Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

[evm1024]

Folks let's put aside the technical aspects of the crossing for a moment.

(and while I'm thinking about it - speaking of strawmen - when did the thread go from shifting course 1 degree to pass 180' behind the corner of a lone ship to threading the needle while crossing a string of ships?).

Imagine being in a courtroom after a collision (please let it not happen to anyone) defending yourself after trying to pass 180' CPA behind a ship doing 20 kts.

The prosecuting attorney enters this thread into evidence.

After pointing out all of the times where it was shown to you that crossings like this were ill advised he then points out that you profess that it is only a matter of pucker factor and not an actual risk.

Then he states to the court that not only were you negligent that you were grossly negligent and most likely Criminally negligent.

Your postings are decisive in the courts finding of guilt....

[Exile]

Originally Posted by evm1024

Folks let's put aside the technical aspects of the crossing for a moment.

(and while I'm thinking about it - speaking of strawmen - when did the thread go from shifting course 1 degree to pass 180' behind the corner of a lone ship to threading the needle while crossing a string of ships?).

Imagine being in a courtroom after a collision (please let it not happen to anyone) defending yourself after trying to pass 180' CPA behind a ship doing 20 kts.

The prosecuting attorney enters this thread into evidence.

After pointing out all of the times where it was shown to you that crossings like this were ill advised he then points out that you profess that it is only a matter of pucker factor and not an actual risk.

Then he states to the court that not only were you negligent that you were grossly negligent and most likely Criminally negligent.

Your postings are decisive in the courts finding of guilt....

"Inadmissible hearsay your Honor, objection sustained!" Looks like Rod wisely hired a good lawyer . . . until the prosecutor brings in all the experts to testify that is.

[Juho]

Originally Posted by ramblinrod

On the approach to the lead ship, on a 90 degree course, the give way sailboat would be 180 ft from the ships port transom corner.

One technical note on this. You didn't say in which direction the sailboat is from the ship's transom corner. It could be at 90° (as seen from the ship), at 104.036° (x and y distances in proportion to the speeds, 20 knots and 5 knots), at the ideal crossing angle of 104.478°, or 180° (behind the transom corner).

You can do all four (and you could get even closer if you want), but these four cases have different implications on the CPA. In the first 90° case, if you keep on steaming with 5 knots, CPA will get shorter than 180'. In the 180° case, the CPA has already been shorter than 180'. In the 104.036° case, you are at the CPA right now. In the 104.478° case, also the direction of your sailboat should have been 104.478°, if you wanted to be at the CPA now.

I drew a picture to make the differences clearer. It is a mirror image of the discussed example, but never mind. And I used speeds 20 knots and 14 knots to make the picture easier to draw, and to make differences between different alternatives bigger.

There are two sets of boats. Those that approach the ship at 90° angle, and those that approach at the ideal crossing angle (45°). The ship moves upwards in the picture (only the transom part of the ship is visible in the picture). The circle describes the CPA that we want to have. The boats are not supposed to go inside that circle. All the positions of the boats in the picture are shown in relation to the current position of the ship.

Boats A and B are ahead of schedule in the sense that they must slow down in order to stay out of the circle. If they were on their respective black lines behind them, they could keep full speed (14 knots) all the way. If boat A would keep its full speed, it would follow the red line through the circle.

Boat C is at its CPA. It may keep its full speed, and it will stay out of the circle. It will continue on the black line towards position E.

Same story with boats D and F. The resulting route will be a bit different due to the different angle.

Boat G is now (almost) outside the circle, but it has come to where it is now following the red line through the circle.

Note that if boat D would be on a 90° course, it would have come to where it is now following the red line through the circle.

In some earlier post you said that it would be possible to turn the bow towards the ship when you reach the transom corner of the ship (at CPA distance). In the picture that could mean arriving first to the position of the C boat, using a 90° course, and then making a turn, after which the boat would be where the D boat is now.

If boat C can make a really quick turn to its new course (45°), it could have done it already one notch before, at the black line that is now below it. Boat A could have made the turn (to the same line) even earlier. (Point C is the last moment to start turning if you want to follow the "ideal" route.)

[evm1024]

Originally Posted by Exile

"Inadmissible hearsay your Honor, objection sustained!" Looks like Rod wisely hired a good lawyer . . . until the prosecutor brings in all the experts to testify that is.

If only that were true. A little use of google will show that social media (including this forum) is quite admissible.

One of very many:

A judge explains how Facebook can be used against you in court - Business Insider

and:

https://blogs.lawyers.com/2013/07/so...d-against-you/

[Exile]

Originally Posted by evm1024

If only that were true. A little use of google will show that social media (including this forum) is quite admissible.

One of very many:

A judge explains how Facebook can be used against you in court - Business Insider

and:

https://blogs.lawyers.com/2013/07/so...d-against-you/

Generally for impeachment purposes only. For example, your first link describes a defendant accused of illegal gun possession who denies in court ever having possessed a gun. The prosecutor is then allowed to admit a Facebook pic with the same defendant holding a gun. Doesn't necessarily prove the specific gun crime charged in the case, but is properly admitted to show that the defendant is not credible.

Applying this to your hypothetical here, Rod may be well-advised not to later deny what he has written here on the thread. Doesn't sound like a problem since Rod is steadfastly adhering to the correctness of his positions. So now it's back to the Colregs experts, and whether his actions were "objectively reasonable" within the meaning therein.

[Dockhead]

Originally Posted by ramblinrod

No, you would be foolish to ignore AIS and that would violate Colregs (if it is available). I did not suggest ignoring AIS (yet another strawman).

Rule 5, mandates that "Every vessel shall at all times maintain a proper look-out by sight and hearing..." In this case, "sight" is very important, hence my recommendation to get ones eyes off the screen. AIS could (and should if available) be used for the approach, but in close quarters, nothing is better than eyeballs.



The scenario that I prepared has a degree of risk associated, no matter how one attempts to cross.

The safest approach would be to avoid the crossing.

Any attempt to cross, certainly involves some "Pucker Factor".

In my opinion, your proposal to maintain a minimum 1/4 nm CPA via AIS from the lead vessel is patently unsafe.

Even if the AIS based positions were accurate, the 1/4 nm distance from the port stern quarter would place your vessel dangerously closer to the following ship, needlessly.

Therefore, I believe passing closer to the stern of the lead ship may be safer, than your proposed 1/4 nm CPA, which my proposed solution would keep you further ahead of the bows of the following ship you are crossing.

If the lead ship has standing orders for 1nm CPA min, they are already going to be concerned with your approach, so it makes more sense to stay closer to their stern, and further ahead of the following ship.

So you can post all of the personal attacks, rhetoric, and strawman arguments you wish, but this will not change the fact that your approach to this crossing scenario, IMHO, is patently wrong.

Where did you get anyone saying pass 1/4 mile in front of the ship behind? That would be nuts. I think you need to re-read the responses.

What to do in this case is obvious -- you need to pass as close as you can behind the first ship in order to give as much room as possible to the ship behind. No one said anything different.

But "close as you can" is not 180 feet. What has been explained to you, and Lodesman even DREW it for you, is that you can't get there, without sailing just about under his bows:





You imagine getting up close, then heading off to maintain distance as you pass down his side. But the time when you can still adjust is far before you get there -- by the time you reach his side, you can't move far enough to do any meaningful adjusting. This sparked a really interesting discussion and the geometry was explained in detail. You would benefit by studying those posts.

I guess a very brief summary would be -- if you want to be close enough to be able to sail into 180 feet from his quarter, you have to be 180 feet plus the distance you can sail in the time at takes him to pass you by, from his bow. All your "correcting" would have to take place while his bow is almost dead off your beam (and would be exactly dead off your beam if his course is 14 degrees ahead of perpendicular to yours), and you see both sidelights of his. Meanwhile he's rushing towards your beam at 20 knots.


The other really faulty thing, in this scenario, is the idea that you sail up, correct, keep correcting, then turn off behind him -- it is specifically forbidden to do collision avoidance maneuvers with a series of moves. The reason for that is that the other vessel cannot know what you intend, if you keep jinking around. It is fundamental to proper collision avoidance that you take EARLY action (also specifically mentioned; Rule 8), and then hold your course to the extent you can -- to set up the crossing to be safe and clearly safe from far enough away, that everyone understands what will happen before you get into close quarters.

You count on the ship's holding its course while you jink around him less than a cable away -- what makes you think you can count on this? He is NOT indeed stand on (if he ever was) -- his obligation to stand on ends when he has doubts about the effectiveness of your maneuver, and as the pros in this thread have already commented -- they will doubt it as soon as they detect a CPA of a few cables. And the ship is OBLIGATED to maneuver once he is in an in extremis situation with you -- which is at the very latest a half mile away -- in such a crossing. So that is just one of the many reasons why it is nuts to attempt to get so close to a ship moving at sea speed -- if he sees you, or suspects you are there, at such a close range, it is highly LIKELY that he will attempt to maneuver, and then you are dead meat if he turns the wrong way.


I know that you find it offensive, that I call all this "wrong". I'm sorry about that -- but you should know, that being wrong is not the same as being stupid, and no one ever called you stupid (or any other name). We are all wrong, and quite often -- I'm wrong about something every single day. I am grateful when someone with more knowledge than me, corrects some mistaken belief of mine. I'm sure that if I came into an electrical thread and suggested sticking my fingers into a live 230v outlet, you would straighten me out really fast, and without any "it's a matter of opinion" or "it's subjective" etc. I would be grateful to you!

[Lodesman]

Originally Posted by ramblinrod

Any professional mariner who would needlessly turn to starboard to maintain minimum arbitrary CPA, kicking the stern toward the sailboat, should have their ticket revoked.

I doubt any pro would make a turn needlessly. If you didn't talk to the ship, and just steered at it, it would be very hard for them to tell on the bridge whether or not you would hit. It would only be natural for them to attempt avoiding action - further out, it would be a turn to starboard - this is "strictly encouraged" by the rules. Close in, last minute dodge to keep you from scratching their hull paint, a turn to port might be made. You probably missed my point about hard rudder causing an incipient turn the other way - so the stern would momentarily swing to port, before starting the swing to starboard in a port turn. Bad news for you.

Your scenario is entirely contrived, so I can't imagine making a crossing between ships 1 1/4 miles apart, going 4 times my speed. But let's say I have to be somewhere and can't wait, I certainly wouldn't make this crossing without having talked to both ships by VHF. Even if the second ship is directly behind and you make your 60 yard CPA on the portside transom of the first ship, you'll still have less than 3c CPA on the second ship's bow. If that ship was a little further to starboard, you're toast.

Naw, I don't have to be anywhere that badly.

[ramblinrod]

Originally Posted by Dockhead

Where did you get anyone saying pass 1/4 mile in front of the ship behind?

What?!?!?!?!

I didn't.

In the scenario I prepared, you posted that you would pass 1/4 nm behind the lead ship, of 2 ships 1-1/4 nm apart, in a convoy of ships equidistant behind the other.

Foolish in my opinion.

If one is going to cross, they should maintain the closest possible "safe distance" behind the lead ship, to provide the maximum possible distance from the following ship.

To delay the approach to the transom of the lead ship by 1/4 nm, will put you much closer to the bow of the following ship bearing down on you.

I'm sorry, but in my opinion, that would be truly stupid.

Crossing the bow of a 20 knot ship 1 nm away, may not be bright, but crossing even closer, just for the purpose of being further away from a ship that you have no risk of collision with, because at that moment they are passed you and nothing they can do short of instantly stopping and backing up can hurt you, just isn't wise.

[ramblinrod]

Originally Posted by Lodesman

I doubt any pro would make a turn needlessly. .

Me either.

If you didn't talk to the ship, and just steered at it, it would be very hard for them to tell on the bridge whether or not you would hit.

Part of the prepared scenario was that the ships were called, but nobody answered.

It would only be natural for them to attempt avoiding action - further out, it would be a turn to starboard - this is "strictly encouraged" by the rules. Close in, last minute dodge to keep you from scratching their hull paint, a turn to port might be made.

I believe, that if they would stand on for one to be 1/4 nm away from their port transom, they would likely also stand-on for one to be 180 ft from their port transom.

You probably missed my point about hard rudder causing an incipient turn the other way - so the stern would momentarily swing to port, before starting the swing to starboard in a port turn. Bad news for you.

Nope, I didn't miss that.

Your scenario is entirely contrived, so I can't imagine making a crossing between ships 1 1/4 miles apart, going 4 times my speed.

Any scenario we create is contrived, unless it is a historical record of events.

What started this whole shemozzle, was the instant jump to the worst case contrived scenario by a poster.

But let's say I have to be somewhere and can't wait, I certainly wouldn't make this crossing without having talked to both ships by VHF.

Agreed. Waiting out the convoy is an option, and definitely the safest without question. What we are considering is that if one decides to go, what is the safer approach, 1/4 nm from the lead boat port stern, or 180 ft. (I believe the latter, though I wouldn't necessarily do it, I would be more likely to just go somewhere else.)

Even if the second ship is directly behind and you make your 60 yard CPA on the portside transom of the first ship, you'll still have less than 3c CPA on the second ship's bow. If that ship was a little further to starboard, you're toast.

Even worse if one maintained a minimum 1/4 nm CPA from the lead ship port transom, which is exactly my point.

[ramblinrod]

Originally Posted by Dockhead

What to do in this case is obvious -- you need to pass as close as you can behind the first ship in order to give as much room as possible to the ship behind. No one said anything different.

Yes they did. YOU DID!

Your proposed solution was to maintain a minimum 1/4 nm CPA from the lead vessel (which by the way, would put you closer than 1nm from the following ship, by the time you cross in front of it's starboard side course.

Bad idea. Dumb.

[ramblinrod]

Originally Posted by Dockhead

You imagine getting up close, then heading off to maintain distance as you pass down his side.

Nope, not my plan at all.

My proposed solution was to sail up on a perpendicular course to ship, coming within 180 ft of the port transom. (Originally incorrectly posted as 180 ft aft of the ships port transom. My bad, corrected in earlier post.)

Your proposed solution was to sail up on a perpendicular course coming within 1/4 nm of the ship. That puts you needlessly close to the oncoming ship, crossing it's bow.

[ramblinrod]

Originally Posted by Dockhead

I guess a very brief summary would be -- if you want to be close enough to be able to sail into 180 feet from his quarter, you have to be 180 feet plus the distance you can sail in the time at takes him to pass you by, from his bow. All your "correcting" would have to take place while his bow is almost dead off your beam (and would be exactly dead off your beam if his course is 14 degrees ahead of perpendicular to yours), and you see both sidelights of his. Meanwhile he's rushing towards your beam at 20 knots.!

I have no idea what you are talking about.

The scenario we are discussing is a perpendicular crossing course, as I proposed some time ago. At no time would his bow be dead off my beam.

[ramblinrod]

Originally Posted by Dockhead

The other really faulty thing, in this scenario, is the idea that you sail up, correct, keep correcting, then turn off behind him -- it is specifically forbidden to do collision avoidance maneuvers with a series of moves.

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).

[ramblinrod]

Originally Posted by Dockhead

You count on the ship's holding its course while you jink around him less than a cable away -- what makes you think you can count on this?

I suspect that if the lead ship is not going to move, for your proposed 1/4 nm CPA from their port stern corner, they also will not move for my proposed 180 ft CPA.

Obviously, in both cases, they have determined to stand on. However with your proposed 1/4 nm CPA, this puts you closer to the bows of the oncoming ship. I believe there is far greater risk being run down by a 20 knot ship bearing down directly on you, than being astern (but close) to a ship spreading the distance between you.

[ramblinrod]

Originally Posted by Dockhead

We are all wrong, and quite often -- I'm wrong about something every single day.

And you were wrong to propose the crossing maintaining a 1/4 nm CPA from the stern of the lead ship, at least in my humble opinion.