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-   -   Collision Avoidance, Cones of Uncertainty, and Appropriate CPA (https://www.cruisersforum.com/forums/f90/collision-avoidance-cones-of-uncertainty-and-appropriate-cpa-189919.html)

ramblinrod 29-09-2017 13:26

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by Dockhead (Post 2473898)
In the other thread, a very knowledgeable person suggested exactly making a one degree course correction from 5 miles our for a 180 foot miss, and asserted that this was an adequate maneuver. I demonstrated mathematically that it is not.

Now wait just a minute.

I left this thread feeling I had cleared this up, only to find the falsehood perpetuated.

After I made the statement about the one degree course change, you are referring to, you then arbitrarily selected the worst case conditions (head on high speed approach) to make it implausible.

One need not make course corrections for the worst case scenario, except in the worst case scenario.

I was considering the usual scenario, where a small rec vessel crosses shipping lanes at right angles.

When crossing a shipping lane, making a 1 degree adjustment from 5 miles out is absolutely fine.

Why?

No risk of collision even exists yet.

As I said in prior posts (and you seem to have completely ignored), the sailboat could be planning to turn 180 and reverse course completely in 4 miles. No risk of collision whatsoever. No need to change course and by a large degree from 5 miles out at all.

Additionally, it is extremely highly unlikely that the sailboat is going to maintain constant speed. Even if the sailboat holds original course, due to speed variation may pass well clear of the larger vessel.

Show me a skipper on a small rec boat that can even hold a course within +/- one degree constantly for 5 miles.

So please, stop making statements about my prior post that are untrue.

If I was approaching a vessel head-on with a very high combined intercept speed, my approach and reaction would be far different than when approaching at right angles.

Crossing at right angles, from 5 miles out, I don't have to change course at all. In all likelihood there will be no risk of collision by the time I get to the original intersect point.

It is only when a risk of collision develops, the give-way vessel must take action to avoid a collision, and that a large course change is required. The sole purpose of the large correction is to (hopefully) provide an obvious indication to the stand on vessel that you have identified the risk, are monitoring the situation and are acting appropriately.

(Even still this could be false, the give way may have just been slapped by a wave, and not aware of anything. Surely the stand-on vessel is monitoring the course keeping track of the other vessel. A 30 degree turn, when the vessel has been on a drunken sailor course, may not mean anything.)

If I am on a narrow miss in a crossing situation, when it becomes necessary to make a course change to avoid collision, I may turn 30 degrees off to show clearly that I am making a course correction, and then turn back to X degrees off the original course (as appropriate) to meet my "obligation" to avoid collision.

Whether any vessel watch officer has standing orders to report any vessel closer than 1 nm is not my concern. That is their business, and they can set whatever arbitrary distance they wish. I am not responsible for knowing what that is, considering that, or basing my decisions on that. My responsibility is to follow colregs, and I do.

(IMHO, that 1 nm number under nay condition is stupid, because as we can clearly see, the safe distance away varies depending on the speed and direction of the vessels.)

Regarding the cone of silence, (tongue in cheek) of course I would look at the situation to see if the larger stand on vessel is likely to make a course change to avoid land or another vessel that may then put us on a collision course.

But to go to the extremes you did to make your point, I could just as easily argue that the worst case scenario is that the faster vessel has the capability to run me down no matter what I do, as soon as they pick me up on radar, even if I turned 90 degrees away from my original course at full speed, after spotting them 5 miles away.

So please, stop referring to my post as if it related to the arbitrary worst case scenario you established after the fact. It didn't.

Dockhead 30-09-2017 02:06

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by ramblinrod (Post 2489639)
Now wait just a minute.

I left this thread feeling I had cleared this up, only to find the falsehood perpetuated.

After I made the statement about the one degree course change, you are referring to, you then arbitrarily selected the worst case conditions (head on high speed approach) to make it implausible.

One need not make course corrections for the worst case scenario, except in the worst case scenario.

I was considering the usual scenario, where a small rec vessel crosses shipping lanes at right angles.

When crossing a shipping lane, making a 1 degree adjustment from 5 miles out is absolutely fine.

Why?

No risk of collision even exists yet.

As I said in prior posts (and you seem to have completely ignored), the sailboat could be planning to turn 180 and reverse course completely in 4 miles. No risk of collision whatsoever. No need to change course and by a large degree from 5 miles out at all.

Additionally, it is extremely highly unlikely that the sailboat is going to maintain constant speed. Even if the sailboat holds original course, due to speed variation may pass well clear of the larger vessel.

Show me a skipper on a small rec boat that can even hold a course within +/- one degree constantly for 5 miles.
. . .


I only have two things to say about this:

1. Your idea of when a risk of collision exists is faulty. Talk to a commercial skipper some time and let him explain it to you, since you don't want to listen to anyone here. 5 miles out in open water is a fully developed risk of collision situation and rapidly approaching the end of the window of opportunity for normal maneuvering. Your misconception of this is an absolutely typical WAFI misconception, which is one of the things which drives commercial seaman crazy, about the way we maneuver. You cannot just make a 180, one mile out, and be sure to avoid danger, and you cannot just blithely sail on from 5 miles out to 1 mile out, telling yourself that there is no risk of collision. The reasons were explained in great detail in this and the other thread, and because you were so much more interested in arguing, than in learning anything, you missed it. The fear of appearing to be wrong, is the mortal enemy of knowledge and learning, and this is a really great example of it.

2. You explain very correctly that you can't hold a steady course within one degree on a small boat -- no way to do it within one degree. So how can you say that a one degree course correction in any case (not only head-on), puts you 180 feet into safety? Do you not see the contradiction here? That is PRECISELY the reason why you must not do collision avoidance with one degree course corrections -- they are MEANINGLESS when you are not even holding your course within one degree anyway. You don't know where you will be on a given course, five miles later, or where he will be, within 180 feet. Your course correction must get you out of the cone of uncertainty around the CPA, in order to be effective. We don't even get to worst case, best case crossing geometry here -- it should be totally obvious to you -- isn't it? -- that a one degree course correction when your course itself is not consistent within one degree is utterly meaningless -- just wasting time while you hurtle towards a possible collision.

danielamartindm 30-09-2017 02:42

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by ramblinrod (Post 2489639)
Now wait just a minute.

I left this thread feeling I had cleared this up, only to find the falsehood perpetuated.

After I made the statement about the one degree course change, you are referring to, you then arbitrarily selected the worst case conditions (head on high speed approach) to make it implausible.

One need not make course corrections for the worst case scenario, except in the worst case scenario.

I was considering the usual scenario, where a small rec vessel crosses shipping lanes at right angles.

When crossing a shipping lane, making a 1 degree adjustment from 5 miles out is absolutely fine.

Why?

No risk of collision even exists yet.

As I said in prior posts (and you seem to have completely ignored), the sailboat could be planning to turn 180 and reverse course completely in 4 miles. No risk of collision whatsoever. No need to change course and by a large degree from 5 miles out at all.

Additionally, it is extremely highly unlikely that the sailboat is going to maintain constant speed. Even if the sailboat holds original course, due to speed variation may pass well clear of the larger vessel.

Show me a skipper on a small rec boat that can even hold a course within +/- one degree constantly for 5 miles.

So please, stop making statements about my prior post that are untrue.

If I was approaching a vessel head-on with a very high combined intercept speed, my approach and reaction would be far different than when approaching at right angles.

Crossing at right angles, from 5 miles out, I don't have to change course at all. In all likelihood there will be no risk of collision by the time I get to the original intersect point.

It is only when a risk of collision develops, the give-way vessel must take action to avoid a collision, and that a large course change is required. The sole purpose of the large correction is to (hopefully) provide an obvious indication to the stand on vessel that you have identified the risk, are monitoring the situation and are acting appropriately.

(Even still this could be false, the give way may have just been slapped by a wave, and not aware of anything. Surely the stand-on vessel is monitoring the course keeping track of the other vessel. A 30 degree turn, when the vessel has been on a drunken sailor course, may not mean anything.)

If I am on a narrow miss in a crossing situation, when it becomes necessary to make a course change to avoid collision, I may turn 30 degrees off to show clearly that I am making a course correction, and then turn back to X degrees off the original course (as appropriate) to meet my "obligation" to avoid collision.

Whether any vessel watch officer has standing orders to report any vessel closer than 1 nm is not my concern. That is their business, and they can set whatever arbitrary distance they wish. I am not responsible for knowing what that is, considering that, or basing my decisions on that. My responsibility is to follow colregs, and I do.

(IMHO, that 1 nm number under nay condition is stupid, because as we can clearly see, the safe distance away varies depending on the speed and direction of the vessels.)

Regarding the cone of silence, (tongue in cheek) of course I would look at the situation to see if the larger stand on vessel is likely to make a course change to avoid land or another vessel that may then put us on a collision course.

But to go to the extremes you did to make your point, I could just as easily argue that the worst case scenario is that the faster vessel has the capability to run me down no matter what I do, as soon as they pick me up on radar, even if I turned 90 degrees away from my original course at full speed, after spotting them 5 miles away.

So please, stop referring to my post as if it related to the arbitrary worst case scenario you established after the fact. It didn't.

I enjoy math as much as the next guy; but I'm a simple human being and sailor. It seems to me that despite all of our technical wizardry, collision avoidance boils down in the minds of captains to a psychological approach/avoidance paradigm: the closer you get, the more extreme your actions should become. We can, and should, attempt to quantify that; but in the real world, it all boils down to sentience of one's immediate situation, and pucker factor. Call me a cowboy, I've been called worse.

steve77 30-09-2017 03:41

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
For us poor acronym-challenged sailors, what is a WAFI?

Cheers!

Steve

StuM 30-09-2017 03:43

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Wind Assisted F*ing Idiot.

El Pinguino 30-09-2017 03:46

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by steve77 (Post 2489937)
For us poor acronym-challenged sailors, what is a WAFI?

Cheers!

Steve

I believe that it stands for Wind Assisted ****ing Idiots.... however I must say that in 42 years in the day job I never ever heard it used.

First came across it in the pages of pommy yachting mags and then here on CF..... or maybe the other way around....

steve77 30-09-2017 03:53

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by rramsey (Post 2463809)
I might be wrong but AIS seems to miss something. It doen't tell me if the CPA is ahead or astern of the other vessel. So I don't know if I am supposed to slow down or speed up.

I didn't see where anyone answered his question, so...

Your AIS may give the compass bearing to the other vessel. Our Simrad system does, and I would expect other systems do, also.

Watching the other vessel for a few moments, you should see the bearing change. You can extrapolate this to determine whether the vessel will pass in front or behind you.

For example, a vessel to port with a compass bearing that is increasing will pass in front of you.

Cheers!

Steve

steve77 30-09-2017 03:56

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by El Pinguino (Post 2489943)
I believe that it stands for Wind Assisted ****ing Idiots.... however I must say that in 42 years in the day job I never ever heard it used.

First came across it in the pages of pommy yachting mags and then here on CF..... or maybe the other way around....

Thanks! I figured it was a Brit thing. I was also pretty sure about the FI part.

Appreciate the clarification. Cheers!

Steve

Dockhead 30-09-2017 04:10

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by El Pinguino (Post 2489943)
I believe that it stands for Wind Assisted ****ing Idiots.... however I must say that in 42 years in the day job I never ever heard it used.

First came across it in the pages of pommy yachting mags and then here on CF..... or maybe the other way around....

Heard a lot in American waters as well. And in the pages of the GCaptain forum for commercial mariners.

Dockhead 30-09-2017 04:13

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by steve77 (Post 2489947)
I didn't see where anyone answered his question, so...

Your AIS may give the compass bearing to the other vessel. Our Simrad system does, and I would expect other systems do, also.

Watching the other vessel for a few moments, you should see the bearing change. You can extrapolate this to determine whether the vessel will pass in front or behind you.

For example, a vessel to port with a compass bearing that is increasing will pass in front of you.

Cheers!

Steve

Yes, that's the way I figure it out, on my system.

BUT -- the question is a really good one! I agree that this is one of the basic things, which we should be able to know at a glance. Sometimes the bearing is not changing quickly enough to see this very soon.

OpenCPN (and I believe Vesper separate AIS displays) solve this problem by graphically displaying the crossing geometry.

This is so valuable that in really complicated situations, where you're dealing with multiple targets and really need to see this at a glance, I always use OpenCPN at the nav table while someone else is at the helm and keeping a visual watch.

ramblinrod 30-09-2017 08:50

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by Dockhead (Post 2489911)
I only have two things to say about this:

1. Your idea of when a risk of collision exists is faulty. Talk to a commercial skipper some time and let him explain it to you, since you don't want to listen to anyone here. 5 miles out in open water is a fully developed risk of collision situation and rapidly approaching the end of the window of opportunity for normal maneuvering. Your misconception of this is an absolutely typical WAFI misconception, which is one of the things which drives commercial seaman crazy, about the way we maneuver. You cannot just make a 180, one mile out, and be sure to avoid danger, and you cannot just blithely sail on from 5 miles out to 1 mile out, telling yourself that there is no risk of collision. The reasons were explained in great detail in this and the other thread, and because you were so much more interested in arguing, than in learning anything, you missed it. The fear of appearing to be wrong, is the mortal enemy of knowledge and learning, and this is a really great example of it.

2. You explain very correctly that you can't hold a steady course within one degree on a small boat -- no way to do it within one degree. So how can you say that a one degree course correction in any case (not only head-on), puts you 180 feet into safety? Do you not see the contradiction here? That is PRECISELY the reason why you must not do collision avoidance with one degree course corrections -- they are MEANINGLESS when you are not even holding your course within one degree anyway. You don't know where you will be on a given course, five miles later, or where he will be, within 180 feet. Your course correction must get you out of the cone of uncertainty around the CPA, in order to be effective. We don't even get to worst case, best case crossing geometry here -- it should be totally obvious to you -- isn't it? -- that a one degree course correction when your course itself is not consistent within one degree is utterly meaningless -- just wasting time while you hurtle towards a possible collision.

As it stands, you believe I am wrong, and I believe you are. In my humble opinion making a course correction from 5 miles out when crossing a larger vessels path at right angles is much farther out than necessary. My speed may vary 10 ways from Sunday over the course of the hour it will take me to get to the original course intersect point. Any course correction I make hat far in advance will most likely be wrong.

You believe differently. That is your right and you are resposiblr for your precious cargo and I mine. But before you belittle or chastise others and call them names, as you have done in this thread (totally unbecoming of a forum moderator by the way) consider that perhaps your opinion could be incorrect.

transmitterdan 30-09-2017 09:54

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Speed counts. 5 miles separation from a big ship is not 1 hour. It's more like 15 minutes in many cases and even less for head on situations. So 5 miles is well inside the "risk of collision" window in my opinion. Even 10 miles is in some cases which is why some ships have already maneuvered before we see them.

skipmac 30-09-2017 09:57

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by ramblinrod (Post 2490057)
As it stands, you believe I am wrong, and I believe you are. In my humble opinion making a course correction from 5 miles out when crossing a larger vessels path at right angles is much farther out than necessary. My speed may vary 10 ways from Sunday over the course of the hour it will take me to get to the original course intersect point. Any course correction I make hat far in advance will most likely be wrong.

You still do not answer or address Dockhead's point #2. You claim a 1 degree correction is adequate 5 miles out then you correctly state that a sailboat could not hold a course anywhere close to a +/- 1 degree accuracy.

The only way for another vessel to tell what you are doing and react accordingly is for you to make a large correction, early in the situation.

Dockhead 30-09-2017 10:22

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by ramblinrod (Post 2490057)
As it stands, you believe I am wrong, and I believe you are. In my humble opinion making a course correction from 5 miles out when crossing a larger vessels path at right angles is much farther out than necessary. My speed may vary 10 ways from Sunday over the course of the hour it will take me to get to the original course intersect point. Any course correction I make hat far in advance will most likely be wrong.

You believe differently. That is your right and you are resposiblr for your precious cargo and I mine. But before you belittle or chastise others and call them names, as you have done in this thread (totally unbecoming of a forum moderator by the way) consider that perhaps your opinion could be incorrect.

Well, don't believe me -- that's fine. I suggested that you talk to some commercial seaman, to find out decision points and distances, and ask them what they think about your idea that there's no risk of collision before a mile out. You will find that ships maneuver according to certain particular time and distance frames, and they do not correspond at all to the ones you are espousing here.

And concerning the 1 degree course correction -- I think you got my point. :thumb:

ramblinrod 30-09-2017 23:11

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Quote:

Originally Posted by transmitterdan (Post 2490098)
Speed counts. 5 miles separation from a big ship is not 1 hour. It's more like 15 minutes in many cases and even less for head on situations. So 5 miles is well inside the "risk of collision" window in my opinion. Even 10 miles is in some cases which is why some ships have already maneuvered before we see them.

Any course correction they make would be based on the sailboat speed.

What is the sailboat speed going to be?

All we know for sure is that it is not likely to be constant.

Even if the ship turned 20 degrees to pass well astern the sailboat, if the sailboat slows down from 5 knots to 4, the original ships course may have been fine, and the new ships course may now be a collision course.


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