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Old 01-02-2013, 13:17   #31
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Re: Single CTS or follow the Courseline?

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Originally Posted by Lodesman View Post
I still haven't had a very good answer as to what you do on a CTS when a fix puts you off your planned CMG? Do you steer to regain or maintain your planned CTS?
If you get a fix, convention (read Bowditch) dictates that you start afresh. If you have been dead reckoning you can use the DR position and your fix to determine the current since your departure or the last fix. That can provide you with an indication of the variance between what ever current prediction tables you were using and the reality.
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Old 01-02-2013, 17:37   #32
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Re: Single CTS or follow the Courseline?

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Originally Posted by Dockhead View Post
Why in the world would the NCL boat achieve a faster BSP? uzzled:
See my post #6. or consider this example:

NCL boat vs CTS boat: Expected wind conditions during passage :
1st hour = light wind reach - expected BSP 6kts
2nd hour = moderate wind reach - expected BSP 7.5kts
3rd hour = fresh breeze reach - expected boat speed 8.5 kts
4th hour - fresh breeze run - expected boat speed 10kts

NCL boat has an average BSP of 8kts

CTS boat, as dictated by the CTS calc must travel at a constant speed. That constant speed must be achievable over the whole course. The only speed achievable over the whole course is 6kts. The CTS boat is therefore restricted to a BSP of 6kts.

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Why don't you make a mathematical case for, for example, if you are off by 1 degree in your CTS calculation, you will already lose your advantage, in this or that scenario. That would be interesting.
Take a look at my Post #3. The second tab of the spreadsheet titled 'Tide Table Error' in that post does just that. Total set and drift errors (including other leeway errors other than tide) can be entered into cells P4 and P5. (Perhaps I should have titled the tab 'Total Leeway Error')

It shows that with only a very small error makes a big difference. A 2 degrees error in drift and a 2% error in drift (0.04kts in a 2kt tide) results in the CTS boat falling behind.


These two issues are the heart of the problem:

1. How do you beat a boat that can sail faster than you?

2. How do you gather boat-specific leeway data to such accuracy and then predict what will actually happen during the course of your passage to the same accuracy?

I put it to the forum that the answer to both these questions in the vast majority of cases will be that you can't.
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Old 01-02-2013, 17:56   #33
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Re: Single CTS or follow the Courseline?

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OK, here's a proof of sorts. I'm on another computer and can't post a scan of my work, but I'll walk you through it. I am doing this stepwise, treating each hour (or fraction) independently.

First, the easy "crabbing along the rhumbline" solution.

Hour 1: (5 kts BSP, crosscurrent of 2 kts). I didn't bother to work out the CTS angle, but we have a triangle with 2 miles for the current, 5 miles for the CTS, and progress along the rhumbline of 4.583 miles (square root of the sum of the squares).

Hour 2: the triangle is 3 miles (current) 5 miles (CTS) and 4 miles along the rhumbline.

Final 0.7456 hour: the triangle is 1.491 miles (current), 3.728 miles (CTS), and 3.417 miles (along the rhumbline).

The total distance along the rhumbline is 12 miles, the CTS distance is 13.643 miles, and the elapsed time is 2.746 hours.

Now, we see how far we can get in the same time (2.746 hours), with a constant CTS:

Vectors are 6.491 miles (current), 12 miles (rhumbline), and 13.730 miles (CTS, 2.746 hours @ 5 kts). The end of the CTS vector is 0.087 miles *beyond* the destination.

We reach the destination sooner with a constant CTS, than by crabbing. The distance sailed crabbing is longer than the constant CTS distance.

Does anybody see a flaw in my analysis?
Sorry Paul - had to jump in here. I see a flaw in the analysis - no consideration for any source of leeway other than tide and therefore not realistic of an actual passage. There there will always be leeway from other sources.

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Please note that this is purely a math exercise, and this assumes perfect information and performance.
Precisely...but is this an achievable goal?
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Old 01-02-2013, 18:16   #34
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Multi hour CTS using conventional sparse tidal data is not nor never was meant to be some sort of catch all sail off on the CTS type of solution. Very few situations , leaving aside X Chanel symmetrical reversing tides ( where a lot f errors get canceled out ) are amenable to long multi hour single CTS. In practice I've never seen it done. Why

(A) you most then often have quite sparse tidal data , such data must be manually interpolated and is only valid if you hit that tide in that position , at that predicted time, in practice over many hours these errors are considerable , leaving aside any debate about the precision of the maths.

(b) you must be able to determine your ground track in advance. You cannot clear the route otherwise. Only in rare cases have you considerable tidal change and stacks of sea room.

(C) computing , a. Say 10 hour CTS , and then sailing miles off on the CTS track in the initial hours is generally very foolish. Wind changes , sea conditions etc can mean the destination could become incredibly difficult to reach. In the vast majority of cases the tides are not symmetrical

(d) multi hour CTS incorrectly computed , or whose ground track is non intuititive etc leave one open to making serious errors of position. The method compounds initial errors and I do not regard large multi hour CTS computations as " fail safe "

In practice few small boat mariners use anything more then 1-2 plots , staying reasonably close to the rhumb line and recomputing CTS based on progress, wind and wave , boat handling etc.

There is nothing worse onboard a boat then a theory expert.


So the debate about GPS track to the destination or multi hour CTS is simply nonsense. The solutions is strategic use of appropriate mechanisms taking into account. Boat capability , sea & wind conditions , crew competency , sea room , obstacles tiredness etc etc

There is no one size fits all.

In practice , outside of poster child X Chanel symmetrical reversing tides , most people make hour to hour allowances for the tide and stay close to the desired ground track.

If you wanted efficiency, buy a high speed RIB , or a 747.


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Old 01-02-2013, 19:34   #35
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Re: Single CTS or follow the Courseline?

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These two issues are the heart of the problem:

1. How do you beat a boat that can sail faster than you?
PHRF

Quote:

2. How do you gather boat-specific leeway data to such accuracy and then predict what will actually happen during the course of your passage to the same accuracy?
John Rousmaniere has a table in his classics that estimates leeway for three different hull configurations and different points of sail at various wind speeds.

Or - take a bearing on your wake, compute the reciprocal, and compare that to your ship's compass after accounting for deviation.

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Old 01-02-2013, 19:42   #36
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Re: Single CTS or follow the Courseline?

You folks are looking for a precision that does not exist without integrated navigation systems not found on any production boat. Try an ACC boat.


For the regular sailor, the currents you are discussing are within sight of land. Get a fix, determine how far off course you are and work your way back on course. Your best navigation instrument in coastal cruising is a Mark I eyeball.

The use of current vectors is to get you pointed in the right direction. And that is how I use them.
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Old 02-02-2013, 02:10   #37
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Re: Single CTS or follow the Courseline?

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Sorry Paul - had to jump in here. I see a flaw in the analysis - no consideration for any source of leeway other than tide and therefore not realistic of an actual passage. There there will always be leeway from other sources.
Sure, but that wasn't the question being asked, nor the one I solved.

I agree that steering a compass course without knowing leeway and other confounding factors will result in uncontrolled errors. I believe that these sensitivities need to be analyzed independently of the underlying "ideal math" solution.

One way to get a reasonable handle on your leeway is to take a bearing on your wake and measuring how far off your stern it is. This will of course change with wind, sea state, point of sail, sail configuration, etc, but at any given point you can measure it with some accuracy.
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Old 02-02-2013, 02:55   #38
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Re: Single CTS or follow the Courseline?

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The use of current vectors is to get you pointed in the right direction. .
This is what I always understood the method to be. A non-gps method to get you from A to somewhere near B. Maintain the BSP, maintain the HDG and in x hours you should be somewhere near your course line and near your destination. By then, you have presumably managed to get a fix as not many destinations are out of sight of land.
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Old 02-02-2013, 03:15   #39
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Re: Single CTS or follow the Courseline?

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One way to get a reasonable handle on your leeway is to take a bearing on your wake and measuring how far off your stern it is. This will of course change with wind, sea state, point of sail, sail configuration, etc, but at any given point you can measure it with some accuracy.
I put the word 'measure' in bold in quoting your post because this is a key point. With the CTS method, you can't measure, you must predict. So somehow you must have data for each point of sail, each sail configuration, in each sea condition, in each wind condition.........

....and then whatever data you select must be the conditions that you actually experience on passage.

Unachievable goal IMHO.
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Old 02-02-2013, 03:24   #40
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Re: Single CTS or follow the Courseline?

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This is what I always understood the method to be. A non-gps method to get you from A to somewhere near B. Maintain the BSP, maintain the HDG and in x hours you should be somewhere near your course line and near your destination. By then, you have presumably managed to get a fix as not many destinations are out of sight of land.
Yes but to many "students" this message above is applied literally

First multi hour CTS, generates non intuitive ground tracks and hence it is often overlooked as to exactly what is safe and what is not.

Secondly the method can result in significant excursions from the rhumb line and make laying the destination difficult if conditions change.

Thirdly , plotting,computation or data extraction errors can result in significant out of position errors.


If I had a student candidate, work up a complex say 10 hour CTS and then suggest sailing off on an extreme ( though technically correct) CTS course simply because it seemingly anticipated events hours in the future, I would be extremely suspect of their skippering ability. I would want to far more prudent approach and if that approach sacrificed some efficiency in passage time , I would not be worried.

Its worth noting that the RYA YM practical exam, focusses far more on the simple practical issues in conning a yacht and dealing with passage making, The theory course is the one that concentrates on the maths.Yet it is the practical exam that gives you the ticket.

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Old 02-02-2013, 05:24   #41
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Re: Single CTS or follow the Courseline?

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Quote:
Originally Posted by Paul Elliott
One way to get a reasonable handle on your leeway is to take a bearing on your wake and measuring how far off your stern it is. This will of course change with wind, sea state, point of sail, sail configuration, etc, but at any given point you can measure it with some accuracy.


I put the word 'measure' in bold in quoting your post because this is a key point. With the CTS method, you can't measure, you must predict. So somehow you must have data for each point of sail, each sail configuration, in each sea condition, in each wind condition.........

....and then whatever data you select must be the conditions that you actually experience on passage.

Unachievable goal IMHO.
You can certainly measure your leeway under various conditions and use that in future CTS predictions. This is obviously what I was saying above.

As far as I can tell, your argument is that we shouldn't bother to use mathematically correct (or more correct) solutions because the real-world conditions don't support the illusory precision we gain. This may be true, but I don't see that it has been demonstrated.

Any navigation method can result in disaster if it is improperly applied. Does this mean we should reject all navigation methods? Of course not, and I don't believe you are saying this. We should understand the various methods, the effect of errors on these methods, and the conditions that favor particular methods. Obvious? I hope so!
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Old 02-02-2013, 05:44   #42
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Re: Single CTS or follow the Courseline?

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Originally Posted by bewitched View Post
I put the word 'measure' in bold in quoting your post because this is a key point. With the CTS method, you can't measure, you must predict. So somehow you must have data for each point of sail, each sail configuration, in each sea condition, in each wind condition.........

....and then whatever data you select must be the conditions that you actually experience on passage.

Unachievable goal IMHO.
Only if you cast your cts in stone beforehand. In reality doing another quick vector drawing on passage with real boatspeed takes just a few moments, and can save you miles of extra distance through the water compared with crabbing along the rhumb line.
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Old 02-02-2013, 05:45   #43
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Re: Single CTS or follow the Courseline?

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You can certainly measure your leeway under various conditions and use that in future CTS predictions. This is obviously what I was saying above.

As far as I can tell, your argument is that we shouldn't bother to use mathematically correct (or more correct) solutions because the real-world conditions don't support the illusory precision we gain. This may be true, but I don't see that it has been demonstrated.

Any navigation method can result in disaster if it is improperly applied. Does this mean we should reject all navigation methods? Of course not, and I don't believe you are saying this. We should understand the various methods, the effect of errors on these methods, and the conditions that favor particular methods. Obvious? I hope so!
The errors in multi hour single CTS can accumulate significantly, Hence the method has often poor applicability in real life.

Its not a question of underlying mathematical precision, I would argue the
method isnt a good one in general. 1,2, maybe 3 hour plots, 10 hour plots ,No.

even if they do a 10 hour plot most people re compute after a few hours anyway , resulting in a 1-2 hour plot anyway.



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Old 02-02-2013, 05:52   #44
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Re: Single CTS or follow the Courseline?

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Only if you cast your cts in stone beforehand. In reality doing another quick vector drawing on passage with real boatspeed takes just a few moments, and can save you miles of extra distance through the water compared with crabbing along the rhumb line.
This is a mis characterisation. Firstly errors in time or position may force you re reextract and reinterpolate the tidal data, if you are working up 8-10 hours of CTS this is a reasonable amount of time at the chart table.

Secondly large multi hour CTS can result in a complicated non intuitative ground track that cannot easily be checked for safety.

Thirdly the effects of future tides can have a dramatic bearing on the initial CTS, so you are in effect, betting in the future, right now.

The method is one of these great in practice, but often practically unusable in reality.

while crabbing along the rhumb line has its disadvantages , its does have

(a) You know where you are going , even if slowly

(b) There is not much ongoing recomputation

in most cases tides are weak in comparison to boat speed and complex. I often just use the 1 in 60 rule and in effect plot on hour CTS.

The question is not to ignore CTS calulations, just to ignore certain parts of it in certain situations.
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Old 02-02-2013, 06:19   #45
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Re: Single CTS or follow the Courseline?

I've been following this and the other threads on the subject, and I'm going to jump in with a stupid question here (yes, they exist. I've seen them.)

If you're sailing a multi-hour CTS and you do get a reliable fix, how do you know where that is compared to your planned course? As I understand it, there is no DR plot with this method. How do you know whether you're on course or not?

I understand that the prudent navigator will start anew from here, and that is certainly my practice. But there has been talk about "correcting" a course from here, and even, IIRC, of calculating the actual set and drift by comparison with some expected position. How do you do that, if you do?

Forgive me if this has been explained before. I didn't see it.
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