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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)

Exile 19-10-2017 10:42

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

Originally Posted by Dockhead (Post 2502554)
Yes. And the judgement also has to be somewhere in sync with what others are doing. This lack of sync is one of the main complaints commercial mariners have against us.

And in sync with what others are doing in similar circumstances. Passing a cable length or two behind one of the big ship's transoms in order to stay outside of 1nm of the bow of the next ship may be deemed reasonable & necessary in the channel, but (possibly) unnecessarily & unreasonably unsafe in the more typical 2-ship encounter in open waters. Contrast this with the rules of the (actual) road, where stopping at a red light is unquestionably required whether it is during rush hour or the dead of night.

Simplistic examples, but it seems much of the misunderstandings arise from the mindset that necessarily gets ingrained from basic traffic laws on the road.

ramblinrod 19-10-2017 10:51

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

Originally Posted by Exile (Post 2502546)
It's only "subjective" in the sense that each vessel has to make their own judgment. But that judgment still has to be within the realm of what is objectively reasonable, namely what a prudent mariner would do faced with similar circumstances.

Gimme a break. That there is no defined value and the distance is based on the judgement of the individual, makes it a subjective issue by definition.

Else one could claim that everything is objective because people should always be reasonable.

Sorry if it affects anyone's Pucker Factor, but what is "safe distance" most definitely is "subjective" and at the slippers discretion unless mandate elsewise or ruled after the fact.

markpierce 19-10-2017 11:27

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

Originally Posted by Dockhead (Post 2500160)
By "won't press stand-on obligation in a potential collision situation" -- do you mean "won't FULFILL stand-on obligation"? I suggest this might not be a good idea, while the obligation exists, but that might be only brief period in a crossing in such waters.

Lots of burdened commercial vessels don't take early and obvious course changes for pleasure boats in potential collision situations, so as a "stand-on" I'm obligated to make a course change.

Dockhead 19-10-2017 11:35

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

Originally Posted by Exile (Post 2502568)
And in sync with what others are doing in similar circumstances. Passing a cable length or two behind one of the big ship's transoms in order to stay outside of 1nm of the bow of the next ship may be deemed reasonable & necessary in the channel, but (possibly) unnecessarily & unreasonably unsafe in the more typical 2-ship encounter in open waters. Contrast this with the rules of the (actual) road, where stopping at a red light is unquestionably required whether it is during rush hour or the dead of night.

Simplistic examples, but it seems much of the misunderstandings arise from the mindset that necessarily gets ingrained from basic traffic laws on the road.

Indeed.

Exile 19-10-2017 11:38

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

Originally Posted by ramblinrod (Post 2502578)
Gimme a break. That there is no defined value and the distance is based on the judgement of the individual, makes it a subjective issue by definition.

Yes, but that obvious subjectivity is expressly limited by a codified, enforceable rule of law. If your individual judgment is outside what is considered objectively reasonable, then you are in violation (to say nothing of potentially giving pro mariners heartburn :smile:). And here it's important to understand that "violation" and "enforcement" are not mutually exclusive. You can potentially wait until 180' to make successful collision-avoidance maneuvers all day long without any consequences other than perhaps some derisive chatter about your boat on Ch. 13, but that doesn't mean you haven't violated the Rules.

Else one could claim that everything is objective because people should always be reasonable.

Yes they should be reasonable, but often are not. That's the whole point of promoting uniformity & predictability through a law. Otherwise it's too easy to turn your comment around & say it's all subjective with no objective standards, in which case we're back to a pre-Colregs era free-for-all.

Sorry if it affects anyone's Pucker Factor, but what is "safe distance" most definitely is "subjective" and at the slippers discretion unless mandate elsewise or ruled after the fact.

Pucker factor has no place in this analysis, although I wouldn't doubt it exists in some of the crossings in the English Channel that have been described! Otherwise it's best left to a racing scenario.

Dockhead 19-10-2017 11:40

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

Originally Posted by ramblinrod (Post 2502578)
Gimme a break. That there is no defined value and the distance is based on the judgement of the individual, makes it a subjective issue by definition.

Else one could claim that everything is objective because people should always be reasonable.

Sorry if it affects anyone's Pucker Factor, but what is "safe distance" most definitely is "subjective" and at the slippers discretion unless mandate elsewise or ruled after the fact.

What is an acceptable RISK is an individual decision. What risks result from what decision point and what CPA in a given crossing is NOT subjective.

And even the first thing is not actually individual, because it involves the other vessel. You are endangering the career and possibly freedom of the ship's watchstanders, with kamikaze 180 foot CPA's, even if you don't mind the risk yourself.

"It's a matter of judgement" doesn't mean "whatever you are brave or stupid enough to risk". There is good and bad judgement.

Dockhead 19-10-2017 11:45

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

Originally Posted by markpierce (Post 2502593)
Lots of burdened commercial vessels don't take early and obvious course changes for pleasure boats in potential collision situations, so as a "stand-on" I'm obligated to make a course change.

Sure. That's reasonable. I just couldn't understand what you were saying before.

Exile 19-10-2017 11:48

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

Originally Posted by markpierce (Post 2502593)
Lots of burdened commercial vessels don't take early and obvious course changes for pleasure boats in potential collision situations, so as a "stand-on" I'm obligated to make a course change.

My experience as well, at least in inland waters. But now I'm questioning whether some of these vessels may in fact be stand on to us, either because they are RAM's (and I missed the signal), or because they assumed I was motoring with sails up (need to remember to fly that cone). Does Class A AIS transmit RAM status for e.g.?

[Edits in bold (sorry).]

Exile 19-10-2017 11:55

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

Originally Posted by Dockhead (Post 2502607)
What is an acceptable RISK is an individual decision. What risks result from what decision point and what CPA in a given crossing is NOT subjective.

And even the first thing is not actually individual, because it involves the other vessel. You are endangering the career and possibly freedom of the ship's watchstanders, with kamikaze 180 foot CPA's, even if you don't mind the risk yourself.

"It's a matter of judgement" doesn't mean "whatever you are brave or stupid enough to risk". There is good and bad judgement.

This is the clearest explanation I've read (or written!) yet.

ramblinrod 19-10-2017 11:58

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

Originally Posted by Dockhead (Post 2502607)
What is an acceptable RISK is an individual decision. What risks result from what decision point and what CPA in a given crossing is NOT subjective.

And even the first thing is not actually individual, because it involves the other vessel. You are endangering the career and possibly freedom of the ship's watchstanders, with kamikaze 180 foot CPA's, even if you don't mind the risk yourself.

"It's a matter of judgement" doesn't mean "whatever you are brave or stupid enough to risk". There is good and bad judgement.

That there is human decision involved at the time the "Safe Distance"
decision is made is clearly subjective. I'm not saying that I would pass
The transom within 180 ft, but I guarantee that as I approach the crossing, I will subjectively determine the minimum safe distance I need to give the lead boat transom port corner, and if that distance is determined to be 180 ft by me, that s how close I will come. I will also turn to port as soon as I clear that corner, by the degree required to get no closer to the lead boat, and maintain maximum distance from the following boat, until I am the hell out of its path.

To me, in this example, that is the "safe distance" and it s
completely subjective. Handle it.

Dockhead 19-10-2017 12:44

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

Originally Posted by ramblinrod (Post 2502621)
That there is human decision involved at the time the "Safe Distance"
decision is made is clearly subjective. I'm not saying that I would pass
The transom within 180 ft, but I guarantee that as I approach the crossing, I will subjectively determine the minimum safe distance I need to give the lead boat transom port corner, and if that distance is determined to be 180 ft by me, that s how close I will come. I will also turn to port as soon as I clear that corner, by the degree required to get no closer to the lead boat, and maintain maximum distance from the following boat, until I am the hell out of its path.

To me, in this example, that is the "safe distance" and it s
completely subjective. Handle it.

Sure, Rod.

And all of these guys:

Darwin Awards: 2014 Darwin Awards

also subjectively believed that what they were doing, was perfectly safe, too. But their subjective beliefs did not change how safe or how dangerous it was, that they were doing.

And the COLREGS also don't talk about your subjective beliefs. They talk about "effective action", "safe speed", "ample time", and "safe distance". They don't say "action which the skipper thinks might be effective, no matter how wrong he is", nor "safe distance in the opinion of the skipper, no matter how badly he misunderstands the risks." It's a matter of judgement, sure, but not even yours -- if there's an accident, it will be the judge whose judgement is applied (in some countries and cases, jury, based on proofs offered by the two sides).

Dockhead 19-10-2017 12:46

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

Originally Posted by Exile (Post 2502613)
My experience as well, at least in inland waters. But now I'm questioning whether some of these vessels may in fact be stand on to us, either because they are RAM's (and I missed the signal), or because they assumed I was motoring with sails up (need to remember to fly that cone). Does Class A AIS transmit RAM status for e.g.?

[Edits in bold (sorry).]

Yes, Class A does communicate nav status. It's unfortunate that commercial plotters show it only if you query the target.

OpenCPA (:smitten:) actually puts up the dayshape symbol above the target caret, so it's immediately visible.

Dockhead 19-10-2017 12:51

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

Originally Posted by Exile (Post 2502617)
This is the clearest explanation I've read (or written!) yet.

Thanks, but it's very basic lawyer stuff. We prove in court every day, as a fact, what is "reasonable", for example. Bread and butter.

Collision avoidance cases -- and I've read a thousand of them by now, I guess -- are full of proof about what is a safe distance, and what is "ample time." You can go to jail if you kill someone because your subjective judgement about what is a "safe distance" does not correspond to what is objectively safe enough, which the prosecutor will prove.

evm1024 19-10-2017 12:52

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Just for fun I ran some calculations describing a crossing at 90 degrees between a ship and boat. There are lots of assumptions like the boat is pencil thin and the ship has a square bow. Also keep in mind that we start with the boat directly in front of the ship and end with the ship directly astern of the boat. There are no waves speed variations and the like. This does not give the CPA. It is only calculated for the ship being directly astern of the boat by a given distance. CPA will be less than this distance.

OK, the boat is doing 7 kts and is 50 feet long. The ship has a beam of 110 feet and is doing 15 kts. (for sake of understanding say the boat is on 090 true and the ship is on 000 true)

So if we want the boat to just miss being hit by the ship the boat needs to be 171 feet ahead of the ship when it crosses the ships bow. The ship will just miss the boat after 6.77 seconds. (hey, this is a good crossing! Just my pucker factor is less obvious than yours)

If we want to have 180 feet between the ship and the boat when the ship is directly astern of the boat the boat needs to be 557 feet ahead of the ship when crossing the bow. It will take 22 seconds for the ship to move to the place where the boat was when the boat was directly ahead. (great crossing, 22 seconds is a long time - I'm listening on headphones, I don't hear anything)

If we want a quarter of a mile between us then we need to be 3426 feet ahead of the ship when we cross her bow and it will take the ship 2.2 minutes to get to where we crossed her bow. (Easy peasy - what pucker factor)

If we want a half mile between us then we need to be 6681 feet (a nm and a touch) ahead of the ship when crossing her bow. The ship will take 4.4 minutes to get to the crossing point. (I have no clue why he keeps sounding his horn)

Lastly if we want 1 nm between us then we need to be 2.17 nm ahead of the ship when crossing her bow. The ship will take 8.7 minutes to reach the point of crossing.

If we take the last crossing and expand it to become a full crossing rather than just an escape we start with our boat 1 nw away from crossing the ships path. Still doing the same speeds and a 90 degree crossing aspect we get a CPA of 0.9 nm 15.4 minutes later. This crossing starts with the ship 4.45 nm away from us 77 degrees off our bow. We will be directly ahead of the ship in 8.7 minutes.....

Exile 19-10-2017 12:56

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

Originally Posted by Dockhead (Post 2502648)
Yes, Class A does communicate nav status. It's unfortunate that commercial plotters show it only if you query the target.

OpenCPA (:smitten:) actually puts up the dayshape symbol above the target caret, so it's immediately visible.

Very cool. Another reason why, when my helm plotter finally craps out, I'll be in the market for a waterproof monitor to run OpenCPA above & below decks. :thumb:


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