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Old 02-02-2013, 11:14   #61
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Re: Single CTS or follow the Courseline?

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Originally Posted by David M View Post
The shortest distance between two points is going to be fastest, unless the current changes where it becomes a following current AND the vector component of that current towards your destination has a speed/distance greater than the additional distance added to a straight line you otherwise would have taken.

If covering great distances and you are not altering course to find better winds or are weather routing to minimize high seas, just stay on the great circle route and adjust course to vector into the current and effects

of windage in order to stay on your great circle route. That's how professionals on ships do it.
Well, a ship making 16 knots will feel much less effect of current vectors, than a yacht making 6 or 7. The effect of 2 knot currents on a ship making 16 knots will be very slight and maybe not worth calculating.

And anyway, I bet professional navigators on a ship crossing the Gulf Stream, for example, will certainly do a CTS calculation. There you can have a lot more than 2 knots. I know for sure they do it in the English Channel.

As Dave said, the shortest distance is a straight line -- through the water.
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Old 02-02-2013, 11:16   #62
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Re: Single CTS or follow the Courseline?

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Originally Posted by jackdale View Post
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.


Or if you are not off by much, just carry on the old CTS and work it out on arrival.
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Old 02-02-2013, 11:35   #63
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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.
Not neccessarily true unless it is bad steering or the coriolis effect.
Anyways, in a comparison of arguments, Drift however summed, applies to both boats equally. Throwing out neglible factors is perfectly ok where both boats are neglibly affected.
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Precisely...but is this an achievable goal?
If current or windrift or any other drift is a major factor, it can be calculated well-enough to illustrate the arguments.

It's easy to demand evermore precise data, but it need be shown HOW the more precise data will CHANGE the outcome of the arguments. Just "saying so" is not a valid argument.

That said, the model should be comprehensible.
Where Drift due to current is given here as
"at" the zero hour, current =0
"at the first hour, current = 2.
at the second hour current =3...
the actual "sum of drift due to current" SIMPLY PUT is
(0+2/2)=1 mile in that elapsed hour.
(3+2)/2=2.5 miles in that elapsed hour.
Sum drift for 2 hours (where direction is the same!)=3.5 miles.

More complicated, but I think is" truer"where tides are sinusoidal, in the entire period from MAX to Slack, a boat will drift .707 X Max current .
.707 is the sin of 45 degrees.
therefore, if max current=3 knots in the current cycle "max->slack" period of 3 hours, the sum displacement =.707*3*3hours=6.363 miles
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Old 02-02-2013, 11:37   #64
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Re: Single CTS or follow the Courseline?

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Originally Posted by bewitched View Post
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.
These assumptions are erroneous. You would never slow down on purpose just to stick to your CTS plan. If you find yourself doing better than you thought, you change your plan and run the numbers again.

You might of course get a lift or knock by sailing a little different from a GPS track boat. That's randomly distributed so doesn't count. But sailing a constant heading you might very well be able to trim your sails better, compared to jerking around trying to stay on the rhumb line. If anyone has a boat speed advantage, I think it would be more likely to be a boat sailing a constant heading.

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Originally Posted by bewitched View Post
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.
I accept that this would be true in a case like Lodesman where there is only the slightest variation of current and so only a very slight advantage of sailing a constant heading. But if you were to run the same procedure on other cases we have analyzed here (try the original Captain Force one, or any of SLW's excellent cases, or Jackdale's English Channel crossing), you will see a very different picture. You will not lose a half hour advantage (or in case of English Channel crossing, the advantages can be hours) with a one degree error in CTS, for sure.


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Originally Posted by bewitched View Post
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.
The first question isn't relevant as far as I can see. Remember that the advantage of a CTS path is different in different circumstances. I know some crossings where, at springs, you could not beat me across in a well-sailed Open 60, on the rhumb line.

The second question has two answers:

a. First of all, leeway can be figured with quite a bit of accuracy, actually. There are various techniques, and I, for one, use them. Read your wake with a hand bearing compass. Know your leeway from checking heading against COG slack tide in various conditions and points of sail.

b. Second, no one says you have to stick to your course over the whole passage. You are allowed to make corrections. Of course, every time you change course, you are departing from an ideal passage and losing time. But in many cases, you will still be far, far ahead of the poor guy crabbing along the rhumb line. Again, it depends on the particular passage, and depends on the size of your error, so there's no a priori answer which suits all cases.


Dave has said a few times that the English Channel is the "poster child" for CTS passages. Well, certainly here you are dead meat if you stick to the rhumb line. At springs in a slowish boat there is a high risk that you will simply never arrive, attempting to stay on the rhumb line. So CTS is mandatory here.

But I think many more places than here. The Gulf Stream is another good example. Anywhere with big variation in the current over the course of your passage you will have a very large advantage which you will not lose with even quite a lot of error.
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Old 02-02-2013, 11:59   #65
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Re: Single CTS or follow the Courseline?

...and so, many of the multi-value drifts here are wrong if they are "values of current speed at that time". However we can accept ALL the representations if they are... "Miles displaced in the period to that time"

Likewise, the RYA,Bowditch,etc. model need be interpreted as "expected sum of drift at that time" and not "current at that time".
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Old 02-02-2013, 13:08   #66
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Re: Single CTS or follow the Courseline?

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what bill352 means is that since the ground track is not easy to compute in a long multi hour CTS, it is therefore difficult to know how you are progressing , not that you cant find where you are. its that you dont know, where you are, is where you should be?
Thank you, Dave. Couldn't have said it better myself, and obviously didn't.
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Old 02-02-2013, 14:08   #67
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Re: Single CTS or follow the Courseline?

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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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I don't believe this has been answered - it's the same question I've been asking.
Of course it has been answered.
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Old 02-02-2013, 15:18   #68
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Quote:
Originally Posted by DeepFrz

Of course it has been answered.
No it hasn't. Given the difficulty in determining ground track , bills questions remains valid , few mariners have the competency in vector maths to construct complex ground tracks.

Dave
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Old 02-02-2013, 15:20   #69
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Re: Single CTS or follow the Courseline?

Oh, okay, throw in another red herring. Few mariners... Few boats... What else do you want to throw in. Don't forget this is a text book case. Ask the questions, state the knowns and that is that.
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Old 02-02-2013, 15:23   #70
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Quote:
Originally Posted by Dockhead

I accept that this would be true in a case like Lodesman where there is only the slightest variation of current and so only a very slight advantage of sailing a constant heading. But if you were to run the same procedure on other cases we have analyzed here (try the original Captain Force one, or any of SLW's excellent cases, or Jackdale's English Channel crossing), you will see a very different picture. You will not lose a half hour advantage (or in case of English Channel crossing, the advantages can be hours) with a one degree error in CTS, for sure.
I will rigorously contradict the ascertain that one degree of CTS has any meaningful effect. In practice real life CTS is unlikely to be accurate +-5 degrees. Hence almost inevitably you have to correct a CTS work up in almost all cases. Believing there is precision when in practice the " system" cannot get anywhere near such precision.

Dave
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Old 02-02-2013, 15:35   #71
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Re: Single CTS or follow the Courseline?

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Of course it has been answered.
Not exactly. How many threads have been generated now over what we've been told is a flaw in the RYA 'multi-tidal set single CTS' - that it doesn't aim to destination, but up to half-hour away and you'll need to "wing it" to get to destination. If any other method of determining a 'multi-tidal set single CTS' will also require one to "wing it" to get to destination, then it seems that these threads are all sound and fury, signifying nothing. We've had a couple 'sort of' answers, but no-one has stated unequivocally what is expected during the execution of the single CTS, be it RYA, SWL or other method.

Do you: (a) plot the expected CMG for each tidal set and change course to maintain that?;
(b) take periodic fixes and recalculate a new CTS based on previously-planned and possibly incorrect tidal data;
(c) take periodic fixes and recalculate a new CTS based on tidal data adjusted by observation;
(d) stick to the planned CTS with faith that it will all even out by the end, fixing and running DRs/EPs to ensure navigational safety;
(e) stick to the planned CTS and only pansies fix; or
(f) other?
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Old 02-02-2013, 15:41   #72
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Re: Single CTS or follow the Courseline?

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Ask the questions, state the knowns and that is that.
This is a very interesting topic. Thanks. Similar to this post, as an "engineer" I must contribute that many of the posts have questioned the validity of a method by questioning the validity of some of the variables used in the calculations. That's simply not a fair approach to "debate" the concepts being discussed. For instance, questioning the validity of the currents information. It either is or is not correct, and mid-course corrections should always be made anyway.

Great stuff.
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Old 02-02-2013, 15:43   #73
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Re: Single CTS or follow the Courseline?

Read SWL's thread. She explains it all very thoroughly.
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Old 02-02-2013, 15:56   #74
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Re: Single CTS or follow the Courseline?

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Originally Posted by goboatingnow View Post
No it hasn't. Given the difficulty in determining ground track , bills questions remains valid , few mariners have the competency in vector maths to construct complex ground tracks.

Dave
Do you really think there are only a few? Given that only hourly tidal data is available ( for most of us anyway) it's really not that hard to draw up some vectors. Though agree, accuracy will start to suffer quickly. Not that I've sat down with many and discussed it at length, but if that is the case then few mariners can keep an EP going, which is basically the same thing using actual boat speed/course.
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Old 02-02-2013, 16:17   #75
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I despair in some of the comments.

Constructing ground tracks in multi hour CTS is not something people are taught nor is it intuitive.

Whether one uses the RYA or the SWL method is irrelevant, you cannot monitor your progress unless you know where you are supposed to be. In a multi hour CTS that's not easy.

Secondly you may even be on your computed ground track ,but early or late. Hence you have to now evaluate what tides are valid. In some cases you could find significant course changes are required its not simple and bill raises a valid point that has not been addressed

Mention has been made of criticising the criticism of the variables . It is entirely appropriate to do so. This are models and like all models you have to relate them to real life and in practice they do not work well.

Furthermore the RYA method does not leave you swinging in the wind for the last 30 minutes nor does it " get you close ". The method correctly applied gives you a theorctical. CTS to the estivation. The discussion is over the method used ( hence where the variables argument vibes in)

People here keep giving glib answers ( you just correct as you go along and repot etc etc )
The fact is correctly a multi hour CTS is not easy. Firstly it requires accurate computation of the ground track. I suspect des here could do that. Secondly errors in time ( progress) and position can invalidate the tide data, forcing re computation of data. I don't about you but extracting and interpolating tides is time consuming.

Them there is the issue that the new course may be quite different from the old CTS , because of tide changes

Bills question is appropriate and the answer is simple. Multi hour CTS does not map well to the real world. That's why it is not included in some of the worlds finest navigation text books ( bow ditch et al)

Dave
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