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Old 02-02-2013, 22:33   #91
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Re: Plotting the expected ground track

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Originally Posted by Seaworthy Lass View Post
The SWL method will, however, at least get you within a degree of two of the destination if the data is correct.

"If" is really rather conditional.

When navigating I give the helmsman a course to steer that is multiples of 5. And that requires a very good helmsman.

Quote:

The RYA method can be 10 or 20 or 30+ degrees in error.
Only in your extreme hypothetical scenarios. I still have not seen you present a realistic scenario. My scenarios between Ediz Hook and Victoria show no significant difference.
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Old 02-02-2013, 22:33   #92
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Re: Single CTS or follow the Courseline?

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The only problem with LJH defintions is that some are idiosyncratic.

SOG is a GPS term refering to a momentary speed based on a the average of a some fixes. The term used in vectoring is SMG.
Sorry, no, I do not think this is correct.
SOG is not the same as SMG.
SMG is relative to the destination, SOG is not.

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It is essential that watch navigators use standard terms and symbols.
I agree .
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Old 02-02-2013, 22:35   #93
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Re: Single CTS or follow the Courseline?

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Sorry, no, I do not think this is correct.
SOG is not the same as SMG.
SMG is relative to the destination, SOG is not.
Good I am saying they are not the same as well.
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Old 02-02-2013, 22:53   #94
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Re: Plotting the expected ground track

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Only in your extreme hypothetical scenarios. I still have not seen you present a realistic scenario. My scenarios between Ediz Hook and Victoria show no significant difference.
Jackdale, it hasn't been for lack of trying .

The only charts I have on board are of the Med. Messina Straits would fit my extreme examples beautifully going from Messina to Reggio Calabrio (5.5nm heading 140 approx, with current almost N-S), but the stong current there is very wind dependent and difficult to predict accurately sometimes, so I know you would dismiss this example.

I searched for hours yesterday morning, but although I could find plenty of tide tables, I could find no tidal stream data online.

I know from a couple of decades of sailing a month each year in Australia's far north east, that tidal streams encountered can easily be 4 knots and cross passages are common. So my data for currents selected over 3 hours in my examples of 3.5 knots, then 1.5 then 0.5 are not at all unreasonable.

There was only one example I gave where I chose unusually high values for current. Dismiss that example if you like, I think the rest of the examples are perfectly valid for the real world. And these do show that a large error can be present in the RYA method for determing CTS.
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Old 02-02-2013, 23:10   #95
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Safety

I would like to address this issue as it is an important one:

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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 doesnít 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 Iím 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.
With the CTS method, the 'expected ground track' is also plotted on the chart.

I feel it is essential to do so once you have determined the CTS (not only to check for hazards, but to check for deviation along the way). I have repeated this over and over in the other thread.


Once you have plotted the expected ground track, checking for hazards along this track is not difficult. Any deviation from this can be just as easily monitored as it can for the NCL method.

Despite the comments GoBoatingNow made in post # 44, the expected ground track can be plotted easily and quickly checked once you have determined your CTS (as I have explained in post #84).

In narrow quarters you must of course abandon this method and do as Dockhead suggests:

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Originally Posted by Dockhead View Post
If you need to follow a straight line through obstacles or through a narrow gap somewhere or a narrow channel, then obviously you need to sail the rhumb line and forget about the efficiency of constant heading passages. I like to use "track mode" on my pilot in such cases.
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Old 02-02-2013, 23:30   #96
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Leeway

Addressing the issues raised regarding leeway:

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Originally Posted by bewitched View Post
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 (itís 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.
It is critical to allow for leeway in any CTS computation.

Leeway is dependent on the shape of the boat (underwater and above), so it is very specific to the boat you are on. As well, numerous outside factors will affect it.

The RYA Navigation Handbook has two pages dealing with leeway.
It gives a formula to enable it to be estimated when sailing:
Leeway = ( Heel / speed squared) x K
where Heel is measured by an inclinometer and "K is a constant primarily related to the boat's design that can be found by measuring the leeway in one set of conditions, and then put into the formula to calculate leeway in other circumstances".

I don't know how reliable this formula is, but yes, leeway can certainly be estimated!

The better you can determine the amount of leeway for your boat for the expected conditions, the more accurate your CTS will and therefore quicker you will get to your destination. If you are racing it is important to take the time to get a good estimate for K.
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Old 03-02-2013, 00:17   #97
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BSP = Vboat = S

Last (for now ), but not least:

Quote:
Originally Posted by bewitched View Post
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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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 Ė 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, Iíve crunched some numbers based on my own boatís 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?
Of course you go as fast as you possibly can in each boat!

As the BSP increases unexpectedly, the advantage of the CTS method diminishes. At some point the increase may even even result in the same time taken as the NCL method, or it may be worse (even if you recompute your 'expected ground track' and remember Expedition is probably doing this instantaneously for you).

But, even if the BSP increases unexpectedly the CTS method may still have you ahead .

However, if the BSP drops, using the NCL method can be a huge disadvantage as your only option is to continue navigating the straight line to the destination. In light wind you may end up going backwards if the water is too deep to anchor! You may well still be making progress in the CTS method as you are sailing on a different heading.

Bewitched, you have not addressed the issue at all of strong current relative to boat speed that is predicted in the latter portion of a long leg. The boat sailing on a CTS will have allowed for this and will still be making progress, while you will be furiously crabbing and making little or no progress or worse still going backwards if you can't anchor!

In this instance how frustrating would it be to see a rival go sailing past in the distance and arrive at the mark and then the finish line well before you? (Perhaps you could fly the protest flag because they were using voodoo mathematics to win the race LOL )
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Old 03-02-2013, 00:59   #98
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Re: Single CTS or follow the Courseline?

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(Perhaps you could fly the protest flag because they were using voodoo mathematics to win the race LOL
Love it...
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Old 03-02-2013, 03:36   #99
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Re: Single CTS or follow the Courseline?

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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
I actually did answer in some detail, and based on real experience.

I'll do it again.

You do NOT plot any ground track in order to check your progress except in case of assumed perpendicular tides (more about anon). You simply rerun your CTS calculation from scratch. If the solution is the same CTS you started out with, you are on track. If not, then you go over to the new CTS. It is really very simple. If you're plotting it by hand, it's a quick and easy solution which is not too bad to do every hour or two, so long as you are not single handing . Especially with SWL's excellent method. Or even easier, with a computer program like Neptune.

If you are assuming perpendicular tides, as we usually do in the Channel, it is even easier. Your original calculation gave you a set in miles for every hour of your passage. I you just finished Hour 3 of your passage, you simply add up the set you calculated for Hours 1, 2 and 3 and check that against XTE on your plotter -- et voila, a very accurate check of your progress. If you are off, you run the numbers again. It is really not rocket science, once you have a bit of practical experience!


A few smallish corrections to your CTS will hardly affect the efficiency of your passage, and the calculations are not laborious. The whole thing is not daunting at all, to those who actually practice this technique!
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Old 03-02-2013, 03:41   #100
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Old 03-02-2013, 03:49   #101
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Re: BSP = Vboat = S

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Originally Posted by Seaworthy Lass View Post
Last (for now ), but not least:





Of course you go as fast as you possibly can in each boat!

As the BSP increases unexpectedly, the advantage of the CTS method diminishes. At some point the increase may even even result in the same time taken as the NCL method, or it may be worse (even if you recompute your 'expected ground track' and remember Expedition is probably doing this instantaneously for you).

But, even if the BSP increases unexpectedly the CTS method may still have you ahead .

However, if the BSP drops, using the NCL method can be a huge disadvantage as your only option is to continue navigating the straight line to the destination. In light wind you may end up going backwards if the water is too deep to anchor! You may well still be making progress in the CTS method as you are sailing on a different heading.

Bewitched, you have not addressed the issue at all of strong current relative to boat speed that is predicted in the latter portion of a long leg. The boat sailing on a CTS will have allowed for this and will still be making progress, while you will be furiously crabbing and making little or no progress or worse still going backwards if you can't anchor!

In this instance how frustrating would it be to see a rival go sailing past in the distance and arrive at the mark and then the finish line well before you? (Perhaps you could fly the protest flag because they were using voodoo mathematics to win the race LOL )
+1 to all of this.

Why in the world would sailing an intentionally longer path possibly be inherently faster in any way? I find this idea to be simply bizarre.

Of course you push your boat as hard as you can. You NEVER slow down to make your plan, as I already posted a couple of pages back -- where did anyone get such an idea? When making your initial calculations, you forecast your average speed, and often in three variants (I always do 7, 8 and 9 knots for Channel crossings). If you get out there and see you're averaging 9 instead of 8, you just punch the "+1" or "-1" on your pilot a couple of times and carry on.

It will be only cases of very weak currents or very consistent currents, that you can come even close crabbing down the rhumb line. I think SWL actually understates this case!
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Old 03-02-2013, 05:22   #102
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Re: BSP = Vboat = S

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Of course you go as fast as you possibly can in each boat!
I must confess, I'm not familiar with your method, but the RYA method (which is the one I have made the comparison with) requires a constant boat speed in order to work. It's not a variable in the calculation. If it changes during the passage, your navigation will be off.

Quote:
Originally Posted by Seaworthy Lass View Post
Bewitched, you have not addressed the issue at all of strong current relative to boat speed that is predicted in the latter portion of a long leg. The boat sailing on a CTS will have allowed for this and will still be making progress, while you will be furiously crabbing and making little or no progress or worse still going backwards if you can't anchor!)
I didn't realise I'd been asked this question. If you look back to my post#3, you will see that I am absolutely convinced that maintaining a single CTS is faster than the NCL method IF all the variables are considered accurately (and I've yet to see in an example where they are actually considered at all) and those variables are representative what actually happens on the passage (and I believe the correlation between the two has to be very close).

So, if you believe you can meet the 'ifs' then the CTS boat wins hands down.

If there a small fraction of error or omission in the calculation data or a small difference between the conditions expected and the actual passage itself and I would be wouldn't be putting money on either.

The longer the leg, the more I'd be thinking about backing the NCL boat.
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Old 03-02-2013, 05:58   #103
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Re: Plotting the expected ground track

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Bill (and everyone else who is puzzled), the method is simple.
SWL - That is a beautifully clear presentation, and thank you for taking the time to respond. Just to clarify, I never doubted that something like this would be done by the prudent navigator, but it was never obvious that this step was part of the calculation and implementation of the complete CTS method; and, judging from other posts, it may not be so in all cases.

Since apparently you did explain all this at the beginning, I'm going to be embarrassed now when I go back and see that it just went right over my head. I can only plead confusion caused by the immediate and vigorous introduction of peripheral issues following your original post.

Thanks to you, I am quite satisfied now on the procedure, although I see that the discussion may not be entirely over:

Your post #95 - "With the CTS method, the 'expected ground track' is also plotted on the chart. "

And Dockhead's #99 - "You do NOT plot any ground track in order to check your progress except in case of assumed perpendicular tides (more about anon)."

But I think I'll sit this one out for now. Thank you again for taking the time and trouble to respond.

Bill
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Old 03-02-2013, 06:14   #104
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Re: BSP = Vboat = S

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I must confess, I'm not familiar with your method, but the RYA method (which is the one I have made the comparison with) requires a constant boat speed in order to work. It's not a variable in the calculation. If it changes during the passage, your navigation will be off.
Both for the RYA method and mine you will be constantly recomputing the CTS when you are racing.

Quote:
Originally Posted by bewitched View Post
I didn't realise I'd been asked this question. If you look back to my post#3, you will see that I am absolutely convinced that maintaining a single CTS is faster than the NCL method IF all the variables are considered accurately (and I've yet to see in an example where they are actually considered at all) and those variables are representative what actually happens on the passage (and I believe the correlation between the two has to be very close).

So, if you believe you can meet the 'ifs' then the CTS boat wins hands down.

If there a small fraction of error or omission in the calculation data or a small difference between the conditions expected and the actual passage itself and I would be wouldn't be putting money on either.
I will work on an example. I got started this afternoon, but was then distracted by the sunshine .

Quote:
Originally Posted by bewitched View Post
The longer the leg, the more I'd be thinking about backing the NCL boat.
I doubt it. The longer the leg, the worse off you would be I think (eg racing across the English channel to Cherbourg with the tidal stream data Jackdale gave in post # 383 of Inaccurate RYA Teaching : CTS - Quest For a New Method
at 6 knots would put you about 2.7nm behind the leader following the CTS.
Also, the leader following the CTS method would have followed roughly one heading most of the way, resulting in far less sail trimming and crew fatigue.
If your speed was less then this difference could be considerably more.

Remember that on a long journey like this with almost zero total current the CTS does not depend a lot on BSP (really almost just leeway needs to be allowed for and this correction can be adjusted along the way if the wind is not as predicted.

Sailing using the CTS method does not mean one CTS the entire way, just as sailing using the NCL method does not mean not deviating from the rhumbline the entire way (you may be chasing puffs of breeze, heading up in the gusts and bearing away in the lulls if the the sailing angle is marginal, avoiding collisions etc etc). The straight track you need to follow would be adjusted in the NCL method, just as the CTS would be frequently recalculated recalculated and adjustments made in the CTS method. Your expected ground track that was predicted at the beginning will not be followed exactly for either method .
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Old 03-02-2013, 06:35   #105
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Re: Plotting the expected ground track

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Originally Posted by bill352 View Post
Thanks to you, I am quite satisfied now on the procedure, although I see that the discussion may not be entirely over:

Your post #95 - "With the CTS method, the 'expected ground track' is also plotted on the chart. "

And Dockhead's #99 - "You do NOT plot any ground track in order to check your progress except in case of assumed perpendicular tides (more about anon)."

But I think I'll sit this one out for now. Thank you again for taking the time and trouble to respond.

Bill

Let me clarify, and harmonize these different statements.

You do NOT need any ground track plots for checking and correcting your CTS. It would be a ridiculously backwards and awkward way to do it. Plotting a CTS solution is a water-oriented method; your ground track is not part of the workings at all. So the simple and logical way to check your progress is simply plot a new one from scratch from where you are. Simples.

You DO need to plot a ground track, as SWL suggests, in case there is any question that you may run into any hazards. This is a separate exercise having nothing to do with an efficient CTS, or checking or correcting your CTS.

With Neptune, you get a ground track spit out automatically, and if your computer is hooked up to GPS, then it will display in real time where you are over ground relative to your plan. With a computer, of course, you can do it this way, but if you're plotting it's not reasonable since it would require, as Dave said, a lot of number crunching. If you're plotting, then for most cases where you have some concern about hazards you can just do an hourly EP plot and connect the dots. I can't imagine where you would be sailing where that would not be enough.

Also don't forget simply watching the plotter. It's easy to see where you're heading and whether you are heading into danger, if you keep an eye on the plotter. I think most careful sailors always keep an eye on the plotter, to see whether you are running into danger or not, in order to have situational awareness about where you are relative to dangers, whether you're sailing a CTS passage or not.

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