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Old 25-10-2017, 23:39   #646
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

Quote:
Originally Posted by robbievardon View Post
DH in post 583 you again incorrectly quote the Cregs despite the fact that I had already corrected you some time back for your misquote for which you appologised and thanked me. I am not sure why 20 Kn speed is being used as the norm for the merchant vessel for as you have a few posts back stated that due to fuel costs the more realistic speed is around 14Kn. In fact if you look at the performance data for MAN diesel engines for marine application they give max speed as 15 Kn.There is further agreement from within the industry that Slow Steaming is now the norm.
Hi Robbie: Post 583 is not mine. Which post of mine are you referring to?

I actually had not seen Post 583, which is here:

Quote:
Originally Posted by ramblinrod View Post
Actually Colregs does not specifically forbid a series of moves, it recommends they should be avoided (to make intentions clear).

If I am sailing a course to cross a ships course at 90 degrees, and my speed is varying, you are damn right I will make course corrections as required to not run into the side of the lead ship, or delay the crossing and end up a sitting duck in front of the following ship. To hold the course because of 8 (b) and run into either vessel would be supremely stupid.

Instead, adjusting course to make a close pass of the lead ship's stern would be the most prudent, (if one was committed to the crossing in the first place).
Concerning a series of small alterations of course, the Rules say:

"[A] succession of small alterations of course and/or speed should be avoided."

Whether this can be fairly characterized as "it is specifically forbidden" or not is questionable; maybe not.

But in any case, it is extremely bad collision avoidance practice to sail into a crossing while jinking around trying to judge the crossing by eye, especially in close quarters. The Rules do require actions to be "[P]ositive, made in ample time, and with due regard to the observance of good seamanship." It is fundamental to proper collision avoidance that you take action early enough to pass safely as perceptible from a safe distance away -- so that you are holding a steady course and speed as you get close. That is not only to make your intentions clear. It is so that you can be sure that you are well clear before you get so close that it starts to become impossible to get out of the way. It is also so that the other vessel has a chance to decide whether he is happy with your solution or not -- he has the right to the chance to maneuver himself if you have miscalculated. Of course you have the right to a course correction if you made a mistake or if you are not passing safely, but you should not sail into a crossing with the plan of changing course in the last seconds, or of continually changing course as you get close.

The thing that all of the pros on here abhor about the 180 foot crossing, and the thing which Rod never visualized or understood, is that the setup and approach to such a crossing is a collision course. To get within 180 feet of the quarter of a fast moving ship requires passing less than a cable from the bow of that ship, and that cannot be set up with that degree of precision. Passing part of a cable behind, part of a cable ahead is all the same from a couple miles away. And so the ship rushes towards your beam, you may very well be passing ahead, rather than behind, and the plan is to get up REALLY close, close enough to see it by eye, and then just turn away if necessary. The problem of course is that once you are close enough to see it by eye, you are already too close to get out of the way, if you are on the wrong side or not clear enough. And that's the whole problem and the whole reason why we don't do it this way.
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Old 25-10-2017, 23:51   #647
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

Quote:
Originally Posted by Juho View Post
Yes, changes in course, speed, understanding, misunderstanding, measurement accuracy, visual observations, weather conditions and device failures may lead to major problems, from few miles distance all the way to the CPA. I don't have a clear idea yet on how visualise that (other than just calculating the possible impacts of the likely errors, worst case scenarios and bad luck), but I'll comment more if I find some more elegant way to do it.
We're talking about two different things here --

One is the cone of error which will allow you to plot the area on the maneuvering board where there is a good chance he may be by the time you get there. You can't account for everything (like a sudden change of course), but you can understand the limits of accuracy of your plot and visualize a safe CPA.

We don't normally go through an exercise like this, because most people understand instinctively that when you set up a crossing, you have to set it up to cross a certain distance which is capable of absorbing the normal variations in your relative paths.

But if you want to get into the nuts and bolts of what makes a safe CPA and what is not a safe CPA -- then this is extremely useful.

The other thing we are talking about here is the limits of what you can discern with your naked eyes. I don't think we should attempt to incorporate this into the cone of error -- it would be too wild and in the end not that useful. Basically if you do collision avoidance with your naked eyes you need such a large CPA that you have an obviously changing bearing from some distance to some other distance, at which point you need an obviously safe aspect. It would be extremely useful to calculate these things based on what eyes can actually see in terms of resolution, stereoscopic distance perception, etc. -- some guidelines and warning could be developed which could really help recreational sailors.

For a normal analysis of a cone of error I think we would take our best data, which is AIS, which should give us our range to him within a ship's length (with the antenna location being the largest unknown factor) with a high degree of confidence.
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Old 26-10-2017, 01:20   #648
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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Originally Posted by Juho View Post
I drafted one set of parameters that can be used to model the dangers and uncertainties when moving around fast big ships. This is just one approach, there could be many more. Modelling uncertainties is much fuzzier than modelling the ideal cases.

The parameters in the model, and their values in the first drawing are as follows.

size of the ship = 300m x 40m (see the small red area at the origin)
speed ratio = 4:1

Some uncertainty and observation error related parameters:

max error in the observed course of the ship = 5°
max error factor in the speed ratio = 1.2 (up or down)
min distance at which the ship might turn 90° = 4 NM

Some ship specific parameters for our safety:

min CPA = 200m
min allowed distance to approaching ship = 3 NM

The values of the parameters may vary a lot, e.g. if we make visual observations vs by AIS. The estimate on how likely the ship is to turn may depend on its earlier observed behaviour in AIS. If we want to prepare for one quick surprise turn, we could also increase the value of the "max error in the observed course of the ship" parameter.

The "min allowed distance to approaching ship" parameter depends quite a lot on one's personal preferences. It determines the position of the widest point of the "danger zone" that is in front of the ship in the drawing (around the ship and forward all the way to 9.4 NM). The pointy end of the danger zone shows when you should start moving away when the ship approaches you.

The drawings have been made for approaching or escaping the track of the ship in a 90° angle. Drawings for ideal crossing angles are quite similar when the speed difference is high enough.

The two alternative sharp heads of the danger zone are for estimated last minute escape (the shorter one), and the same with the given error margin ("max error factor in the speed ratio").

The three alternative tracks for approaching the track of the ship behind the ship are the estimated track and optimistic and pessimistic tracks (again based on the "max error factor in the speed ratio")

With these parameters you can see that if you are fishing in the middle of a shipping lane, you should move away when the approaching ship is 9.4 NM away. You can return to your fishing spot no later than 3.9 NM behind the ship. With these parameters it is not possible to pass the transom of the ship at the "min CPA" distance (without being inside our danger zone). (In one earlier post I presented a similar example.)

In the second drawing only one parameter has been changed. The "speed ratio" is now 2:1 instead of 4:1. In the drawing you can see that with this faster boat we might be able to pass the ship at the "min CPA" distance if we so wanted.

The ship stays in the origin all the time. Boat's position (along the curves) is shown in relation to the position of the ship.

Is there any feeling of naturalness in these parameters? Are the drawings helpful in visualising the dilemma of staying away from the danger zone and not being able to cross the track right behind the ship? The boats in the given examples do not want to take big risks. Are the parameter values ok for such an attitude, when meeting a large fast ship?
Wow, this is incredibly impressive work, and I think hugely valuable.

By "min allowed distance to approaching ship" -- you mean the decision point, right? The last moment to leave a collision course before executing a collision avoidance maneuver? If I'm understanding that correctly, then I think that's about right. That's near the end of the normal stand-on, give-way maneuvering in open sea without complicating factors (like other ships).

I find it interesting that you modeled possible turns by the ship. A really important reason for taking early and positive action is to give yourself room to respond in case of an expected change of course of the ship.

But this parameter will be so different from the others, that I think it would be useful to model them separately. I think the first question we want to answer is "How much CPA do I need in a given situation to avoid him under any combination of the normal variations and uncertainties about his and my course, speed, position, etc."


Are you based in Helsinki? I'll be in Helsinki next week on business. I would be happy to sit around a table with you with maneuvering boards and bottle of Koskenkorva, if this sounds like fun and you can spare the time.
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Old 26-10-2017, 03:31   #649
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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Originally Posted by ramblinrod View Post
Cool.



Tomorrow will be another day,

With another issue fought and won,

But for me my friends, without a doubt,

This one is pondered, perpended...done.

Suggested take-aways from this thread.

1. Never cross a bow you don't have to.
2. Beware applying an old gestalt to a new situation.
3. The large bolts be in the engine, but the biggest nut, is always behind the wheel.
Why don't you answer the questions? Why can't you answer even one question? You've vociferously argued that it's safe and effective to pass large ships as close as 180' on their stern, so now's the time to put your money where your mouth is.

How do you measure a 180' passing by eyeball?
Is there a minimum distance that you would pass? If yes, then what is it and how do you ensure that by eyeball?
How close have you ever passed a large ship underway?
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Old 26-10-2017, 03:54   #650
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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Originally Posted by Lodesman View Post
. . . How do you measure a 180' passing by eyeball? . . . Is there a minimum distance that you would pass? If yes, then what is it and how do you ensure that by eyeball? . . .
In a sincere, friendly effort to help Rod visualize and understand this crossing --

Rod, can you draw what this looks like? Where will you be as you approach this crossing from 3 miles out, 2 miles out, 1 mile out? What will you see from each of those points? At what point do you expect it to be possible to visualize what kind of correcting you plan to do? When do you plan to do all this correcting you talk about, and based on data from what source? I really think you will be able to see it if you will just draw it.


I'm guessing from the wrong way you calculated CPA earlier in the thread, and from your use of terminology like "starboard course line", that you've never done any plotting. Am I right? Nothing shameful about that; few recreational sailors have any experience with it. It's not difficult, and it's an extremely useful skill to be able to plot and solve relative motion problems on paper. There are lots of resources online, but here's the classic one: http://msi.nga.mil/MSISiteContent/St...RNM/310ch6.pdf

Another good one:

http://www.boatswainsmate.net/BM/MOBOARDS.pdf


And you can download the classic 1920 U.S. Navy Maneuvering Board somewhere; if you Google "maneuvering board printable" you can find it. It can be printed out any size as long as the proportions are not distorted, but bigger is better. All you need to use it is some kind of protractor; best of all a Portland Plotter. A small ruler, dividers, soft very sharp pencils, and LOTS of good erasers -- Art Gum is the best.

Plotting it on a maneuvering board is incredibly useful for visualizing what a crossing will look like.


The key here is you cannot just magically appear a part of a cable off his bow in a position which will allow you to shave his quarter at 180 feet - you have to get there somehow. How do you get there? From what distance do you have to set it up? What data do you have to set it up, and how accurate is it? How and when will you correct any errors? Where is the danger zone in front of his bow? You will see it all if you just plot it.


Here's where you need to be at one mile out:

Click image for larger version

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Views:	52
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ID:	158328


One unit is one cable and Own Ship is at the center. Ship is to scale and it the same one Lodesman was using -- a Neo Panamax vessel 366 x 50.

This is the perfect "glide path" to pass 180 feet past his quarter, that's a CPA of 0.29 cables. From where you are, the place you are trying to get to bears 14 degrees ahead of your beam, and his bow bears 19 degrees ahead of your beam. It requires you to pass within 0.7 cables of his bow. If you are behind the position shown, in relation to the Relative Motion Line, then you can't get there. If you are ahead, then you are on track to pass closer, and might be able to correct by heading off or slowing down. So if you plan to "correct" along the way anywhere, then you must be ahead of the position shown, which means approaching with LESS than 0.29 cables CPA, and then opening it up later by heading off or slowing down.

But at 1 mile out, and with Relative Motion Speed of 20.6 knots, TCPA is only 175 seconds off. During this time you can only move 2.4 cables in total LESS the amount of time it takes you to turn and if necessary get back up to speed. Halfway there and 87 seconds TCPA, you can only move 1.2 cables. At one cable off and 17.5 seconds (!) TCPA, you are already under his bows, because the ship is almost 2 cables long.

This is why you just can't do this maneuver. The Relative Motion Line is not written in the sea, and you can't see it with the naked eye, and besides that, it's changing all the time. Passing this much behind -- 180 feet behind his quarter, 0.7 cables from his bow -- looks exactly the same -- even to radar, much less to the naked eye -- as passing that much ahead -- you can't see the difference. Even AIS is not accurate enough to tell you. Every degree of variation in his course changes the CPA by 16 meters -- 3 degrees turns a 180 foot CPA into a 0 CPA. If he turns by 10 degrees to port at this point, from this position, you cannot get out of his way, if you can even detect it at all and know which way to turn

So not only can you not do this maneuver, you must not be there, where you would need to be, a mile off. The position shown in the maneuvering board is a COLLISION COURSE -- 1 mile off, 0 CPA (0.3 cables is not recognized as a positive CPA, much less a safe one*), and 175 seconds TCPA -- is a desperate, in extremis situation. The ship, unless they have simply not seen you, will not let you be there. They will sound 5 short and turn hard, if you somehow appeared in this position, and it may already be too late to avoid collision considering the maneuvering characteristics of large ships (if he turns hard to starboard, the whole ship will move first out to port, before the the stern swings out and ROT comes up).

Again, this is a very useful and interesting discussing, forcing us to think through in detail things which we take for granted as obvious. Which is always useful.



* You can see from the maneuvering board, how you can't plot a 180 foot CPA -- the thickness of a pencil lead already throws it off. That should tell you something. Radar, even a big ship's X band radar, does not have enough range or bearing resolution. If you watch AIS in a close crossing, you will see it jump around +/- a cable or more. That reflects the unknowability of a crossing to that degree of accuracy.
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Old 26-10-2017, 07:19   #651
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

Quote:
Originally Posted by Dockhead View Post
Concerning a series of small alterations of course, the Rules say:

"[A] succession of small alterations of course and/or speed should be avoided."

Whether this can be fairly characterized as "it is specifically forbidden" or not is questionable; maybe not.
"Should be avoided" / "specifically forbidden" - hmmmm, are these synonyms. No. Another case of "alternate facts".

Quote:
"It is extremely bad collision avoidance practice to sail into a crossing while jinking around
..."

Hmmm, I don't recall using the term "jinking" or any term similar. (strawman).

Quote:
It is fundamental to proper collision avoidance that you take action early enough to pass safely as perceptible from a safe distance away so that you are holding a steady course and speed as you get close.
So if you approached the crossing as YOU said you would, and realized your plan was flawed, or the set-up wasn't perfect, or one of the many parameters affecting the crossing varied, you would hold fast and purposefully run into the lead or following ship rather than change course or speed to compensate?

Dude, you're digging the hole deeper.

Quote:
The thing that all of the pros on here abhor about the 180 foot crossing, and the thing which Rod never visualized or understood, is that the setup and approach to such a crossing is a [I][B]collision course.
Strawman, strawman, strawman.

Dude, I know full well what that set-up looks like.

Not the preferred way to approach a ship for sure.

But that wasn't the question.

The question was, given the scenario, if you needed to cross, how would you?

If the parameters remained constant, my solution would work.

Under the same circumstances, yours would most certainly get you pasted against the starboard bow of the following ship.

You knew (or should have known) that the crossing was risky before you made that stupendous blunder.

You had all the time in the world to consider and plan a solution, not like in real life. You blew it. Get over it.

(Grade School Argument Tactic #56 - After the test is failed, claim the entire test was at fault instead of your answer.)
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Old 26-10-2017, 07:45   #652
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

Quote:
Originally Posted by ramblinrod View Post
"Should be avoided" / "specifically forbidden" - hmmmm, are these synonyms. No. Another case of "alternate facts".

..."

Hmmm, I don't recall using the term "jinking" or any term similar. (strawman).



So if you approached the crossing as YOU said you would, and realized your plan was flawed, or the set-up wasn't perfect, or one of the many parameters affecting the crossing varied, you would hold fast and purposefully run into the lead or following ship rather than change course or speed to compensate?

Dude, you're digging the hole deeper.



Strawman, strawman, strawman.

Dude, I know full well what that set-up looks like.

Not the preferred way to approach a ship for sure.

But that wasn't the question.

The question was, given the scenario, if you needed to cross, how would you?

If the parameters remained constant, my solution would work.

Under the same circumstances, yours would most certainly get you pasted against the starboard bow of the following ship.

You knew (or should have known) that the crossing was risky before you made that stupendous blunder.

You had all the time in the world to consider and plan a solution, not like in real life. You blew it. Get over it.

(Grade School Argument Tactic #56 - After the test is failed, claim the entire test was at fault instead of your answer.)
Sounds like a confession.
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Old 26-10-2017, 07:49   #653
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

Quote:
Originally Posted by Dockhead View Post
In a sincere, friendly effort to help Rod visualize and understand this crossing --

Rod, can you draw what this looks like? Where will you be as you approach this crossing from 3 miles out, 2 miles out, 1 mile out? What will you see from each of those points? At what point do you expect it to be possible to visualize what kind of correcting you plan to do? When do you plan to do all this correcting you talk about, and based on data from what source? I really think you will be able to see it if you will just draw it.


I'm guessing from the wrong way you calculated CPA earlier in the thread, and from your use of terminology like "starboard course line", that you've never done any plotting. Am I right? Nothing shameful about that; few recreational sailors have any experience with it. It's not difficult, and it's an extremely useful skill to be able to plot and solve relative motion problems on paper. There are lots of resources online, but here's the classic one: http://msi.nga.mil/MSISiteContent/St...RNM/310ch6.pdf

Another good one:

http://www.boatswainsmate.net/BM/MOBOARDS.pdf


And you can download the classic 1920 U.S. Navy Maneuvering Board somewhere; if you Google "maneuvering board printable" you can find it. It can be printed out any size as long as the proportions are not distorted, but bigger is better. All you need to use it is some kind of protractor; best of all a Portland Plotter. A small ruler, dividers, soft very sharp pencils, and LOTS of good erasers -- Art Gum is the best.

Plotting it on a maneuvering board is incredibly useful for visualizing what a crossing will look like.


The key here is you cannot just magically appear a part of a cable off his bow in a position which will allow you to shave his quarter at 180 feet - you have to get there somehow. How do you get there? From what distance do you have to set it up? What data do you have to set it up, and how accurate is it? How and when will you correct any errors? Where is the danger zone in front of his bow? You will see it all if you just plot it.


Here's where you need to be at one mile out:

Attachment 158328


One unit is one cable and Own Ship is at the center. Ship is to scale and it the same one Lodesman was using -- a Neo Panamax vessel 366 x 50.

This is the perfect "glide path" to pass 180 feet past his quarter, that's a CPA of 0.29 cables. From where you are, the place you are trying to get to bears 14 degrees ahead of your beam, and his bow bears 19 degrees ahead of your beam. It requires you to pass within 0.7 cables of his bow. If you are behind the position shown, in relation to the Relative Motion Line, then you can't get there. If you are ahead, then you are on track to pass closer, and might be able to correct by heading off or slowing down. So if you plan to "correct" along the way anywhere, then you must be ahead of the position shown, which means approaching with LESS than 0.29 cables CPA, and then opening it up later by heading off or slowing down.

But at 1 mile out, and with Relative Motion Speed of 20.6 knots, TCPA is only 175 seconds off. During this time you can only move 2.4 cables in total LESS the amount of time it takes you to turn and if necessary get back up to speed. Halfway there and 87 seconds TCPA, you can only move 1.2 cables. At one cable off and 17.5 seconds (!) TCPA, you are already under his bows, because the ship is almost 2 cables long.

This is why you just can't do this maneuver. The Relative Motion Line is not written in the sea, and you can't see it with the naked eye, and besides that, it's changing all the time. Passing this much behind -- 180 feet behind his quarter, 0.7 cables from his bow -- looks exactly the same -- even to radar, much less to the naked eye -- as passing that much ahead -- you can't see the difference. Even AIS is not accurate enough to tell you. Every degree of variation in his course changes the CPA by 16 meters -- 3 degrees turns a 180 foot CPA into a 0 CPA. If he turns by 10 degrees to port at this point, from this position, you cannot get out of his way, if you can even detect it at all and know which way to turn

So not only can you not do this maneuver, you must not be there, where you would need to be, a mile off. The position shown in the maneuvering board is a COLLISION COURSE -- 1 mile off, 0 CPA (0.3 cables is not recognized as a positive CPA, much less a safe one*), and 175 seconds TCPA -- is a desperate, in extremis situation. The ship, unless they have simply not seen you, will not let you be there. They will sound 5 short and turn hard, if you somehow appeared in this position, and it may already be too late to avoid collision considering the maneuvering characteristics of large ships (if he turns hard to starboard, the whole ship will move first out to port, before the the stern swings out and ROT comes up).

Again, this is a very useful and interesting discussing, forcing us to think through in detail things which we take for granted as obvious. Which is always useful.



* You can see from the maneuvering board, how you can't plot a 180 foot CPA -- the thickness of a pencil lead already throws it off. That should tell you something. Radar, even a big ship's X band radar, does not have enough range or bearing resolution. If you watch AIS in a close crossing, you will see it jump around +/- a cable or more. That reflects the unknowability of a crossing to that degree of accuracy.
I am disregarding this entire post on the grounds that it is yet another plethora of strawman arguments, and nothing more than an attempt to deflect attention away from...

Basic Navigation Test - Close Crossing # 3

Me: 180 ft aft (dx increasing rapidly) / 1500 ft ahead (dx decreasing rapidly)

Grade: A+ (max safety under circumstances)

EMV1024: 600 ft aft of lead / 1200 ft ahead of following

Grade: B (may work, but closer to most "certain" dangerous aspect of crossing)

Dockhead: 1800 ft aft / guaranteed contact with following vessel

Grade: F (guaranteed collision)
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Old 26-10-2017, 08:01   #654
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

Quote:
Originally Posted by ramblinrod View Post
"Should be avoided" / "specifically forbidden" - hmmmm, are these synonyms. No. Another case of "alternate facts".

..."
And what do you consider the significance of the difference to be? In any case, it is BAD PRACTICE, for reasons which have been explained.


Quote:
Originally Posted by ramblinrod View Post
Hmmm, I don't recall using the term "jinking" or any term similar. (strawman).
We're going to have to report you to the Society for the Prevention of Cruelty to Strawmen Do you even know what a strawman is? I'm starting to doubt it.

In any case, this has nothing to do with terminology -- this is the specific tactic recommended -- getting up close and then maneuvering around in close quarters. Call it what you like -- I call it "jinking". You may call it something else but the name doesn't change the substance.




Quote:
Originally Posted by ramblinrod View Post
So if you approached the crossing as YOU said you would, and realized your plan was flawed, or the set-up wasn't perfect, or one of the many parameters affecting the crossing varied, you would hold fast and purposefully run into the lead or following ship rather than change course or speed to compensate?

Dude, you're digging the hole deeper.
I already stated that there is no safe way to do this crossing.

The way I had earlier suggested doing it, based on a mistake in plotting it, was the closest feasible and somewhat safe way to cross with the lead ship. Since it turned out that the CPA with the following ship would be only slightly more than a cable, it's not safe. So there is no safe way to do it, as I stated.



Quote:
Originally Posted by ramblinrod View Post
Dude, I know full well what that set-up looks like.
Obviously you do not. When it was explained to you, you even denied that the ship would be approaching from your beam. I gave you a friendly suggestion about how to plot it so you could visualize it -- really, you should do it.

Quote:
Originally Posted by ramblinrod View Post
Not the preferred way to approach a ship for sure.

But that wasn't the question.

The question was, given the scenario, if you needed to cross, how would you?

If the parameters remained constant, my solution would work.
No, your solution would not work at all, as by now many people besides me have explained to you. It requires approaching the lead ship on a collision course PLANNING to pass less than a cable from its bow. But you can't implement that plan, anymore than you could execute such a crossing PLANNING to pass one inch from his quarter -- you cannot make that approach with even vague knowledge that you are not passing ahead or running straight into a collision, and by the time you figure it out, you will not be able to get out of the way. What's even worse is that you propose to do it by eye, "getting your head out of the electronics", so relying on less accurate information. You can't even set it up -- you can't even get anywhere near where you would need to be -- doing it by eye.

The only way you could do it would be if both you and the ship were running on rails, and you had God's knowledge of his position, course and speed, and you're own, so that you could set it up from far enough away that with your speed, you can get into position -- several miles at least. You don't, so you can't. Without God's prescient knowledge, a 180 foot CPA is a collision course -- it's the same as 0 CPA, just like a one inch CPA is.

Quote:
Originally Posted by ramblinrod View Post
Under the same circumstances, yours would most certainly get you pasted against the starboard bow of the following ship.
My solution is to ask the following ship to alter course to port by 10 degrees, then to pass the lead ship with a CPA of 2 or 3 cables. Without alteration of course by the following ship, the crossing cannot be done even in a cowboy fashion.



Quote:
Originally Posted by ramblinrod View Post
You knew (or should have known) that the crossing was risky before you made that stupendous blunder.

You had all the time in the world to consider and plan a solution, not like in real life. You blew it. Get over it.

(Grade School Argument Tactic #56 - After the test is failed, claim the entire test was at fault instead of your answer.)
What are you talking about? Weird.
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Old 26-10-2017, 08:19   #655
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

Quote:
Originally Posted by ramblinrod View Post
I am disregarding this entire post on the grounds that it is yet another plethora of strawman arguments, and nothing more than an attempt to deflect attention away from...

Basic Navigation Test - Close Crossing # 3

Me: 180 ft aft (dx increasing rapidly) / 1500 ft ahead (dx decreasing rapidly)

Grade: A+ (max safety under circumstances)

EMV1024: 600 ft aft of lead / 1200 ft ahead of following

Grade: B (may work, but closer to most "certain" dangerous aspect of crossing)

Dockhead: 1800 ft aft / guaranteed contact with following vessel

Grade: F (guaranteed collision)
I went and reread posting number 1 just now. As this is Dockheads thread he gets to set the direction and topic Rod. I'm sure you well know that.

In re-reading that post I see that you are arguing from the basis of an entirely different thread. Sounds like a lack of flexibility to me.

You should remember that being a bit of a bully does not make you right.

As for the one proposed crossing where I set up for a 2 cable CPA to the oncoming ship - I would not give it a B. At best it is a D. The crossing situation as a whole sucks - but you know that....

You continue to be very heavily invested in being right. You give your 180' CPA solution an A+ (tisk tisk ego) all the while ignoring the real problems in actually getting that close to the 4th or 5th ship in line. And ignore the very real problem of that ship taking collision avoidance measures and throwing your crossing into disarray - can you say SNAFU nay make that TARFU.

You fail to realize that the goal in this cross is not to get as close to the leading ship as possible so as to keep as far away from the trailing ship as you can.

Indeed the goal is to get as far away from the leading ship as possible while maintaining a safe distance from the trailing ship. It is a balance where the risks of the leading ship and in balance with the risks of the trailing ship.

I should point out that in the 600' CPA I did the math on we end up being about 1 nm ahead of the trailing ship when crossing the ships centerline.

It is plainly clear that you have not been 180' away from a 600' long ship doing 20 kts. No need to answer that question. Your silence speaks volumes.

It is also plainly clear that Dockhead pissed you off and much of your posts follow on that. FWIW I'm fairly sure that Dockhead did not realize how pissed off you would become and is quite sorry.

But you are entrenched in your position and will not and cannot budge one bit. But let me try to nudge you just a little.

Rod, what do you think about opening up your proposed CPA to 200' does it make it better or worse than 180'?
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Old 26-10-2017, 08:50   #656
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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Originally Posted by Dockhead View Post
And what do you consider the significance of the difference to be?
Anyone with reasonable comprehension of English, would most likely understand:

A) "should be avoided" to mean "This is not the normal or preferred approach, but under certain conditions may be acceptable."

B) "specifically prohibited" to mean, "though shalt not under any circumstances."

I see a big difference, do you see a big difference?

Quote:
This has nothing to do with terminology -- this is the specific tactic recommended -- getting up close and then maneuvering around in close quarters. Call it what you like -- I call it "jinking". You may call it something else but the name doesn't change the substance
.

This is like saying "Afro-American" and n-word" doesn't change substance.

"Course correction" / "jinking around"

Big difference. Only a person desperate to deflect attention away from a huge blunder, like running into a 600 foot ship, would attempt this.

Quote:
I already stated that there is no safe way to do this crossing.
Common knowledge before attempting. As I demonstrated and proved in post # 591 calcs, my solution has success potential, yours does not.

Quote:
The way I had earlier suggested doing it, based on a mistake in plotting it, was the closest feasible and somewhat safe way to cross with the lead ship.
Since when is being pasted against the starboard bow of a 600 foot ship, "somewhat safe".

Quote:
Since it turned out that the CPA with the following ship would be only slightly more than a cable, it's not safe. So there is no safe way to do it, as I stated.
Incorrect! You suggested a CPA from the lead ship of 2-3 cables. At the 3 cable limit of the range you proposed, you would most definitely be run down by the following ship if they held course and speed.

Quote:
No, your solution would not work at all, as by now many people besides me have explained to you. It requires approaching the lead ship on a collision course passing less than a cable from its bow.
Incorrect, the geometry is certainly possible. The solution is based on everything remaining constant. As I mentioned, in reality, where everything is not constant, course /speed alteration may be necessary to address variables along the way. But it is possible.

Quote:
My solution is to ask the following ship to alter course to port by 10 degrees, then to pass the lead ship with a CPA of 2 or 3 cables.
Sorry, you are not Captain Kirk, and this is not the Kobayashi Maru scenario, where you get to change the test. The original scenario included inability to communicate with the ships. You obviously understood this, as communication with the ships was not part of your solution, you just plotted a course for 2-3 cables from the lead ship.

You may claim all you wish that my solution would put me close to the bow of the lead ship. I already knew that very, very, well.

But I would far sooner be close to, but not in the direct path of, the lead ship, (my solution) rather than being most definitely hammered by the following ship (your solution).

Bang, glug, glug, glug.
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Old 26-10-2017, 08:58   #657
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

By the way, one thing this thread is about is "what is a safe CPA"? But there is a completely different question which is also highly relevant to this whole topic -- what is a MEANINGFUL CPA? This is a question about the Accuracy and Precision of CPA calculations.

Again, it's fantastic that we have been forced to think about this -- we all take for granted that we can't rely on a CPA, calculated by whatever means, which is too close. This covers two completely different things -- uncertainty about the present (what do we know about his position, course and speed, and our own, and how well can we plot it), and uncertainty about the future (variations in the basic parameters between the time of calculation and time of closest approach).

Let's make no mistake about it -- until we are close enough to have accurate stereoscopic vision, AIS is by far the most accurate information we have, with position of both vessels known to a few meters, and highly accurate data about COG and SOG, plotted almost instantaneously based on data received as often as every two seconds (as transmitted by his Class "A" set -- his data about us will be far worse, limited by our Class "B" transmissions at 30 seconds).

Juho, I am sure, will be able to tell us a lot about the accuracy and precision of this data, but there is actually a very, very simple way to see it almost graphically:

Just watch the CPA displayed by AIS in a close crossing. If you've ever been in a close crossing with a fast ship, close enough that CPA is displayed in meters, you can see it changing every few seconds: 300 meters, then 25 meters, then 600 meters, then -250 meters, etc., etc. There you have the limitation of accuracy, so that the display in meters is actually false precision.

AIS will not be usable once you are within a ship's length of a target -- because of the antenna placement problem. But in the approach, when you are still at least a few cables off, it will show you exactly, what is the limitation of accuracy. This will vary a lot with sea conditions -- because it is our course-keeping which varies the most, and our speed if we are under sail.

AIS is a fantastic tool for trying to make a close crossing, because it will show you exactly how far you can trust it -- you will see that you have to steer until the CPA no longer jumps between a planned CPA which you think may be safe, and a 0 CPA or negative one (crossing the other side), or a small one measured in meters. It reports itself, as it were, on its own accuracy.

If you are trying to pass as close as two cables from a fast moving, large ship, however, you cannot ignore the antenna placement problem (a Triple E is two cables long). It's frustrating that we don't get that part of the static data, on our Class "B" sets. I think one would need to assume that the antenna is somewhere in the accommodation block, and add the estimated distance from there to the part of the ship you expect to come closest to, to your CPA.
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Old 26-10-2017, 09:35   #658
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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Originally Posted by ramblinrod View Post
Incorrect! You suggested a CPA from the lead ship of 2-3 cables. At the 3 cable limit of the range you proposed, you would most definitely be run down by the following ship if they held course and speed.
And then Dockhead found his error, and told us about it. He admitted that his proposed solution was unsafe.

He admitted his error. This is a tactic you might very well consider for yourself sometime.
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Old 26-10-2017, 09:43   #659
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

The thread would gain value by discussing real world crossings and approaches. I would like to see less discussion around 50 ships in a line. Thanks for your consideration; from someone trying to learn something useful.
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Old 26-10-2017, 09:48   #660
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

Let's all remember that this 180' business was called for WAY before the 'line of ships doing 20' was brought up. It was all down to 'pucker factor', remember?

Now, it's changed. Now, it's 'it's the only way to make the 'line of ships' crossing. Well, even though it's not possible to do, that was not the context of the original argument.

This whole business started early in the thread, and the 'line of ships' is being used as cover for a poor argument.

This kind of maneuvering is just nuts. The thing that I don't like about it, is that there may be some semi-experienced folks around who think 'Hey, I've got a big pair and I know I'm an awesome navigator-It's perfectly fine to make this kind of a close pass with a ship'.

Friends, it isn't. Not on the open sea (which is what we're talking about), It's just not.

Every pro, every math whiz-all agree. This is not ok. Forget the 'line of ships' scenario. That's a true straw man, being leaned on now. That's not how this started.
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