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Old 24-10-2017, 14:51   #616
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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Originally Posted by Dockhead View Post
If Juho will do some math to show what you can and what you cannot see -- that would be terrific. But otherwise I think we won't really get anywhere further without a simulator.
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.
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Old 24-10-2017, 14:56   #617
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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I think you are right.

I too have had one of those moments where the ship was closing fast and it looked huge 1/4 mile away. Was it going straight and going to run me over or was it going to turn to port and head for Victoria Harbor? I had no indications and even with the USCG intervening on the VHF we could not get an answer from the ship.

That was just one ship.

Rod is very heavily invested in being right. It appears to me that he must prove that crossing behind a ship with a 180' CPA is nothing (if you have big enough balls?) and the right way to do it in the BS example we are given.

You are right that it appears that he cannot visualize what a line of ships doing 20 kts 1 nm apart would look like.

10 miles out you are not even going to see them. Heck they form a line over 66 nm long.

At 5 miles you might see them from the deck of a sailboat. The would look like a string of pearls with each pearl (ship) about 3.6 degrees apart from each other directly ahead of you. Let the convergent lines take care of themselves.

At 2 miles of course you would see them. They would be about 32 degrees apart and covering that 32 degrees in 3m 45s.

At 1 mile they would be 51 degrees apart when directly ahead and covering that distance in the same 3m 45s. Just flying by....

Let me repeat that Flying By

At 1 mile out you will not be crossing under the stern of the ship directly ahead of you. It will be long gone. You will not be passing under the stern of the next ship (2nd) or the next (3rd) or the next (4th)....

You will be lining up to pass under the stern of the 5th ship. You are a little too late for the 4th ship and a bunch too early for the 5th. So you have to fudge your speed or angle to a little faster to make your 180 CPA on the 4th ship or slow a bit to make the 5th.

Is there any doubt folks how this crossing plays out?

We are debating a bogus scenario designed to bolster the arrogance and ego of someone who does not have the experience to understand what the scenario is like and cannot visualize the reality of the scenario.

In the meantime we as a group are getting something out of this, at least for those who can visualize the scenario. All the while we are missing out on a better understanding of cones of uncertainty and how they apply to night time or low visibility crossings.

- You are in a maze of twisty little passages all alike -
Excellent closing statement, council.

The jury has now deliberated, and found Rod guilty of lack of imagination, vision and humility, and sentences him to being run down at sea.

Jim
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Old 24-10-2017, 15:24   #618
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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I too have had one of those moments where the ship was closing fast and it looked huge 1/4 mile away.
My most frightening moment was when we were on a shipping lane, in a sailboat with no electronics, a huge ship coming behind in our direction, with us already moving towards the edge of the shipping lane in due order, being hit by an insane shower with maybe 15 m visibility. The ship became quickly much more frightening than it was 10 seconds ago. We were supposed to end up between the shipping lane and some nearby rocks. What to do? We tried to keep our current course until we estimated that we might be somewhere between the shipping lane an the rocks, or hopefully at least somewhere near the buoy that was supposed to be there, and then slowed down and waited for the shower to end in few minutes. The ship had turned to another lane, away from us, and the sun was shining again.
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Old 24-10-2017, 15:59   #619
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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Originally Posted by ramblinrod View Post
If one can consistently duck the transom of a racing sailboat, perhaps going twice their speed, within a hairs breadth, they can certainly duck the transom of a ship at 4 times their speed, within 180 ft.
For the third time, I ask the as yet un-answered question:

"Have you ever actually sailed close to a large ship travelling at speed?"
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Old 24-10-2017, 16:01   #620
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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Excellent closing statement, council.

The jury has now deliberated, and found Rod guilty of lack of imagination, vision and humility, and sentences him to being run down at sea.

Jim
I can't disagree with the verdict

However, I have a mea culpa of my own.

I just plotted this again with care, and the 3 cables CPA I got was an error. If you have a 2 cables CPA with the lead ship, then you cross the following ship's course line at about 3.5 cables. HOWEVER, because of the very acute angle of the Relative Motion Line, the actual CPA with the following ship is only a little more than a cable.

My comments about the danger of attempting a 180 foot CPA with the lead ship stand -- it requires a kamikaze collision course approach with LESS than a cable CPA with the lead ship, and so can't be done with any safety at all. You can't know enough to be sure not to get under the bow of the lead ship.

But the minimum safe CPA with the lead ship results in an unacceptably dangerous CPA with the following ship. So I was wrong -- there is NO safe way to do this crossing. So I'm guilty of the same thing Rod has been doing -- projecting experience from one situation on another, different situation. I do a lot of threading through lines of ships, but I'm making 8 to 9 knots, and that looks totally TOTALLY different.

This is yet another argument for not assuming this crossing is like the last one, not doing it by eye or by feel. Collision avoidance is a science.
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Old 24-10-2017, 16:03   #621
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

Btw, instead of trying to dive between two huge ships, maybe the safest strategy is after all to take a steady 90° course long before reaching the shipping lane, make sure that you have a radar reflector, big white sails up, and AIS on. Then close your eyes, listen to the wind and waves, and trust the large ships. They quite certainly have better equipment, more experienced crew, and they have seen you already long time ago, have noticed your steady 90° course, and are happy that you are not trying to make any silly tricks. When you reach the shipping lane, all ships will be at the other side of the lane, and when you reach the other side, all the ships will be on this side. Proper CPA guaranteed. Should work at least in the English Channel where all the ships must keep their eyes open anyway. Or do they hate us small boats?
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Old 24-10-2017, 16:17   #622
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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Btw, instead of trying to dive between two huge ships, maybe the safest strategy is after all to take a steady 90° course long before reaching the shipping lane, make sure that you have a radar reflector, big white sails up, and AIS on. Then close your eyes, listen to the wind and waves, and trust the large ships. They quite certainly have better equipment, more experienced crew, and they have seen you already long time ago, have noticed your steady 90° course, and are happy that you are not trying to make any silly tricks. When you reach the shipping lane, all ships will be at the other side of the lane, and when you reach the other side, all the ships will be on this side. Proper CPA guaranteed. Should work at least in the English Channel where all the ships must keep their eyes open anyway. Or do they hate us small boats?
That's one way to do it, and one GREAT advantage is that you are holding a steady course and speed which at least allows the ships to caculate an effective maneuver. It's vastly safer and more seamanlike than jinking around in close quarters.

I would not just sail and pray, however. I would figure out which ship I had the biggest problem with, and call on the VHF in plenty of time (probably 5 miles out) and ASK for more room, and agree on passing arrangements. That would definitely be the safe way to do it.
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Old 24-10-2017, 16:19   #623
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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Excellent closing statement, council.

The jury has now deliberated, and found Rod guilty of lack of imagination, vision and humility, and sentences him to being run down at sea.

Jim
The speed of and distance between ships (which was 20 knots and 1-1/4 nm) doesn't change due to the CPA value one chooses.

They are a scenario constant.

I proposed a 180 ft CPA and proved by simple calculation how that will clear the lead ship by a margin, and clear the following ship by a significantly greater margin.

Another poster proposed a range of 2-3 cable CPA, that will mean a greater than necessary clearance of the lead ship and a much closer and more dangerous crossing with the following ship, AND CERTAIN COLLISION AT 3 CABLE CPA, even if executed flawlessly.

This is proven by the calcs I provided in post # 591.

Ignore the math if you wish; these are the facts.
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Old 24-10-2017, 16:31   #624
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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. . . I proposed a 180 ft CPA and proved by simple calculation how that will clear the lead ship by a margin, and clear the following ship by a significantly greater margin..
You proved nothing. Your calculation assumes that you have God's prescient knowledge about where he will be in the future, within a few feet, and from a good distance away. You don't have that knowledge, and therefore can't make that assumption. The same reason why you can't pass with a 1" CPA, is the reason why you can't pass with a 180 foot CPA. Lining up with the lead ship to pass 180 feet behind his quarter is a collision course, pure and simple. You would be less than a cable away as his bow passes you if you pass exactly as you planned. The chance of ending up directly under his bow, or even passing ahead, is high.

That's why there is, actually, such a thing as a safe CPA. But I fear this is getting repetitive. Good night, y'all.
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Old 24-10-2017, 16:35   #625
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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I can't disagree with the verdict

However, I have a mea culpa of my own.
Thank you for fessing up.

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I just plotted this again with care, and the 3 cables CPA I got was an error.
Yup, guaranteed collision if the ships maintain course and speed.

Quote:
If you have a 2 cables CPA with the lead ship, then you cross the following ship's course line at about 3.5 cables.
I haven't done the additional math, but I suspect you are referring to the following ships course centreline. If so, you will actually be much closer to the ships starboard course line.

Quote:
But the minimum safe CPA with the lead ship results in an unacceptably dangerous CPA with the following ship. So I was wrong -- there is NO safe way to do this crossing.
Again, "safe" is subjective. Under normal circumstances, I wouldn't atempt this crossing at all, so I was surprised you actually proposed a crossing.

However, if my wife was ill and needed urgent medical attention on the other side of that convoy, I'm going, and I will be very close to the port stern corner of the lead ship. This will allow a little room for error in my maneuvers, (actually making a greater CPA than planned) while still not being run down by the following ship.

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So I'm guilty of the same thing Rod has been doing -- projecting experience from one situation on another, different situation.
That's certainly a backhanded error admission, but I'll take it.
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Old 24-10-2017, 18:14   #626
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

well, I'll joine this just to show a picture of what the worlds largest ship, Emma Maersk, looks like at 1 nm distance.

We were the stand on and he was the give way. He turned to giv e us 1 nm room, but I can tell you that looking at him coming at us with 19 knots was enough to us to say "Jesus - he's huge!"
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Old 24-10-2017, 18:46   #627
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

All these hypotheticals about CPAs to two ships in line in a totally unrealistic scenario are really a waste of time.

The bottom line is that anyone who has actually been anywhere close to a large ship travelling at speed is well aware that attempting to get anywhere within 50 metres of one in a slow moving small craft would be madness. And certainly not prudent seamanship!
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Old 24-10-2017, 18:56   #628
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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Originally Posted by ramblinrod View Post
The speed of and distance between ships (which was 20 knots and 1-1/4 nm) doesn't change due to the CPA value one chooses.

They are a scenario constant.

I proposed a 180 ft CPA and proved by simple calculation how that will clear the lead ship by a margin, and clear the following ship by a significantly greater margin.

You failed to show how you could actually do this in the real world. Talk about 'straw men"!

Another poster proposed a range of 2-3 cable CPA, that will mean a greater than necessary clearance of the lead ship and a much closer and more dangerous crossing with the following ship, AND CERTAIN COLLISION AT 3 CABLE CPA, even if executed flawlessly.

This is proven by the calcs I provided in post # 591.

Ignore the math if you wish; these are some of the the facts.
You continually ignore the fact that the process you describe is not feasible to carry out, and then arrogantly state that the case is closed. The only thing closed around this thread is your mind.

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Old 24-10-2017, 19:11   #629
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

FWIW. Cockcroft:
"In the open sea it is suggested that a stand-on vessel should not allow a give-way vessel to approach to a distance of less than about twelve times her own length in a crossing situation without taking avoiding action."
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Old 24-10-2017, 19:21   #630
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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FWIW. Cockcroft:
"In the open sea it is suggested that a stand-on vessel should not allow a give-way vessel to approach to a distance of less than about twelve times her own length in a crossing situation without taking avoiding action."


Exactly. It is madness to approach a 20 knot ship of any size within 180' in open or closed waters. It cannot be done without collision or near certain collision. If no collision happened it would be shear luck. To argue otherwise makes no sense.
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