken caught my eye and made me reach for pen and paper (and calculator). When crabbing instead of following a single CTS in the last example you presented, the time taken if constant current is assumed for the last partial hour (the same assumption the SWL method makes) is 2.706 hr (2 hr 42.4 min).

If I mathematically work out the CTS using the SWL, I get 2.69 hr (2 hr 41.4 min).

(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.

Interesting......
Gathering thoughts, I will comment more later.

[HappySeagull]

SWL, yes, but keep that saltshaker handy(the grain of salt) Big mistake in my drawing...

I should have put the drift on the other side of the rhumb, and then AUTO would re-steer for Destination....so... a better drawing is wanted

My so-called autopilot plot here is really just a "Hapless-on-Caffeine" plot....Autopilots can't think ahead (not mine, anyways) and really are off-course before they correct to Destination.

[HappySeagull]

...for an autopilot, my drawing would suit one that had the hourly drift increments dialed in....

[Seaworthy Lass]

Originally Posted by HappySeagull

SWL, yes, but keep that saltshaker handy(the grain of salt) Big mistake in my drawing...

I should have put the drift on the other side of the rhumb, and then AUTO would re-steer for Destination....so... a better drawing is wanted

My so-called autopilot plot here is really just a "Hapless-on-Caffeine" plot....Autopilots can't think ahead (not mine, anyways) and really are off-course before they correct to Destination.

Makes no difference which vector you draw first for the purposes of the diagram .

Yes, you need to be off course before the autopilot will make a correction (speed of correction depends on the sensitivity you select), but this just results in a small lag. A track close to the rhumb line can still followed quite well.

[Seaworthy Lass]

Originally Posted by HappySeagull

...for an autopilot, my drawing would suit one that had the hourly drift increments dialed in....

My calculations for crabbing were based on average hourly data too. In an earlier nav thread it was shown that making constantly corrections when crabbing actually increased not decreased the advantage of the single CTS. I doubt we would be talking about big differences in time taken for this example though.

[HappySeagull]

mo-bettah! HERE's an autopilot...that has a lot of lag!


say, three hours because it still hasn't arrive at the time here

[HappySeagull]

here, the boat steers to Destination at outset and after every 15 minutes given the POSITION. (my autopilot is not capable of "position" so explains why I dialed something in a la "Hapless" to crab" in the previous example...)

Yes, it don't suit a real life per time of corrections but it certainly agrees with my Faith- The last mile will show the ignorant sinner , straining against the Worldly Stream, while the Methodists have previously skated happily across it into the Breakwaters of Good Haven...(albeit,only recently !)

added: you can see why I have to put vectors on the other side...being past tense!

[Seaworthy Lass]

Originally Posted by HappySeagull

here, the boat steers to Destination at outset and after every 15 minutes given the POSITION. (my autopilot is not capable of "position" so explains why I dialed something in a la "Hapless" to crab" in the previous example...)

Yes, it don't suit a real life per time of corrections but it certainly agrees with my Faith- The last mile will show the ignorant sinner , straining against the Worldly Stream, while the Methodists have previously skated happily across it into the Breakwaters of Good Haven...(albeit,only recently !)

Happy I am thoroughly confused. Why are you 'down current' so much the whole time?
Love your coloured diagrams regardless of inaccuracies though, they beat my pencil sketches hands down!

[HappySeagull]

Originally Posted by Seaworthy Lass

Happy I am thoroughly confused. Why are you 'down current' so much the whole time?
....

because "autopilot" has no "foresight" and cannot look ahead to an hours worth of current to vector against and thereby resolve to Rhumbline at the end of the period.

This happens whatever the reaction time as to exactly how much it delays the boat versus single CTS is interesting. I'm interested because currents are faster than my boat lots of times hereabouts.

So, blow by blow, Boat starts out at 4 knots with this .8 (averaged ) cross current heading to Destination along rhumbline 337. 15 minutes later, it finds itself off course by 15 minutes drift(.2 miles)... It alters again to point at Destination- say 338. 15 minutes later, it finds itself off course again...This time, destination bears 340. 15 minutes later, despite steering 340, destination bears 342. 15 minutes,343.. VMG gets smaller and smaller. Worse,as the drift-rate increases (as it does here), that 15 minutes off-course becomes larger and VMG ever smaller....until, finally the autopilot is motoring directly up-current.

Obviously, setting my compass autopilot to Rhumb, would be a pathetic display in this scenario and just have me 3.3 nm downstream after 3 hours.