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Old 07-02-2013, 22:54   #226
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Re: Single CTS or follow the Courseline?

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Originally Posted by Dockhead View Post
I'll do it later today.

Neptune will do that calculation on the computer.
I know that programs like Neptune can do that calculation but this is a test for traditional, non-electronic navigation on charts or some other form of paper.
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Old 07-02-2013, 22:54   #227
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Re: Single CTS or follow the Courseline?

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Originally Posted by Seaworthy Lass View Post

Using such small increments of time there is no need for either the RYA or SWL methods to determine the CTS. The CTS calculated mathematically would have you arriving within a couple of minutes if the data was correct .
Exactly. That will be the acid test of both methods. We cam compare them both to an exact calculation.

The next scenario will be to try to simulate a real passage with realistic speed errors, to see how meaningful the differences in accuracy are.
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Old 07-02-2013, 22:56   #228
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Re: Single CTS or follow the Courseline?

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I know that programs like Neptune can do that calculation but this is a test for traditional, non-electronic navigation on charts or some other form of paper.
We're only using 5 minute tides for the precise baseline calculation - the benchmark. We'll use one hour tides for the hand methods.
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Old 07-02-2013, 22:57   #229
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Re: Single CTS or follow the Courseline?

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Exactly. That will be the acid test of both methods. We cam compare them both to an exact calculation.

The next scenario will be to try to simulate a real passage with realistic speed errors, to see how meaningful the differences in accuracy are.
But no electronics are to used. That is what traditional basic navigation is about.
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Old 07-02-2013, 23:00   #230
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Re: Single CTS or follow the Courseline?

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Originally Posted by Dockhead View Post
We're only using 5 minute tides for the precise baseline calculation - the benchmark. We'll use one hour tides for the hand methods.
Thanks. I still question the accuracy of 5 minute currents.

Dave made a great point about the difference between precision and accuracy. I was reminded of my education in statistics and strategic planning (studies of the future).
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Old 07-02-2013, 23:06   #231
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Re: Single CTS or follow the Courseline?

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Thanks. I still question the accuracy of 5 minute currents.

Dave made a great point about the difference between precision and accuracy. I was reminded of my education in statistics and strategic planning (studies of the future).
I don't question their accuracy (UKHO is the gold standard of hydrography), but it's irrelevant - it's a theoretical test of the two hand methods. We'll take one hour tides built from the 5 minute data and see how well they work compared to an electronic calculation which will approach mathematical perfection (particularly, no partial hour, which is the bane of the hand methods).

Then we'll see how well they deal with input errors. I think it will be extremely interesting.
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Old 07-02-2013, 23:11   #232
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Re: Single CTS or follow the Courseline?

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Originally Posted by Seaworthy Lass View Post
Using such small increments of time there is no need for either the RYA or SWL methods to determine the CTS. The CTS calculated mathematically would have you arriving within a couple of minutes if the data was correct .
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Originally Posted by Dockhead View Post
Exactly. That will be the acid test of both methods. We cam compare them both to an exact calculation.

The next scenario will be to try to simulate a real passage with realistic speed errors, to see how meaningful the differences in accuracy are.
Oh, I see, this is just being done for a comparison of the two methods .

If the average tidal stream component is very small compared to the expected boat speed (eg 0.2 knots compared to 6 knots for a 60nm channel crossing), then it won't show much. The RYA and SWL methods and will give an almost identical result here.

Dockhead, are you able to pick just a portion of the journey over only a couple (or several) hours where the RYA and SWL methods give a different result?
It would be interesting to see that comparison.
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Old 07-02-2013, 23:20   #233
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Re: Single CTS or follow the Courseline?

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I don't question their accuracy (UKHO is the gold standard of hydrography), but it's irrelevant - it's a theoretical test of the two hand methods. We'll take one hour tides built from the 5 minute data and see how well they work compared to an electronic calculation which will approach mathematical perfection (particularly, no partial hour, which is the bane of the hand methods).

Then we'll see how well they deal with input errors. I think it will be extremely interesting.
I have one small thing to add to the SWL method. It is how to handle a partial hour at the beginning.

I included it in the email I sent Tim Bartlett a few days ago, but I have not had a chance to go back to my other thread and include it there. This is what I wrote (the alteration to the method I posted at the beginning of that thread is highlighted):

Step 1: Mark the departure and destination points on a chart. I will refer to these as A and B respectively.

Step 2: Measure AB (= the course distance) to enable you to start gathering data on currents. It need not be measured very accurately at all unless you need to know your predicted SMG for some reason (the time taken is computed separately and does not require this distance to be accurately known).

Step 3: Divide the course distance by the expected boat speed. This gives time of the journey if there is no current or leeway.

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).
This gives you a starting point for how many hour lots of current you need to consider (or half hour lots if this current info is provided). For simplicity I will describe hourly data here, but the technique works equally well for any segment you care to look at.
Divide the course into an equal number of sections as per the number of hours you estimate and determine the current in each segment for the specific hour you will be there.
I know this is only an approximation as the current will not be constant over the hour, but we need to work with what we have.

Step 5: For this next step, it is important to draw all distance displacement vectors to the same scale as the chart (I think this is actually being taught during RYA course already, as if the tidal stream is strong relative to boat speed it is difficult to accurately estimate the time taken and results can be hit and miss).
Determine what proportion of the first hour the first lot of tidal stream data will apply and work out how much the boat would be displaced by this.
Draw this amount starting from A in the direction the current will take you.


Step 6: Continue adding on hourly displacement amounts from the tip of each current displacement vector until you can arc off a speed displacement vector towards B that is getting close to reaching B (eg if you have marked off 2.5 lots of tides and your speed is 5 knots, then you are arcing off 12.5 nm).
If the arc off from the tip of your subsequent current displacement vector then extends past B, almost all the hard work has been done. You know you will arrive sometime during this last hour of current.
(If the arc exactly crosses B, mark this off. This is your CTS. The time taken for the journey is the number of hours of current vectors you had to use.)

Step 7: Your arc is unlikely to exactly coincide, so draw a line from the start of this final current displacement vector to B. Arc off the distance vector for this time and mark S (for short)

Step 8: Draw a line from the end of the final current displacement vector through B. Arc off the distance vector for this time and mark L (for long)

Step 9: Look at the proportion between SB and BL. This will immediately
give you an idea how much of the final current needs to be applied.
Determine this proportion accurately by calculating:
= SB / SB+BL
Multiply this by the strength of the current for this last hour and you now know the boat displacement due to current for this period.
Mark that point on the final current vector and label it K

Step 10: Join K to B, extending the line past B

CTS = the angle of the line KB before you have made allowances for compass variation and for leeway.
Distance travelled through water = the length of KB
Time taken = the length of KB divided by the boat speed.
SMG (if you need to know this for some reason) = the length of AB divided by the time taken.

Step 11: Plot the expected ground track by adding the boat displacement due to boat speed to the tip of the first current displacement vector. This is your position after the first bit of current finishes applying. Mark this point.
From this point add the next lot of displacement due to current, then again add the displacement due to boat speed to the tip of that. This is your position after the next bit of current finishes applying. Mark this point.
Continue doing this, and the last point will coincide with the destination.
Join all the marked points and this is your expected ground track. This can also be very easily done on a chartplotter or using a program such as OpenCPN.
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Old 08-02-2013, 01:42   #234
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Re: Single CTS or follow the Courseline?

[QUOTE=bewitched;1145165]In recent months on this forum, there has been a growing support in favour of opting for a Single Course to Steer (CTS) method of navigation when sailing across a varying tidal flow. The proponents of this method claim that it is faster than ‘crabbing along the rhumbline’. Which perhaps could be more kindly termed navigate along the course line (NCL)?

To make sure we are all on the same page, here is a brief outline of the two methods:

CTS: .....provides the basics of the technique which essentially sums the tidal vectors over x hours, adds the boatspeed (BSP) vector from the end of the tidal vectors to the intersect with the course line. This is the estimated position after x hours.

NCL: This method uses GPS data to ensure that the boat stays on the course line. In very simple terms, the technique involves ensuring that the course over the ground (COG) achieved matches the desired course. It is simply a case of following the course line that has been drawn on the chart as shown on the attached picture.
/QUOTE]

I did not read all 16 pages of comments or calculations on comparing ‘CTS’ to ‘NCL’ so may have completely missed the point of this discussion.

In my opinion, neither is the most efficient method of navigating.
Instead fully understanding the dynamics of tidal influences and wave form as they are affected by barometric pressures, sea states and the vessels own sailing characteristics, will provide the most efficient passage plan from Points A to B.

In reality, it is the awareness of both CTS and NCL based on real-time monitoring of optimum speed and distance towards point B that has allowed for adjusted tidal factors, topographical opportunities and an optimum course that best suits your helm characteristics, which will provide the navigator with his Holy Grail.

Even with GPS, this part is still an Art Form.
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Old 08-02-2013, 02:40   #235
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Re: Single CTS or follow the Courseline?

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Im a fan of PFM, Im told is also explains heavier then air flight,

Dave
PFM ???
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I'm gonna guess PAG = Plot And Go
AWTY = Are We There Yet?
If so, then I an signing up to Boaty's method
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Old 08-02-2013, 02:54   #236
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Re: Single CTS or follow the Courseline?

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PFM ???

If so, then I an signing up to Boaty's method
PFM = Pure Freaking Magic
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Old 08-02-2013, 02:57   #237
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Re: Single CTS or follow the Courseline?

[QUOTE=Pelagic;1151151]
Quote:
Originally Posted by bewitched View Post
In recent months on this forum, there has been a growing support in favour of opting for a Single Course to Steer (CTS) method of navigation when sailing across a varying tidal flow. The proponents of this method claim that it is faster than ‘crabbing along the rhumbline’. Which perhaps could be more kindly termed navigate along the course line (NCL)?

To make sure we are all on the same page, here is a brief outline of the two methods:

CTS: .....provides the basics of the technique which essentially sums the tidal vectors over x hours, adds the boatspeed (BSP) vector from the end of the tidal vectors to the intersect with the course line. This is the estimated position after x hours.

NCL: This method uses GPS data to ensure that the boat stays on the course line. In very simple terms, the technique involves ensuring that the course over the ground (COG) achieved matches the desired course. It is simply a case of following the course line that has been drawn on the chart as shown on the attached picture.
/QUOTE]

I did not read all 16 pages of comments or calculations on comparing ‘CTS’ to ‘NCL’ so may have completely missed the point of this discussion.

In my opinion, neither is the most efficient method of navigating.
Instead fully understanding the dynamics of tidal influences and wave form as they are affected by barometric pressures, sea states and the vessels own sailing characteristics, will provide the most efficient passage plan from Points A to B.

In reality, it is the awareness of both CTS and NCL based on real-time monitoring of optimum speed and distance towards point B that has allowed for adjusted tidal factors, topographical opportunities and an optimum course that best suits your helm characteristics, which will provide the navigator with his Holy Grail.

Even with GPS, this part is still an Art Form.
I think you might profit from reading the 16 pages of comments

If after reading all that, you still believe in doing it that way, then I'll race you from Needles to Cherbourg at spring tide for pink slips You will use your art form, and I will do a precise CTS calculation. I'd love to have a 65' Van Hellman schooner
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Old 08-02-2013, 03:15   #238
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Re: Single CTS or follow the Courseline?

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Originally Posted by Seaworthy Lass View Post
I have one small thing to add to the SWL method. It is how to handle a partial hour at the beginning.

I included it in the email I sent Tim Bartlett a few days ago, but I have not had a chance to go back to my other thread and include it there. This is what I wrote (the alteration to the method I posted at the beginning of that thread is highlighted):
Quote:
Originally Posted by Seaworthy Lass View Post
Determine what proportion of the first hour the first lot of tidal stream data will apply and work out how much the boat would be displaced by this.
Draw this amount starting from A in the direction the current will take you.
Good inclusion. It can make a difference, for example you are leaving point A at HWT plus 2.8 Hrs for a 1.5 hr trip. And the currents are:
HWT +2 313/1.0
HWT +3 168/0.2
HWT +4 149/0.9
(actual currents from Imray chart C-15 point A)

Quote:
Originally Posted by Seaworthy Lass View Post
Step 11: Plot the expected ground track by adding the boat displacement due to boat speed to the tip of the first current displacement vector. This is your position after the first bit of current finishes applying. Mark this point.
From this point add the next lot of displacement due to current, then again add the displacement due to boat speed to the tip of that. This is your position after the next bit of current finishes applying. Mark this point.
Continue doing this, and the last point will coincide with the destination.
Join all the marked points and this is your expected ground track. This can also be very easily done on a chartplotter or using a program such as OpenCPN.
This is very easy in OpenCPN - 4-7 mouse clicks for each current segment and then join the points to create the route. I would hat to try to do it on my RL70!
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Old 08-02-2013, 03:38   #239
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Re: Single CTS or follow the Courseline?

DockHead,

The real test ( of any CTS method) will be seeing if the EP error workup, which results in a error circle at the end of the single CTS , encompasses both calculation methods, if it does then its a precision versus accuracy argument.

I would wager that the error of position circle resulting from simply sailing the same exact CTS (+- helm errors) and allowing for speed variations, and leeway, will be far greater then either theory error circle. Good luck with that.

If however the error of one or the other methods lies outside the EP error , then the one that lies inside is better.

The difficult of course is how can you model a real crossing simulating maintaining a real single CTS and mathematically derive any meaningful result.

of course , we can initially compare the two theory methods to closer inspection by simple comparion of a 5 minute CTS workup compared to both methods

I strongly disagree with SWL comments that one should pick extreme situations, initially we should look at common sitiuations, Reductio ad absurdum can come later ( many models work fine for general use but fail to explain certain things at the extremes , see Newton)

Right now we can deal with the "Ad ignorantiam" issue.

Dave
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Old 08-02-2013, 03:41   #240
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Re: Single CTS or follow the Courseline?

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Interesting discussion of VMG as it relates to ETA.

http://www.thesailinggps.com/VMGarticle.pdf
jackdale,

Thanks for the link to the interesting article. It explains nicely why I have not used that function in my chart plotter. I have always calculated an actual speed of advance towards my waypoint and/or destination. Still something to be said for handraulic navigation.

It is a good warning to make sure that you know the terminology being used and how it has been applied. I find my VMG using trig from my Vboat.
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