Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

[El Pinguino]

Originally Posted by Dockhead

Yes, let's get back to the original premise of this thread. I see it went way down a couple of rabbit holes while I was traveling.

The whole point of the original thread was this -- how do we account for what we can't know, when we are doing collision avoidance maneuvers? What we can't know can be represented with a cone of uncertainty, which was actually plotted by someone some pages back in the thread.

A one degree alteration from five miles out, say, might really change a collision course to a safe pass, if you do simple math and figure the distance represented by that one degree over five miles. The only problem with this is that we don't know to that degree of certainty where either we or the ship will be five miles later. In fact, if you think about it, a one degree alteration of course is meaningless, totally meaningless, anyway, because we can't hold a course to that degree of accuracy, nor can you expect the ship to hold its course to that degree of accuracy. Then you add in speed, and errors in estimating position, etc., and you soon see that making a one degree alteration of course is completely useless.

As to the 180 foot pass -- I think we can all agree that passing close behind can be safe -- if you can see the transom of a ship moving much faster than we are, then no matter how close or how far you are, there is no problem, guaranteed. But the whole problem with this is getting there. How do you set it up? How close to the BOW of the ship, would you have to get, in order to pass 180 feet behind, when there is such a different in speed? What does it even LOOK LIKE, being on a course to pass 180 feet behind a fast moving ship?

And so there is such a thing as a safe CPA -- and a safe CPA is one which you can set up from a safe distance away, and which provides enough margin of error that it can absorb the uncertainties in calculating your maneuver. 180 feet is not a safe CPA under any circumstances, when crossing in open water with a fast moving ship, because there is actually no such thing -- we don't have enough information to set it up. It's the same as a collision course or 180 feet passing ahead. You have to set up a crossing so that you know you are passing, the way you think you are passing, and that needs to be a distance bigger than the cone of uncertainty around the positions of the two vessels when they cross.

That's what we were talking about, and I think most people on here got it.

Is it just me that's having a déjà vu moment all over again?

How long is it until the spring thaw??

Surely it is long past the time that this thread should have been put down....

[Paul Elliott]

Originally Posted by El Pinguino

Is it just me that's having a déjà vu moment all over again?

How long is it until the spring thaw??

Surely it is long past the time that this thread should have been put down....

It would die a natural death is you wouldn't keep posting to it!

(Ok, you and a few others. Some of who apparently still find it interesting.)

[Dockhead]

Originally Posted by El Pinguino

Is it just me that's having a déjà vu moment all over again?

How long is it until the spring thaw??

Surely it is long past the time that this thread should have been put down....

The subject is interesting to some of us, and thanks for bumping it up by posting to it!

[TJ D]

Originally Posted by El Pinguino

Is it just me that's having a déjà vu moment all over again?

How long is it until the spring thaw??

Surely it is long past the time that this thread should have been put down....

Well, it's sort of like an accident (or a collision at sea, based on some of the posts) that you really want to look away from, but you just can't...

I've been entertained for hours by this thread! Most times when I log on, this is the first thread I check to see what fireworks have been launched by some of my favorites.

It probably lost any usefulness a couple of weeks ago, but sometimes CF can be a little dry. It's good to have this kind of thing to spice things up.

Carry on!

[jackdale]

OK here is Rule 8

Action to avoid a collision.
(a) Any action taken to avoid collision shall be taken in accordance with the Rules of this Part and shall, if the circumstances of the case admit, be positive, made in ample time and with due regard to the observance of good seamanship.

(b) Any alteration of course and/or speed to avoid collision shall, if the circumstances of the case admit, be large enough to be readily apparent to another vessel observing visually or by radar; a succession of small alterations of course and/or speed should be avoided.

(c) If there is sufficient sea room, alteration of course alone may be the most effective action to avoid a close-quarters situation provided that it is made in good time, is substantial and does not result in another close-quarters situation.

(d) Action taken to avoid a collision with another vessel shall be such as to result in passing at a safe distance. The effectiveness of the action shall be carefully checked until the other vessel is finally past and clear.

Can some one please explain to me how they can recognize a 1 degree course change as being "readily apparent", especially since I do not know a single helmsperson (including an autopilot) who could steer with that precision even under power, let alone under sail. A one degree course change is a small alteration.

There are three ways to give way

1) Alter course (to cross astern)
2) Alter Speed (by slowing down)
3) Both

The thread seems only to cover the first.

[evm1024]

Originally Posted by jackdale

OK here is Rule 8



Can some one please explain to me how they can recognize a 1 degree course change as being "readily apparent", especially since I do not know a single helmsperson (including an autopilot) who could steer with that precision even under power, let alone under sail. A one degree course change is a small alteration.

There are three ways to give way

1) Alter course (to cross astern)
2) Alter Speed (by slowing down)
3) Both

The thread seems only to cover the first.

The primary focus was (is?) a course change. But there were a few posts that talked about speed changes or speed and course changes. Rule 8 was also debated....

[Lodesman]

Originally Posted by Uricanejack

The courts in Canada will apply the standard of due diligence. Should a court case result due to a collision between a commercial vessel and a small vessel.

Due Diligence? How would a reasonable person expect a reasonable person to reasonably act. The level of knowledge and certification will certainly form part of the question was due diligence exercised and will apply.

So would it be reasonable to expect the operator of a small vessel with a PCOC to understand the full meaning of the requirement to not impede or rule 2 or the actions of a stand on vessel?

Change the operator of the small vessel to MM. ask the same question and the answer about what might be reasonable may be different.

I have no expertise in legal matters, but am of the opinion that due diligence plays into the defence:

Judge: Do you dispute the claim that you ran down the small vessel?
Defendant: No your Honour, it's plain as day; his rigging was hanging from my anchor, bits of fibreglass all around, and the skipper's DNA splattered all over my bow. But I did my due diligence. It was daytime, good vis; we were at 12 kts, under the 14 kt limit, but not so slow that we'd hamper the vessels behind. The engine was ready for manoeuvre and the helmsman was steering in hand, I had 2 lookouts posted and both radars were operating and tuned properly. I observed the small boat motoring from about 5 miles on my port bow, and took gyro bearings and observed it steady. He hadn't changed course, so identified from AIS and called him on VHF at about 3 miles distance. I explained that I was constrained to the buoyed deep draught channel and asked his intentions. The skipper told me he'd altered by 1º and was planning to pass my stern at 180 feet. I told him that I not happy with that passing and asked that he pass by 500 yards. He started screaming "strawman, strawman" over the radio. What was I to do? I couldn't turn without hitting the rocks either side of the channel - I had 80,000 tonnes of fuel oil; it would be an environmental disaster. If I crash stopped, the sheer to starboard would put me onto the rocks, so I ordered the engine stopped and sounded 5 short blasts. I launched a boat right away, but there was nothing left.

Found this online: https://en.wikibooks.org/wiki/Canadi.../Due_Diligence

So I take the point that "all reasonable care" might involve a few more steps for a MM that has ARPA, 2 radars, the facility to increase the number of lookouts, etc, etc. than what would be expected by some yokel in a 20-foot runabout. But put that MM in a 20-foot runabout too, and the standard of "all reasonable care" is no different than that expected of the yokel. So RigglinRod's notion that it's better to be an unqualified ignoramus so you don't get punished as severely when you eventually get busted, is inane.

[Lodesman]

Originally Posted by Uricanejack

The original premise of this thread. Referred to the 1 degree alteration and 180ft.
I have not read the original context in the original thread, I haven't found it.
Most of you seem to have concluded this was a reckless or foolish statement.
You are all to focused on hard numbers.

Passing close astern is very different from passing close ahead.
If you alter course early rather than later you don't have to alter course nearly as much to have the same effect.


As has been stated just do the math. Apply the geometry or trig.

The numbers are just rather extreme examples of a fairly simple perfectly safe and reasonable approach to collision avoidance.

You need to read the whole thread. One poster stuck by a hard and fast figure. First it was 180 feet astern until it was pointed out that the actual CPA would be under 40 feet, then it was "of course 180 foot cpa, strawman strawman..." There was also the matter of whether or not this 1º alteration was done at 5 miles as you and I (and just about everybody else) would gauge it, or if it was the poster's subsequent insistence that he meant "5 miles from the invisible X in the ocean indicating the future point of collision". There was quite a lengthy diversion into whether or not you could see a vessel at 20+ miles, and even then if you are able to ascertain a risk of collision necessitating a 1º course alteration.

[Dockhead]

Originally Posted by jackdale

OK here is Rule 8



Can some one please explain to me how they can recognize a 1 degree course change as being "readily apparent", especially since I do not know a single helmsperson (including an autopilot) who could steer with that precision even under power, let alone under sail. A one degree course change is a small alteration.

There are three ways to give way

1) Alter course (to cross astern)
2) Alter Speed (by slowing down)
3) Both

The thread seems only to cover the first.

We did talk about altering speed -- taking way off or heaving-to. It's an important tactic for a sailboat. Note that ships rarely alter speed to change CPA -- too much trouble compared to putting the helm over. Note also that taking way off has a really serious downside -- you change the relative speed against you, ending up with less control and less ability to make subsequent changes to the CPA. If you are on something like a collision course but actually you are passing somewhat ahead, something you often can't recognize if you're not using AIS, or if the ship alters course in the wrong direction after you take way off, then taking way off can leave you a sitting duck.

As to one degree course changes -- yes, besides being meaningless for the purpose of creating a safe CPA, since you can't hold a course that closely anyway, it is a violation of Rule 8. And taking that further -- going into a crossing trying to make a very close pass with a much faster vessel, intending to make multiple small corrections along the way, is a violation per se of Rule 8 and is just the wrong way to do it. We should make large and obvious course changes at a safe distance, so that we can hold our course and speed as we get closer, and the other vessel (and we) can see the whole time that the crossing is safe.

We did talk about this, but I understand it may be difficult to dig it out of the large volume of posts.

[Uricanejack]

Originally Posted by Lodesman

You need to read the whole thread. Rod stuck by a hard and fast figure --- well as much as Rod sticks by something - he tends to move the goalposts. First it was 180 feet astern until it was pointed out that the actual CPA would be under 40 feet, then it was "of course 180 foot cpa, strawman strawman..." There was also the matter of whether or not this 1º alteration was done at 5 miles as you and I (and just about everybody else) would gauge it, or if it was Rod's subsequent insistence that he meant "5 miles from the invisible X in the ocean indicating the future point of collision". I don't know if he walked back on the that one - there was quite a lengthy diversion into whether or not you could see a vessel at 20+ miles, and even then if you are able to ascertain a risk of collision necessitating a 1º course alteration. Then there was his ridiculous scenario of 50 ships steaming in a perfect line, requiring you to steam head on into a wall of steel, while the hole of the needle, broad on your beam blows by you at 20 knots.

Admittedly I have not read the entire thread only about half of it.
After half the thread I came to the conclusion Rod was being characterised a bit unfairly and is responding to this defensively.

The OP posted excerpts from Rod. from another thread and stated a belief about Rods post.

Both sides of this 70 plus page argument appear to agree a 1 degree alteration is by the small sailboat not actually possible.

So I can only conclude its a theoretical or hypothetical argument not intended as a real practical action. I did not see any suggestion from Rod he would actually apply this in a real situation or a suggestion anyone else should.


I took Rods comment to be a theoretical mathematical computation of a the effect of a 1 degree alteration from a distance of 5 miles.

Rods further comments sum up as take your eyes up away from the electronics and look out at the situation, take early action, make it obvious, avoid crossing ahead.

I get the impression. From reading Rods posts. On This thread. When Rod is out on his boat on Lake Ontario he keeps a good lookout. Using visual references or giving visual references higher priority than RADAR or AIS.
Applies the rules appropriately. Responding as a giveaway vessel early and making it obvious to the other vessel he intends to pass astern.

I don't disagree with The OP's suggestion of using a significant margin of error for collision avoidance. I just think its a bit unfair to pick this one guys comment and use it in this way.

I do disagree with the suggested mile CPA in open water with large vessels.
To my mind it depends on the circumstances. A mile is quite appropriate in some circumstances. In some like crossing ahead of the theoretical containership doing 20 knots. It might not be quite far enough.

[JPA Cate]

For those of you who have been wondering, this thread has recently undergone a removal of disrespectful, or rude, or intentionally provocative posts. If you are unaware what the standards actually are, it's great to combat ideas, but not attack other posters--even if you feel angry and enjoy fighting; the actual rules are at the bottom of the page, under the heading "Community Rules."

Ann, for the mod team

[Lodesman]

Originally Posted by Uricanejack

After half the thread I came to the conclusion Rod was being characterised a bit unfairly and is responding to this defensively.

Rod made an unequivocal statement about altering by 1º and passing, literally, 180 feet astern of a large, fast moving merchantman. This was the basis of the thread, and is no different than any other thread where someone posits controversially. Rod hasn't helped himself by being rude and obnoxious to many who have contributed to the thread.

Both sides of this 70 plus page argument appear to agree a 1 degree alteration is by the small sailboat not actually possible.

Well one would think so, and Rod himself has said as much, but he's also stuck to it being a valid course of action.

So I can only conclude its a theoretical or hypothetical argument not intended as a real practical action. I did not see any suggestion from Rod he would actually apply this in a real situation or a suggestion anyone else should.

Actually he claims that he has, but has offered no details. You can decide for yourself if you believe it.

Rods further comments sum up as take your eyes up away from the electronics and look out at the situation, take early action, make it obvious, avoid crossing ahead.

Aiming to pass 60 yards under the stern of the other vessel from 5 or 20 miles out, is hardly obvious. That has been central to the OP's point.

I don't disagree with The OP's suggestion of using a significant margin of error for collision avoidance. I just think its a bit unfair to pick this one guys comment and use it in this way.

Do you consider it unfair that Rod singled out the OP's comments on minimum CPA to disparage the OP? Feel free to check the original thread: http://www.cruisersforum.com/forums/...-189419-7.html

I do disagree with the suggested mile CPA in open water with large vessels.
Why the heck would you want to pass closer, if there is nothing constraining you (open water)? Frankly most 1st world pro's will tend to make passes in open water of at least 1 mile - I would be highly suspicious of one that came past closer than that (assuming he's the give way).

To my mind it depends on the circumstances. A mile is quite appropriate in some circumstances. In some like crossing ahead of the theoretical containership doing 20 knots. It might not be quite far enough.

Yes, it depends on circumstances - but the OP specifically stated he was talking about open water, with no other pressing factors.
BTW, a "theoretical containership" can be passed ahead by 1mm; it's the real ones that need more room

[Dockhead]

One of the three things mentioned in the title of the thread was "Appropriate CPA". In all of these many pages, we haven't directly engaged that point. The idea was that the difference between a safe and unsafe CPA is related to the different uncertainties in the information we have when we set up the crossing. The further away the ship is, the less we know about where it -- and we -- will be at the moment when we are closest.

So one possible approach to collision avoidance is this, and we've heard it argued for (not necessarily in this thread) -- don't worry about it, wait until you're closer, then you'll see whether it's a problem or not (see even with your bare eyes), and you can just adjust as you go along.

But the problem with that is twofold -- first of all, in some situations, by the time you're close enough to see it with your bare eyes, it may be hard or even impossible to get out of the way; and secondly, it's not safe to "adjust as you go along" -- you are required by the Rules to make a large and apparent maneuver from a safe distance.

"Just keep a good watch, and correct as necessary" is a natural and kind of instinctive way to do collision avoidance, but many collisions, lost ships, and lost lives resulted from this, as a result of which the procedures which are required by the Rules were developed over time. So what we do is NOT indeed to wait until it's obvious with your bare eyes, that you have a problem. What we do is to determine from a safe distance away, generally using other means than bare eyes (hand bearing compass, at least), whether we have a problem. "Have a problem" here means that we are on the way to pass so close to the ship, that it's not obvious FROM THAT DISTANCE that we will pass with a decent margin of error. Then, we make a large and obvious change of course and/or speed to correct the situation -- to set us up to pass with a decent margin of error. "Large and obvious" is so that the ship will be able to see what we've done, AND be convinced that we've solved the problem and that he can carry on his course and speed. The required "large and obvious" maneuver communicates to the other vessel that we took control of the crossing and dealt with it.

A mile is not theoretically needed in most cases -- ONCE YOU ARE CLOSE ENOUGH FOR IT TO BE OBVIOUS EXACTLY WHERE HE WILL BE. But from a decent distance away -- four or five miles -- if you set up the crossing for less than a mile, then depending on how accurate your information is about where he is and what is course and speed are, and how well everyone is holding course and speed, there is a substantial risk that you will not have resolved the problem after all, by the time you get closer, and you will have to correct again.

This is an undesirable situation from several points of view -- first of all, it requires concentration and work, which then can't be spent for example dealing with other traffic. The efficient way to use your resources -- and this is especially important on short-handed cruising boats -- is to solve it once and for all so you're not distracted from watching other traffic. This is important. Secondly, once you get closer, your options start to dwindle fast, and it becomes gradually harder to make an orderly maneuver which convinces the ship that you continue to be in control of the crossing. At some point he might start to maneuver himself, and then you are already doing it differently -- uncoordinated maneuvering with a risk of maneuvering into each other. Setting up the crossing at the appropriate distance and resolving it once and for all at that distance avoids all of this.

Trying to cross at a very close distance, is like trying to make a flight by leaving for the airport at the last minute. Why? Say you're leaving your office in the center of a big city with a flight at a certain time. You can calculate how long it takes to get to the airport, maybe you even have Google Maps calculate it for you (that's like having AIS). So from your desk, you calculate that it will take 42 minutes to get to the airport, and another 23 minutes to get through security and get to the gate. So does that mean it's OK to leave your office 66 minutes before the flight? Well, if your information is exactly correct, you'll get there with a minute to spare (that's like a one cable crossing -- I'm not even going there, to talk about 60 meters). But we don't do this, of course, because as everyone knows, you cannot know, not even with Google Maps and its real time analysis of traffic conditions, exactly how long it will take us -- traffic is unpredictable. We can't know what the lines at security will be like. We might know pretty well in good traffic conditions (middle of the night, airport not busy), or we might not have any idea at all (rush hour; airport mobbed), but even in the best case, we can't know to one minute. THEREFORE, we leave in time to get there an hour or an hour and a half before the flight, and plan on having a cup of coffee when we get here if all goes well, in the knowledge that even if there are some traffic snarls or unexpected problems at security, we have a margin of error to get through that without missing our flight.

The one mile CPA is a lot like that. Crossing closer than that would mostly not mean that we will get run down, just like leaving for the airport with a plan to arrive 40 minutes before the flight will mostly result in our not missing the flight. But we make a habit of crossing at a comfortable safe distance which can absorb different contingencies, variations in speed, inaccuracies in calculations, etc., so that we can do it smoothly and calmly and without any drama, just like we leave a decent margin of error for time when we go to the airport.

[DotDun]

Originally Posted by Dockhead

One of the three things mentioned in the title of the thread was "Appropriate CPA". ......

I think you hit the point that many are missing.

IMO, at 10nm out, a 1nm CPA is very comfortable. At 5nm out, a .5nm CPA isn't uncomfortable, but it may require a little closer watch depending on conditions. I can certainly handle closer CPA, but why?

Again, IMO, this whole discussion shows a lot about human nature. Those with the 'I'll worry about me only' attitude are showing total disregard for what happens on the bridge of a large vessel. It's certainly demonstrates a lot about 'character'. Collision avoidance is a multi-player activity, every player has the same goal, why not help each other achieve the desired outcome?

Planning a .03nm astern CPA is, IMO, not very smart.

[Seaworthy Lass]

In all this discussion I have not heard what people consider is the most reliable tool for estimating if a risk of collision exists. Or to put is another way, what they are most comfortable depending on.

Personally, I rely on a hand bearing compass. It is always around my neck during watches. I can keep track of 2-3 boats fairly easily from memory. Any more, then a pen, paper and often headtorch is needed.

What do others rely on the most?

SWL