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Old 27-10-2017, 04:42   #691
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

Most people who are not trained or lack very recent experience cannot judge distance and trajectory as well as they imagine. An example, go to a big league baseball park and try to catch big league fly balls. Professional players make it look easy. But untrained eyes are easily deceived and most people cannot catch a single ball until they practice a while. The same holds true for judging distance, course and speed of vessels on the water. Most people cannot estimate distance to within +/-25% and speed estimates are often off by 2:1. With training and practice the visual estimates can be improved. AIS and radar can help us to improve our eyeball estimates. But it takes constant "retraining" else, over time, our eyes will deceive again.

And let's face it, the older we get the poorer is most people's eyesight. Acknowledging that is key. Using modern instruments greatly enhances safety if we understand their (and our own) limits.
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Old 27-10-2017, 05:04   #692
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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Originally Posted by transmitterdan View Post
Most people who are not trained or lack very recent experience cannot judge distance and trajectory as well as they imagine. An example, go to a big league baseball park and try to catch big league fly balls. Professional players make it look easy. But untrained eyes are easily deceived and most people cannot catch a single ball until they practice a while. The same holds true for judging distance, course and speed of vessels on the water. Most people cannot estimate distance to within +/-25% and speed estimates are often off by 2:1. With training and practice the visual estimates can be improved. AIS and radar can help us to improve our eyeball estimates. But it takes constant "retraining" else, over time, our eyes will deceive again.

And let's face it, the older we get the poorer is most people's eyesight. Acknowledging that is key. Using modern instruments greatly enhances safety if we understand their (and our own) limits.
One of the several wonderful things about AIS is that it presents its own range of uncertainty itself. All you have to do is watch the calculated data vary in real time, and you can see the range of possible values according to the current input data. And that's how you would set up a crossing like some of the ones we have been discussing here -- your target CPA would need to be enough that the calculated CPA will not be varying into 0 or too close to 0. So I think you can set up a quite close pass behind even a fast ship, with confidence, by ensuring that the calculated CPA is steady enough, and that on top of that you've added enough to account for the antenna placement issue. Based on observation, I guess that might be a couple of cables in really good conditions where you are also able to hold a quite steady course and speed.

In my experience, AIS gives by far the most accurate picture of everything, until you get close enough to worry about antenna placement, at which point radar will give a definitive range, within its own accuracy limitations. Then when you are within a ship's length or so, the naked eye trumps both. But you shouldn't get within a ship's length in open water for the reasons which have been discussed. In my experience, the naked eye is also dangerous because it is extremely easy to fail to understand its limitations -- to think you know more than you do.

The hard part of setting up a close pass with a fast ship is, like landing a plane, the approach, not the landing itself. The flare and power cut is the trivial part of a landing; what is critical is getting into the right position for the flare and power cut in the first place.
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Old 27-10-2017, 05:28   #693
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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Originally Posted by conachair View Post
The Opencpn is , as usual, excellent. Using AIS and thinking it's OK to get so close that the transponder position makes a big difference will end in tears sooner or later..

Definitely. So with 2 cables possible error from the antenna position. You can add another cable maybe from the effect of the current as is demonstrated in your picture. Then another cable to allow for a swing out of the stern should the ship choose to turn just as you pass. All this could eat up all of a 0.4nm safety margin. Working with Murphy's law, that will of course happen one day.
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Old 27-10-2017, 05:39   #694
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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Definitely. So with 2 cables possible error from the antenna position. You can add another cable maybe from the effect of the current as is demonstrated in your picture. Then another cable to allow for a swing out of the stern should the ship choose to turn just as you pass. All this could eat up all of a 0.4nm safety margin. Working with Murphy's law, that will of course happen one day.
That could be leeway as well as current -- how many of us remembers that big ships have leeway also?

Besides leeway, big ships can have a lot of advance in hard turns -- deviation of COG from heading.


Both current and leeway are considered in AIS calculations of CPA, since all the position and motion data is ground-referenced. But you can't assume that the bow is pointed in the direction of COG!


Deviations between heading and CPA of either vessel will completely baffle naked eye observations -- another thing to keep in mind. Another limitation of naked eyes.
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Old 27-10-2017, 05:54   #695
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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I have been fascinated by this discussion and have been following it daily.

So, I gave Rod's "concept" a real life test. One thing that I have been meaning to say during this dialog, and that others now have, is that there have been two “trains” [sic] of thought on this scenario:

1) First is the 180 foot...pass astern close to the first vessel. Period.

2) Second are the second and following ships, which affect the amount of time to cross behind the first ship before the second one "gets there" --- which is mostly, I believe, Rod's entire argument for "needing" to cross close behind #1. But not an issue in this exercise.

For the moment, please, folks, indulge me and please forget about the second ship(s). OK? Thanks.

So, I tried Rod's "method" about two hours ago (yesterday afternoon actually). I was heading north on the west side of Swanson Channel in the Gulf Islands. A BIG ferry that goes across the Strait of Georgia to the mainland was making its way out of the terminal at Swartz Bay, heading north, too.

I have made this ferry trip many times (on the ferry!) and I am comfortable saying that at this point in his trip he was most likely going 15 knots. In the grand scheme of things, less than the 20 knot scenario, but still going three times MY speed.

Beaver Point was to my port, at the narrowest point between Saltspring Island to port and North Pender Island to starboard of both me and THE ferry. Only one, now, please remember. Thanks. That distance on the chart is 1.8 nm. I was quite a lot east of Saltspring, perhaps a third of the way across the "neck." Let's assume for this exercise that the big ferry did the same on his side. Therefore, dividing 1.8 nm by three = 0.6 nm away from each other at the "neck." Having sailed on SF Bay for 40 years, I got pretty good at judging distances for ships of this size, and I have been here actively cruising these Gulf Islands for one summer season, dodging ferries left and right. I'd guessed about a half a mile before I checked on the chart. I also raced for a number of years back there, and am quite familiar with turning marks and sailboat speeds at those marks.

I know the ferries size, speed and routes, which make it easier, but, again, bear with me - and use that assumption again that the ferry is on rails. This exercise is one of a 5 knot sailboat crossing a 15-20 ship. Period.

I also recognize that this is NOT in open sea, he had a route/channel to take, etc. But the ENTIRE purpose of this exercise was to see what happens - in the real world.

I saw him coming at between 5.5 and 6 nm as he rounded Portland Island.

So the “0” CPA would be a line between Beaver and Mouat Points if I was aiming for anything along his length and "trying to tuck in behind him."

He had 5 miles to get to that “0” CPA at 15 knots or 20 minutes. I had essentially 0.5 nm to go at 5 knots or 6 minutes [(60 * 0.5 nm) / 5 knots]

I didn’t start “aiming” at his port quarter until he was about 2˝ miles away, or about half the distance from when I first saw him and got this idea. So it should take him 10 minutes at 15 knots to get to the “0” CPA point, and I had 6 minutes to get there.

Well, whaddaya know: He was LONG GONE by the time I got anywhere close!!!

Why? Any number of possible reasons. I was a bit chicken – I don’t have Rod’s pucker factor! I may have been going less than 5 knots when I first turned to do this exercise, and then sped up to 5 knots. The ferry was going faster than I thought – I checked AIS this morning and by the time those guys get around Portland Island and pass Moresby they are doing 20 knots already. Really. That would have made his Time from 2˝ miles decrease to 7˝ minutes from 10 minutes. I may have been a bit further away than just a ˝ nm. He may not have been on a straight course, and could have been bending to starboard as he rounded N. Pender Island at Mouat Point. Maybe he saw me coming and turned even a little bit to starboard.

Point being that even if you tried to do what Rod has been proposing, and as others have mentioned, only a brain dead conning crew on a ship would let you even get there.


But even if they did, it is really hard to set this up when you're going 5 knots and they are doing 20. Even 15!


Again, forget the whole second train of ships. It’s a smokescreen, or strawman. It is NOT pertinent to a 5 knot sailboat crossing a ship doing 20 knots. Ain’t gonna happen.


I, too, am surprised by Rod's lack of answer as to whether or not he has ever tried this, dead conning crew on the ship or not!


As many of you know, I have a great deal of respect for BOTH Dockhead (his CTS exercise is a textbook example of taking his knowledge and sharing it, as is this thread) AND Rod (his electrical knowledge and sharing of it is superb).

In this case, developing a theory of a train of ships deflected, I believe, from the whole 180 feet passing distance. And the work done by those who provided the plotting tables and graphs is truly appreciated.

What I ALSO learned is that the ferries here, the big ones, truck along very quickly and much faster than I originally thought. Contrary to the slower speed of the shipping in the channels I was used to in San Francisco (and let’s NOT get into channels, TSS, harbors, etc., please) it increased my awareness of the importance of speed, time & distance and safe passing. I also realized I am not going to ever try this again, either!!!


Thanks again so much for this entire thread. Extremely useful, helpful and important for safety.
The line of ships is not a "strawman". It is designed to be a compelling reason to go between the ships rather than wait a couple minutes or turn off just a few degrees and duck the second ship.

I'm sorry you had such difficulty rounding a moving target in close proximity. I would have had to see it, to understand what you did so terribly wrong.
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Old 27-10-2017, 06:26   #696
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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Originally Posted by evm1024 View Post
Rod,

I think you missed my question.



Can you take a moment to answer if 200' would make a better or worse crossing?
Sorry, I've already spent way too much time on this thread.

I am not responding to questions that I feel are just "bait" to keep the thread alive, or have already been asked and answered.
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Old 27-10-2017, 07:02   #697
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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Originally Posted by Dockhead View Post
Yes, yes, yes.

Now we're getting to the heart of the matter.

Look at the 1 mile plot of 5 knots yacht vs. 20 knot ship. Can you see from either vessel, who's passing ahead and who's passing behind? You can't, not by any means, because the planned crossing takes place well within the uncertainty cone. The yacht appears to the ship fine off the port bow, and the ship appears to the yacht less than two points ahead of the beam, and the "nose" can't be on the "right side". Depending on slight variations of course and speed, the yacht may end up ahead or under the bows of the ship, rather than passing close behind.

This is not just a matter of clarity of intentions -- the statement in the Rules against small repeated alterations is intended to require decisive maneuvering from a safe distance so that the other ship has a chance to evaluate and has time to make its own move if it is not satisfied.

That's why you can't approach a ship, sailing into close quarters on a collision course, intending to correct later -- if you are passing behind, it needs to be clear and visible that you are passing behind, from a safe distance away. This is one of the elements of a safe CPA.
Question: So how different would your proposed 2-3 cables CPA compared to my 180 ft CPA look from distance out?

Answer: Negligible.

Please, there would be very little difference in the grand scheme from this aspect whether one chose 180 ft or 2-3 cables CPA.

Where it would make a difference, is if the ships didn't move and the plan was actually executed. I with a 180 ft CPA behind the lead ship would make it, you with a 3 cable CPA would be plastered against the side of the following ship.
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Old 27-10-2017, 07:07   #698
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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Originally Posted by ramblinrod View Post
The line of ships is not a "strawman". It is designed to be a compelling reason to go between the ships rather than wait a couple minutes or turn off just a few degrees and duck the second ship..
Trouble with your "line of ships 20Kts 1.25Nm behind each other " is that it doesn't exist anywhere in the real world, the real world is very different with ships not in a line and travelling at different speeds.

Any discussion about your made up example is pretty much meaningless, the world just isn't like that. Thankfully, if it was no one with any sense would try to dodge through that particular game of frogger.
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Old 27-10-2017, 07:39   #699
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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Question: So how different would your proposed 2-3 cables CPA compared to my 180 ft CPA look from distance out?

Answer: Negligible.

Please, there would be very little difference in the grand scheme from this aspect whether one chose 180 ft or 2-3 cables CPA.

Where it would make a difference, is if the ships didn't move and the plan was actually executed. I with a 180 ft CPA behind the lead ship would make it, you with a 3 cable CPA would be plastered against the side of the following ship.
That's a really good question, which actually gets right to the heart of the matter. And there is a simple answer to it, which is not "negligible":

2 or 3 cables of CPA is enough that you are clearly passing behind.
It means you will pass 3 or 4 cables from his bow which is enough to get you out of the cone of uncertainty at his bow.

180 feet is not. That is a collision course. You can't set up a 180 foot pass just like you can't set up a 6 foot pass or a 6 inch pass -- you don't have enough information or control.

The problem is not what you do when you're already close to the ship. That's the "flare and power cut" part of the landing. If you could be magically teleported to a point 0.7 cables from his bow, I have no doubt that your maneuver could be done, more or less, with only the risk that his stern swings out as you pass. But the whole problem is getting there -- the problem is the landing approach. By the time you get right up to him, you may be heading straight under his bows. So you simply cannot approach him from a course which would get you where you would have to be, in order to pass so close.

So the answer is you have to set up the landing approach at a large enough CPA that you are clearly passing behind. You also need room and time to react in case something unexpected happens. By now in this thread you've heard from several professionals how they would view a yacht coming at them on a 0.3 cable CPA, and their opinion is all the same.


You go on to talk about the ship behind. How you cross with the ship behind is a separate question. If passing safely with the ship ahead means you get run down by the ship behind, that doesn't mean it's OK to attempt an unsafe or impossible pass with the ship ahead. That's a logical fallacy. It rather means you can't do it at all. Or that you can only do it by asking the following ship for a course alteration, which would be the way I would do it.

But in any case, you never calculated the CPA with the ship behind correctly, and you keep repeating these incorrect calculations over and over again. I posted links to good instructions on how to plot it. And here is where you can download a plotting sheet: http://msi.nga.mil/MSISiteContent/St...eringBoard.pdf. This is not really important to the discussion, but for good order.
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Old 27-10-2017, 07:40   #700
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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Paul, can you enlighten me a little on the NavMonPC? I'm running an ancient toshiba in the nav station with CM93, and wouldn't mind beefing up the functionality a bit. It doesn't look like it'll work with Windows 10 from the site. Any feedback?
NMPC works fine on the Win10 computers I have. There are issues with using the "virtual serial ports" on 64-bit systems, but there is an easy install-time fix for that. Here's a shot of NMPC running on my Win10 64-bit machine. You can see how ship dimensions and GPS location is displayed.

FWIW, I think OCPN now does it better in some respects...
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Old 27-10-2017, 07:41   #701
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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In my experience, AIS gives by far the most accurate picture of everything, until you get close enough to worry about antenna placement, at which point radar will give a definitive range, within its own accuracy limitations. Then when you are within a ship's length or so, the naked eye trumps both.
Precisely.

But it is important to get the naked eye on the subject as early as possible, to help make sense of / confirm the AIS and radar data.

As was mentioned in a recent (excellent) post, if you don't train your eye and continue to exercise it, it isn't as good as it could be.
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Old 27-10-2017, 07:43   #702
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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Originally Posted by conachair View Post
Trouble with your "line of ships 20Kts 1.25Nm behind each other " is that it doesn't exist anywhere in the real world, the real world is very different with ships not in a line and travelling at different speeds.

Any discussion about your made up example is pretty much meaningless, the world just isn't like that. Thankfully, if it was no one with any sense would try to dodge through that particular game of frogger.
You're right, but it's still an interesting problem. I have gotten a lot of benefit out of thinking it through and arguing about it.
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Old 27-10-2017, 07:49   #703
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

[QUOTE=Dockhead;2507113]

2 or 3 cables of CPA is enough that you are clearly passing behind.
QUOTE]

Nope.

I can't believe you are still supporting your 3 cable CPA.

From a distance out, a 180 ft and 3 cable CPA, both look very similar; the angles simply aren't that different. Negligible.

If you use a 3 cable CPA, your set-up still appears as a collision course, and in fact you will run into the following ship.

There will be no easy landing for you sir if you attempt this and the ships don't move.
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Old 27-10-2017, 08:04   #704
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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Trouble with your "line of ships 20Kts 1.25Nm behind each other " is that it doesn't exist anywhere in the real world, the real world is very different with ships not in a line and travelling at different speeds.
Any scenario that is not based on historical record, is "hypothetical".

It is common practice to set up hypothetical situations to determine the limits of what is practically possible.

Of course this would not play out in the real world; that isn't the point at all.

The point I was making, was how important it is to get one's head out of their arse and electronics.

We just witnessed an experienced navigator, who had this been real life, would have pasted his vessel to the front of a 600 ft ship travelling at 20 knots.
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Old 27-10-2017, 08:08   #705
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

As for CPA uncertainty (aside from the GPS location issue), yes, my sailboat often yaws too much for a stable CPA solution. I adjust the NMPC filters to smooth my own COG and the calculated CPA/TCPA settle down to useful numbers. Of course no program can realistically anticipate course changes.

One might use the extent of the unfiltered CPA results to generate a Zone of Uncertainty on the plotter screen (using ship dimensions if available), but I doubt that this would prove useful. Call me a luddite, but I trust my own situational analysis using all the available data (AIS, eyeball, and everything in between). I generally dislike the "false sense of security" argument, but in this case it seems appropriate.

But FWIW, most of my ship encounters have been of the "we couldn't hit them if we tried" type.

Here are the NMPC filters, and a displayed CPA:
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