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Old 10-02-2013, 00:14   #631
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by jackdale View Post
Step 6 - Measure CTS = 053 T (CD)

A single CTS will fail here as you will need to keep avoiding all those fishing boats and nets
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Old 10-02-2013, 12:20   #632
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

ok the race is on.
destination bears 90T 6.8mi
displacement hrs 1,2,3 all 180T: 2.8mi, 2mi, 0.8mi
Note that Hapless is just tooling along by considering 1 hour increments of tide displacement...
after 2 hours, Hapless is a little behind Jackdale who has a longer view...
(Hapless bears 222T@150meters from Jackdale) after 2 hours
If Hapless were to engage his electronic deckhand to steer along the rhumb, he'd have to expostulate his theories to explain detailed drift increment and nobody wants that, least of all,Hapless.
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Added Hapless steers 45 first hour, 63 second hour @4knots

(I think SWL -you mean CTS 52 ? not 62? you might want to sharpen that protractor? or not.)
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Old 10-02-2013, 13:05   #633
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by Seaworthy Lass View Post
Tidal stream data:
09:00 2.8 knots 180T (= rough average 08:30-09.30)
10:00 2.0 knots 180T (= rough average 09:30-10.30)
11:00 0.8 knots 180T (= rough average 10:30-11.30)
Quote:
Originally Posted by HappySeagull View Post
ok the race is on.
destination bears 90T 6.8mi
displacement hrs 1,2,3 all 180T: 2.8mi, 2mi, 0.8mi
Note that Hapless is just tooling along by considering 1 hour increments of tide displacement...
after 2 hours, Hapless is a little behind Jackdale who has a longer view...
(Hapless bears 222T@150meters from Jackdale) after 2 hours
If Hapless were to engage his electronic deckhand to steer along the rhumb, he'd have to expostulate his theories to explain detailed drift increment and nobody wants that, least of all,Hapless.

Added Hapless steers 45 first hour, 63 second hour @4knots

(I think SWL -you mean CTS 52 ? not 62? you might want to sharpen that protractor? or not.)
Nope, my protractor is razor sharp along with my pencil (although after three odd weeks of computations my pencil has been sharpened down to a stub LOL).

You are considering the tidal stream data incorrectly I think. If the current is 2.8 knots at 9am and dropping and you are leaving at 9 am, then the average tidal stream for the next hour is NOT 2.8 knots. It is less than 2.8 knots average (I have estimated 2.65) for the next half hour

For the the next hour it will then roughly average 2.0 knots.

Using an average current of 2.8 knots for the first hour of your computations is what produces a substantial error in the CTS that both you and Jackdale have determined.
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Old 10-02-2013, 13:09   #634
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by HappySeagull View Post
ok the race is on.
destination bears 90T 6.8mi
displacement hrs 1,2,3 all 180T: 2.8mi, 2mi, 0.8mi
Note that Hapless is just tooling along by considering 1 hour increments of tide displacement...
after 2 hours, Hapless is a little behind Jackdale who has a longer view...
(Hapless bears 222T@150meters from Jackdale) after 2 hours
If Hapless were to engage his electronic deckhand to steer along the rhumb, he'd have to expostulate his theories to explain detailed drift increment and nobody wants that, least of all,Hapless.

Added Hapless steers 45 first hour, 63 second hour @4knots

(I think SWL -you mean CTS 52 ? not 62? you might want to sharpen that protractor? or not.)

Quote:
Originally Posted by Seaworthy Lass View Post
Step by step explanation of the SWL method of determining the CTS with variable cross current.

More than a week has passed since I promised to post step by step instructions with diagrams for my method of determining CTS with cross current (chores on board have been keeping this lassie occupied and temporarily out of trouble), but here they finally are .

The example I will work through:
Boat speed is 4 knots (log reading with freshly cleaned log)
Calm conditions, no wind, you are motoring at a steady throttle setting
Departure time 09:00
Destination due east (6.8 nm)
Tidal stream data:
09:00 2.8 knots 180T (= rough average 08:30-09.30)
10:00 2.0 knots 180T (= rough average 09:30-10.30)
11:00 0.8 knots 180T (= rough average 10:30-11.30)

What is the expected CTS?
What is the expected time taken?
What is the expected ground track?
Hapless,

I used my electronic brain to help me with this. Have a look at the tide times SWL used. The first tide was for 30 mins, the second a full hour, and the last tide for the remainder. (lower part of attachment)

As a result she steers 062. If she had used a full hour tide for the first two hours she would steer 054! (upper part of attachment) The difference 30 minutes makes. It is also 9 minutes quicker if the tide behaves the way she estimated it.
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Old 10-02-2013, 13:32   #635
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Hapless, are you still with us?

I won this race and have been in the bar sipping celebratory drinks for at least 10 minutes before you and Jackdale got in .

Pity we didn't have a wager. I am getting desperate for Scotch here and since the email I sent last week to the RYA and Tim Bartlett has not elicited a response, I am reluctantly starting to think maybe that case of Scotch I was expecting won't be forthcoming and I need to seek other sources .
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Old 10-02-2013, 13:43   #636
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Agreed, I think the currents were overstated in Jacks and HS's examples. I think we should all use the same assumptions if we're going to compare notes. Using the same assumptions as SWL did regarding the tide displacement.

I drew A to B at 6.8 miles. (90 deg.) Drew a single line at 180 deg. @ 3.6 miles long from A ..and then swung the CTS line ( hypotenuse ) back 8 miles from the end of the set and drift line , It lands at a point .3 of a mile past B on a course of 63 deg.T
(About a 3.5 minutes? further along the Rhumb line than B)

At that point, I would cheat and increase my RPM to arrive with SWL. so SWL wasn't drinking alone :-)
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Old 10-02-2013, 13:49   #637
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by Tempest245 View Post
Agreed, I think the currents were overstated in Jacks and HS's examples. I think we should all use the same assumptions if we're going to compare notes. Using the same assumptions as SWL did regarding the tide displacement.

I drew A to B at 6.8 miles. (90 deg.) Drew a single line at 180 deg. @ 3.6 miles long from A ..and then swung the CTS line ( hypotenuse ) back 8 miles from the end of the set and drift line , It lands at a point .3 of a mile past B on a course of 63 deg.T
(About a 3.5 minutes? further along the Rhumb line than B)
The journey takes me less than 2 hours, that is why your CTS is a little higher, but you are pretty close .

Think we are both in the bar before Hapless and Jackdale .
First shout on me .
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Old 10-02-2013, 13:56   #638
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by Tempest245 View Post
At that point, I would cheat and increase my RPM to arrive with SWL. so SWL wasn't drinking alone :-)
Tempest, had you extended your two hours worth of 'boat displacement due to boat speed' directly towards B rather than the rhumb line (I know, I know this is a cardinal sin according to the RYA, but wanting to arc off to the rhumb line is actually ultimately their downfall), then you would have had a CTS of 62 degrees as I did and no cheating would have been necessary for the final spurt LOL.
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Old 10-02-2013, 14:04   #639
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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
The journey takes me less than 2 hours, that is why your CTS is a little higher, but you are pretty close .

.
Yes, if you back out the 3.5 minutes I sailed past B, I arrive around half a minute after you, but theorecally you'd have the current with you....

I like your solution..I keep looking for a set of real circumstances ( for me) where it would make a substantial difference. Still, it's another tool in the toolbox.

I think It has real implications for those that regularly race where there are predictable currents.
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Old 10-02-2013, 14:13   #640
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by jackdale View Post
The distance DB is approx. .4 miles which at 4 knots is less than 4 minutes.
At 4kts, you cover a distance of 4 cables in 6 minutes.
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Old 10-02-2013, 14:28   #641
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Re: SWL method for CTS: step by step instructions

Quote:
Originally Posted by Seaworthy Lass View Post
Tidal stream data:
09:00 2.8 knots 180T (= rough average 08:30-09.30)
10:00 2.0 knots 180T (= rough average 09:30-10.30)
11:00 0.8 knots 180T (= rough average 10:30-11.30)
Quote:
Originally Posted by Seaworthy Lass View Post
STEPS 1-5

In this case departure is at 09:00 so the average of the last half an hour of tidal data applies (estimate 2.65 knots) for half an hour.

So the boat displacement due to current during this time is 1.3 nm.
Draw 1.3 nm starting from A heading south, the direction the current will take you.
You're playing games with the tides, so not a very fair assessment that you've arrived ahead of Jackdale. Without seeing the entire tidal graph you have no way of determining the average speed for the period 0900-0930. There certainly appears to be no linear correlation between the three listed current speeds.
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Old 10-02-2013, 14:44   #642
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Re: SWL method for CTS: step by step instructions

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Originally Posted by Lodesman View Post
You're playing games with the tides, so not a very fair assessment that you've arrived ahead of Jackdale. Without seeing the entire tidal graph you have no way of determining the average speed for the period 0900-0930. There certainly appears to be no linear correlation between the three listed current speeds.
Hey, I posted the example and immediately gave my step by step computations and results, and all this was several hours before Jackdale posted.

I actually spelled out the times periods that the current applied for.

Unlike my previous examples where the RYA results was out by up to 30+ degrees, this example not meant to show to inadequacies of the RYA method. In fact the RYA method would have given a very similar result here had the data been treated the same way.

All I was doing here was giving step by step directions with step by step diagrams on how to proceed using my method.

The handling of tidal data is a personal choice. I do not advocate any particular method for this, all I did was explain what assumptions I would make and how I would personally deal with the data on hand.

The computation of the CTS is dependent on both the data and the method used. My method just gives a consistent result regardless of the data thrown at it. The RYS method will sometimes be close, but it can also sometimes be wildly out. That is the main thrust of what I have been trying to demonstrate.
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Old 10-02-2013, 15:11   #643
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

...actually, I was thinking we had stopped posting the current except as a sum of "Displacement". Because no previous model posted here can be considered reasonable without it. ....the average current of a cycle from slack to max is not 50%.

SEE:
Single CTS or follow the Courseline?

and
Single CTS or follow the Courseline?
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Old 10-02-2013, 15:25   #644
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

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Originally Posted by HappySeagull View Post
...actually, I was thinking we had stopped posting the current except as a sum of "Displacement". Because no previous model posted here can be considered reasonable without it. ....the average current of a cycle from slack to max is not 50%.
Yes, I agree the average amount of current in any one hour period is not necessarily 50%. Either plot out the tide of the entire cycle and extrapolate the average from there, or make some simple assumptions as I have or find data presented more frequently than hourly.

Deal with the current data however you like. I am not here to argue about that.

But once you have decided on what data to select, the critical thing is which method do you use to compute the CTS?

The instructors here have basically been saying the data is so inaccurate that it doesn't matter how inaccurate the method is (although one instructor still feels that the CTS determined by the RYA method will consistently get you to B).

I think working with an inaccurate method only compounds the errors. Although the RYA method frequently gives a good 'rough and ready' approximation of the CTS, sometimes it fails miserably. My method can handle any data you care to throw at it .
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Old 10-02-2013, 15:51   #645
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Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

...then what are the displacements you want to use? and I will send Hapless out again. I don't expect him to beat anybody, but I am curious as to what advantage can be displayed graphically.
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