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Old 31-01-2013, 20:35   #1
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Single CTS or follow the Courseline?

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)?
In this thread I hope to present the case for my belief that NCL will normally be the fastest and safest way to complete a passage when compared to the CTS method.

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

CTS: Take a look a this video. . It’s not a definitive explanation of the CTS method, but 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.


To avoid presenting the subject in one huge post, I will post separately for each major issue.

So first issue: Safety
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Old 31-01-2013, 20:36   #2
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Re: Single CTS or follow the Courseline?

Safety
With the NCL method, a course line is physically drawn on the chart. As it is the intention to navigate along the course line, the navigator checks to ensure that it doesn’t go too close to obstacles such as islands, headlands, reefs etc. It is an essential part of passage planning and the major reason for drawing the course line in the first place. I think most of us will plot a route, with a course line, before heading out on a significant passage.

The CTS method however largely ignores this course line (other than for the purposes of calculating the CTS). Navigating using the CTS will take you off the course line. A proponent of this method has posted on this forum that on a channel crossing this could be up to 12 miles off the course line. For me this brings in a level of uncertainty that I’m not comfortable with in most situations, particularly when navigating near a coastline or in the vicinity of islands, reefs etc.

I will leave you to decide what navigation method you feel comfortable with from a safety perspective.

Now on to the meat of the subject: Which method is faster?
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Old 31-01-2013, 20:38   #3
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Re: Single CTS or follow the Courseline?

Fastest by Calculation
For those who may want to check my assumptions and calculation method, I have outlined these in the attached MS Word document.

My calcs are presented on the first tab of the MS Excel worksheet, titled ‘Base Case’

The scenario I have modelled represents a increasing and then decreasing tidal set and a veering then backing drift. The tide drift does not get above 3.5 knots. A fair representation of sailing through the peak flow of a moderate tide I hope.

I have purposely not represented strong and wildly swinging tides as these are so rare globally and if encountered, it is usually close to land where other strategies (such as heading for the shallows to get out of the unfavourable tide) are likely to be far more beneficial than either method being considered here. It will probably also slew the data unfairly in favour of one of the methods.

To cut to the chase, on paper, the CTS method is faster every time.

I can tweak the tidal data, the BSP and the distance and the CTS method always wins. Usually not by much and always less than 1nm. But it is faster every time.

This conclusion has been reached by others on this forum and I guess this is why the method has been promoted. On paper, it appears to be faster.

But that’s not the end of the matter. We now must look into the resilience of the CTS calculation. The first issue I’ll address is the effect of inaccuracies in the tidal data.
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Old 31-01-2013, 20:41   #4
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Re: Single CTS or follow the Courseline?

Effect of Inaccurate Tide Data
One of the fundamental differences between the two methods is that the CTS uses predicted tides, from published tide tables or from chart, whereas the NCL method uses real time data from the GPS to establish the leeway caused.

So one method is based on what might happen to the tide over the whole passage, the other is based on what is actually happening as we sail along.

What this means is that if the tide does not do what it is expected to do, the CTS method will be affected as accurate tide data is fundamental to the method. The NCL will be unaffected as it has no reliance on any tide data.

I would imagine that the accuracy of the tide data varies significantly by location. Busy waterways are likely to have more accurate information than the more out of the way places. You must decide yourself how accurate your data is. An easy test is to wait until the next time you have a glass out and then drift in the tide for a while. Record your SOG and COG and then compare it to the Tide Data.

The second tab on the MS Excel workbook titled ‘Tide Table Error’ allows for an error to be introduced to the Base Case CTS calculation. This can be achieved by entering your chosen error for set and drift into cells P4 and P5.

Some sample data is attached to get a feel for the implications of inaccurate tidal data. As you can see it is significant.

So we see that the CTS method is very susceptible to marginally inaccurate tidal data input and in some areas at least, the tidal data is inaccurate to such a degree that the CTS method fails miserably when compared to the NCL.

But that’s not the end of the story; there are also other errors that creep in. I’ll call these inherent errors and impracticalities.
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Old 31-01-2013, 20:44   #5
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Re: Single CTS or follow the Courseline?

Inherent Errors and Impracticalities
The CTS method considers only one source of leeway and that is the leeway caused by tide. It addresses no other causes of leeway, which include waves and swell, wind on the sails and on the structure of the boat, prop walk when motoring to mention just a few. This introduces more inaccuracy into the method.

The quantum of this leeway is boat-specific and will vary with sea state, point of sail, sail selection, sail trim, helm competence and probably a whole host of other considerations. It is therefore a massive undertaking to gather reliable leeway data for your boat and probably the reason it is omitted from the CTS Calc in the first place.

However, to get a feel for your own boats leeway characteristics, record the following when sailing in various sea states and when there is no tide: HDG, BSP, COG and SOG. Then resolve the triangle shown in the attachment to this post. Your leeway set and drift vector is the side which runs from BSP to SOG. This can then be added as further set and drift error on the spreadsheet .

The NCL method suffers from none of this inaccuracy because the combined effect of all influences on the boat and the leeway that results is instantly identified (it’s the angle difference between HDG and COG). But these influences have no effect on the boat as the helm is steering a COG and so taking the combined effect of leeway into consideration with each touch of the wheel.

Helm Skill
How does the helm maintain an accurate course with no other reference than the compass? Remember, the CTS follows no course line or any other reference which indicates whether you are higher or lower, ahead or behind where you should be at any given time.

To get an indication of how critical maintaining an accurate course is, consider for a moment that a 5 degree variation over our 25 mile course will result in missing the waypoint by about 2nm. How confident are you that you can helm a boat within a few degrees over 25 miles of varying sea state using only the compass? Again, I’ll leave you to determine the proficiency of your helming and the quantum of inaccuracy that may creep in.

My final consideration of the CTS method relates to BSP. For me this is the Achilles heel and why I believe that though useful for outline planning, the CTS method is not suitable for making fast passages. This subject deserves a post of its own.
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Old 31-01-2013, 20:46   #6
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Re: Single CTS or follow the Courseline?

The Importance of Boat Speed
Obviously, a fast BSP is the desirable for getting to a destination quickly. With the NCL method, you can push the boat as hard as you see fit. There is nothing in the method itself which restricts the ability to sail as fast as the wind or the engine will take you.

The same cannot be said for the CTS method. This is because this method requires that both for calculation and for navigation, a single BSP must be maintained throughout the passage. If at any stage during the passage you cannot maintain your selected BSP, the CTS method will not take you to your desired destination. For the passage under consideration, a 1kt reduction in BSP will take you off course by about 4 degrees (a couple of miles).

What that means is that the speed selected must achievable throughout the whole passage. This is an incredible drawback to this method if we are trying to make a fast passage. The BSP selected is by definition; the slowest speed that the boat is envisaged to travel during the entire passage.

The CTS method therefore requires you to sail slowly.

If a favourable wind arises that would increase your BSP from say the 6kts you have used in the calculation to 9kts, you cannot take advantage of this good fortune as you must maintain your 6kts in order to reach your destination. Think about that – you must actively slow the boat down or your CTS calc is out the window.

To give an indication of the quantum of this point, I’ve crunched some numbers based on my own boat’s performance and compared it to a CTS calc which requires a 6kt BSP (which incidentally is a speed I use for my own passage planning). See the attached

I cannot think of a single scenario where a CTS boat would have a faster BSP that a NCL boat. Can you?
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Old 31-01-2013, 20:48   #7
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Re: Single CTS or follow the Courseline?

Summary and Conclusion
The Single CTS method looks good on paper if only tide data is used. Once we consider the accuracy of that tide data compared with what is experienced during the passage, the fact that no other cause of leeway has been considered and the ability of the helm to maintain an accurate course with only the compass as reference, we see that significant error is introduced. The net result is that the CTS determined from the calc is probably incorrect by a significant margin.

My rough estimate attached.

This results in the NCL boat being ahead by 7.8nm after the 5 hours. Plug in your own estimates to cells P4 and P5 of the ‘Tide Table Error’ sheet to see what you come up with.

This calculation hasn’t taken into account that the NCL boat almost certainly has a faster BSP than the CTS boat, which would put it even further ahead.

So to summarise the pros and cons of each method:

The CTS method:
1. Is inherently unsafe in my opinion
2. Uses predicted data
3. Only looks good on paper
4. Is highly susceptible to very marginally incorrect tidal data
5. Completely omits to incorporate any causes of leeway other than tide
6. Requires helming and sailing skills which are probably not achievable in many sea states
7. Requires you to sail the boat slowly.

The NCL method:
1. Is inherently safer, from both a planning and navigating perspective
2. Uses accurate, real-time data
3. Is consistently faster once one considers the errors and impracticalities of the CTS method
4. Incorporates all sources of leeway
5. Does not require such a high level of helming or sailing skill
6. Allows you to sail the boat as fast as you desire

If you use the CTS method, you will more than likely be sailing slowly in the wrong direction.
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Old 31-01-2013, 21:43   #8
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Re: Single CTS or follow the Courseline?

You make some assumptions that may not be true. First of all you are assuming that helm error is always in the same direction and that helm error only exists on the CTS method.

Another assumption that you make is that you cannot recalculate your CTS once underway. Recalculating your CTS for speed would again make the CTS speed better than the NCL method. Also, if you are steering by hand and have a GPS unit you will be able to check your XTE at any time and so chart your exact position on the chart. This should allow you to keep clear of any dangers.

However, I do like the idea of steering on the rhumb line for short distances. If steering by hand there also would be steering errors that will have to be corrected on a continuous basis. You would only be able to do this using a GPS unit (which almost everyone has these days). This will make the distance traveled longer than the measured course.
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Old 31-01-2013, 22:58   #9
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Re: Single CTS or follow the Courseline?

Quote:
Originally Posted by DeepFrz View Post
You make some assumptions that may not be true. First of all you are assuming that helm error is always in the same direction and that helm error only exists on the CTS method.
Good point. The helm error could be in either direction, but usually less experienced helm tend to drop off to leeward. I think the helm issue is less for the NCL method since you have the course line as a reference. The helm may wander a bit, but they will always try to get back to the line. The CTS method requires to helm to steer purely by compass, which gives no indication if is trend is higher or lower than his desired HDG. I do believe it is a more difficult task.

Quote:
Originally Posted by DeepFrz View Post
Another assumption that you make is that you cannot recalculate your CTS once underway.
This is true, but the method, as described relies on a SINGLE course to steer. If you need to re-calculate your CTS, aren't you just saying that if conditions change along your route, which is highly probable, then the single CTS calc doesn't work?

Quote:
Originally Posted by DeepFrz View Post
Also, if you are steering by hand and have a GPS unit you will be able to check your XTE at any time and so chart your exact position on the chart. This should allow you to keep clear of any dangers.
I take your point on board, but I'm still happier planning and verifying a safe route before I leave.
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Old 31-01-2013, 23:04   #10
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Re: Single CTS or follow the Courseline?

BSP or SOG?

I've just noticed another significant error in the CTS method. The course vector. Surely that should use SOG... not BSP?
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Old 01-02-2013, 00:18   #11
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Re: Single CTS or follow the Courseline?

Quote:
Originally Posted by bewitched View Post
Inherent Errors and Impracticalities
The CTS method considers only one source of leeway and that is the leeway caused by tide. It addresses no other causes of leeway, which include waves and swell, wind on the sails and on the structure of the boat, prop walk when motoring to mention just a few. This introduces more inaccuracy into the method.
Hi Bewitched
Leeway is certainly not ignored in determining CTS, either in the RYA Navigation text (several ages are devoted to this), or for the method I proposed in the CTS thread I started.

This was noted post #2 when the method was described:

Quote:
Originally Posted by Seaworthy Lass View Post
Step 9: Join K to B, extending the line past B
This is your CTS before you have made allowances for compass variation and for leeway.
I have lots to comment on. You have raised lots of issues. I will try and do this tomorrow. I am off the boat all day today (when hopefully the work on our sails will be completed) and then we are out to celebrate tonight.

One thought for you to ponder on:
What happens if current is encountered (and had been predicted from tidal data) that is significant to (or horrifyingly even greater than) in certain areas of the course than your expected boat speed?

Maybe this is not encountered when racing, but it may certainly be when cruising and trying to make a passage in adverse conditions.

What happens then when you are trying to follow a straight track on the chartplotter?
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Old 01-02-2013, 01:45   #12
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Re: Single CTS or follow the Courseline?

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Hi Bewitched
Leeway is certainly not ignored in determining CTS, either in the RYA Navigation text (several ages are devoted to this), or for the method I proposed in the CTS thread I started.
Hi,

I must confess it has been some time since I did my RYA Yachtmaster and can't remember in any detail what was taught. However, apart from the odd throwaway comments about adding allowances for leeway, I have not seen anything that provides useful leeway data. Probably because this is so boat-specific and difficult to gather.

I don't remember seeing any leeway data going into any of the examples presented in the other threads, but I wasn't following them too closely, and not the first one at all.

If I were to ask the question: If TWA=70deg, TWS = 18kts, moderate sea, No.2 headsail, full main, what is the magnitude and angle of leeway generated on your boat? I doubt many could answer - I couldn't. But this is precisely the information you need if you intend to add the leeway vector to your tidal vector for the purposes of calculating a single CTS.

Quote:
Originally Posted by Seaworthy Lass View Post
One thought for you to ponder on:
What happens if current is encountered (and had been predicted from tidal data) that is significant to (or horrifyingly even greater than) in certain areas of the course than your expected boat speed?

Maybe this is not encountered when racing, but it may certainly be when cruising and trying to make a passage in adverse conditions.
I does happen when racing - when the wind dies. The answer is that you anchor. Otherwise you are going backwards - not much fun
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Old 01-02-2013, 01:51   #13
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Re: Single CTS or follow the Courseline?


Wot settles in for some interesting reading
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Old 01-02-2013, 02:40   #14
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Re: Single CTS or follow the Courseline?

Quote:
Originally Posted by bewitched View Post
BSP or SOG?

I've just noticed another significant error in the CTS method. The course vector. Surely that should use SOG... not BSP?
You are correct. The CTS methods are using the V to B vector length and time to work out speed, or using length and speed to figure out the time, (where V is the end of the current vector and B is the destination). As has been shown, this plotted distance is *not* the through-the-water distance, so SOG should be used.

However, the actual SOG will be somewhat less than if we were actually sailing on the VB line, since our track will have a constantly-varying current-induced sideways ground vector.

I'm not sure how we work out the speed. It's probably SOG * cos(angle BAV), or BSP / cos(angle BAV). Someone should look into this. It could be important!

Or, if we calculate speed by looking at time and the distance AB (which is the actual through-the-water distance for the CTS method), then we can use BSP (speed through the water) in the calculations. I think that works. It's not how I was doing it though.
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Old 01-02-2013, 04:23   #15
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Re: Single CTS or follow the Courseline?

On second thought, I think I was wrong.

We have been using BSP in the calculations, and this is correct. The the segment VB *is* the distance sailed through the water (I was wrong about that in my previous post).

So, we don't use SOG, we use BSP, since we are dealing with distance through the water when working out the VB vector.
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