Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

[El Pinguino]

Originally Posted by Exile

Why take offense when you can respond the way you did, which in turn led to some discussion about potential distinctions that might be drawn btwn sailing in remote areas of the S. Atlantic vs. extremely busy waterways such as the English Channel? Or possible distinctions based on crossings btwn big ships and small boats? I interpreted DH's comment about leaving two miles as related to what was taught and maybe considered good practice by pro mariners. Maybe he was wrong, or maybe this is limited to areas such as the English Channel? Or maybe this is just the opinion of the pros he talked to and others like yourself have a different opinion? The Rules don't specify after all, and Rule 2's "ordinary practice of seaman" clause implies an objective standard that might be interpreted as leaving sufficient distance for a safe and orderly crossing given the body of water & traffic density.

I can understand how you & Rod could take offense at times even if others would not, and don't question your sincerity. But whatever DH did or didn't do to mitigate you & Rod's personal offense doesn't justify the offense subsequently meted out to the ideas that were being discussed & analyzed.

I wasn't offended ... but with everyone else getting offended I didn't want to miss out on my share.
However... early on I was not happy about the tone of the thread and that tone persisted for some time... at least that was my perception. And I was not agreeing or disagreeing with anyone.

Moving right along.... as I have tried to say along the way safe 'distance off' is very variable... and I would suggest that distance would in the normal courseof events be greater when far offshore than in busy coastal waters... just look at the overtaking distances in Singapore Strait at the time of the McCain collision. And overtaking is a far more hazardous business than crossing if only because you may be in very close proximity for a much longer period.

I'm also unsure about the statement '2 miles is taught in college'.... news to me...

From a big ship point of view 'yachts' are a very small part of the 'small craft' mix..... far more common are encounters with the masses of small fishing boats off Dondra Head and unlit fishing boats up to 200 miles of the Peruvian coast.... that sort of thing.

Most of my watch keeping life was spent in 'foreign trade' on runs such as PG - NEurope/Far East , Japan/Straits/India and Australia/Far East so I have had my share of traffic in all sorts of situations.

The last 18 years in the day job this pic below is what used to greet me every second day of my working year at 0315.... before I even had a chance to suck down my first caffeine fix of the day. Two of us are 'pilot exempt' ... moi and the one 4 points to stbd... the rest will be stopping to take pilots...everyone wanting to be up in town in time for the start of the day shift.. and who knows who is outwards....

Thankfully yachts were rare unless arrival was at slack water.... then you would have the occasional 'deep draught' yacht wanting to stick to the main leads...

Oh happy days....

[Lodesman]

Originally Posted by evm1024

The stern of the boat is even with the path of the stb sides of the ships and thus 50' from the centerline of the path. The boat is 40' long so the boat traveled that distance to clear the sides of the ships and thus the center of the boat is (50+40+20) 110' from the centerline of the ships path.

The angle seen from the ship astern to the center of the boat is (arctan(110/6315.5)) 0.9978 degrees off the bow of the ship astern.

If his stern is 50' from the extended centreline and his boat is 40' long, then surely the centre of his boat is 70' from the extended centreline.

But for that, your calculations would make sense. I used radian rule rather than a calculator, but I don't want to give my answer until Rod has had a chance. Rod?

[evm1024]

Originally Posted by Lodesman

If his stern is 50' from the extended centreline and his boat is 40' long, then surely the centre of his boat is 70' from the extended centreline.

But for that, your calculations would make sense. I used radian rule rather than a calculator, but I don't want to give my answer until Rod has had a chance. Rod?

Ah, pooh. I added rather than subtract....



I assumed that you would be using the radian rule (someone spoke of it in the thread as I recall) and left it for you.

[Dockhead]

Originally Posted by evm1024

The 320' of motion of the boat takes 37.9 seconds. In that time the ship astern has traveled (37.9 * 33.76) 1279.5'. So at that point the ship astern is now (1.25 nm or 7595' - 1279.5) 6315.5' from the boat.

The stern of the boat is even with the path of the stb sides of the ships and thus 50' from the centerline of the path. The boat is 40' long so the boat traveled that distance to clear the sides of the ships and thus the center of the boat is (50+40+20) 110' from the centerline of the ships path.

The angle seen from the ship astern to the center of the boat is (arctan(110/6315.5)) 0.9978 degrees off the bow of the ship astern.

So at this point when our intrepid boater thinks he is free and clear of the ship astern. And the bridge crew sees a boat dead ahead just 1.04 nm away.

The bridge crew is in full on collision avoidance mode.

My own bridge time on large ships has been confined as an observer on a 185' research vessel. And my license was no where near that tonnage to boot.

This exercise begs the question: At what point do you consider the crossing complete?

Offhand I would venture that the crossing is complete when no action of the boat (like doing a 180 and running across the ships bow) could cause a CPA of less than (pick some number, oh say) 1 nm.

More experienced hands please chime in.



BTW being 180' perpendicular to the port quarter of the ship ahead is not a 180' CPA. The differences are minor for this calculation and for the most part we all know that.

Yes. This is all about "what is CPA". The place you are aiming to be at the time you are closest to the ship (that's what CPA is) needs to a safe distance away from him -- a place where he won't be with minor variations of everyone's course and speed or inability to measure them.

This is really good lesson in this, because with relative motion only 14 degrees from the ship's course line, the point at which you might theoretically be crossing his course line is meaningless. You've still got a long ways to go before CPA.

It's a lot easier to see this if you plot it, the way you suggested some time earlier.

As to when the crossing is complete -- that's a very relevant and interesting question. I'm not sure it can be answered the way you did, because a much faster ship can always turn around and run you down. I also don't really know what the answer is, but I don't think it's earlier than CPA. Certainly hasn't happened just because you've crossed his course line.

[Dockhead]

Originally Posted by El Pinguino

. . .

Moving right along.... as I have tried to say along the way safe 'distance off' is very variable... and I would suggest that distance would in the normal courseof events be greater when far offshore than in busy coastal waters... just look at the overtaking distances in Singapore Strait at the time of the McCain collision. And overtaking is a far more hazardous business than crossing if only because you may be in very close proximity for a much longer period.

I'm also unsure about the statement '2 miles is taught in college'.... news to me...

From a big ship point of view 'yachts' are a very small part of the 'small craft' mix..... far more common are encounters with the masses of small fishing boats off Dondra Head and unlit fishing boats up to 200 miles of the Peruvian coast.... that sort of thing.

Most of my watch keeping life was spent in 'foreign trade' on runs such as PG - NEurope/Far East , Japan/Straits/India and Australia/Far East so I have had my share of traffic in all sorts of situations.

The last 18 years in the day job this pic below is what used to greet me every second day of my working year at 0315.... before I even had a chance to suck down my first caffeine fix of the day. Two of us are 'pilot exempt' ... moi and the one 4 points to stbd... the rest will be stopping to take pilots...everyone wanting to be up in town in time for the start of the day shift.. and who knows who is outwards....

Thankfully yachts were rare unless arrival was at slack water.... then you would have the occasional 'deep draught' yacht wanting to stick to the main leads...

Oh happy days....

I'm ready -- actually eager -- to be educated on what is considered a safe CPA by pros, in different situations.

According to various things I've read, treatises and articles, 2 miles minimum is taught as a recommendation when there is sea room; maybe less but at least 1 mile in traffic. Obviously you can't set a rigid rule about it, which is why none exists. I read one "near miss" report in CHIPS where one ship was complaining that another refused to give 2 miles of room when requested; CPA over a mile was considered too little by the Board, who scolded the ship which didn't give the room requested.

I can't say whether or not I've observed anyone maneuvering for 2 mile CPAs, but where I sail I see all the time ships maneuvering at 5 to 10 miles out, to get exactly 1.0 mile CPA when crossing with me. It seems to be really standard operating procedure in the Channel and North Sea. I also make it a practice, when I'm at sea, to listen to radio conversations concerning collision avoidance (and I switch channels to listen in), which has been a whole education in itself.


That's about all I know about the "ordinary practice of seaman" concerning safe CPAs, and I'll be glad to be corrected or supplemented.

I'm also interested to know if anyone thinks that safe passing distances should be less or more between ships and small vessels as opposed to ships and other ships.


P.S. -- Concerning fishing vessels -- a remarkably large proportion of the collisions I've read the reports on are with fishing vessels. Maybe half? And quite few with yachts. I guess yachts just make up a very small part of traffic offshore. Nevertheless, a yachtsman or two is killed every few years around here in a collision with a ship. Last year in a collision with a dredger.

[StuM]

Originally Posted by Dockhead

I'm ready -- actually eager -- to be educated on what is considered a safe CPA by pros, in different situations.

Interesting figures in a document you linked to earlier:

https://hrcak.srce.hr/file/267896
"A MODEL OF DETERMINING THE CLOSEST POINT OF APPROACH BETWEEN SHIPS ON THE OPEN SEA "

During the last two years the Faculty of Maritime Studies in Rijeka was the leading project manager for the EU research project “Avoiding Collision at SeaACTs”. One of the goal of the project was to conduct research related to determining the CPA within the unlimited area of navigation.
The research conducted in 2014 involved 1,530 respondents, but for the purpose of this paper only the results obtained from those with at least one year of navigation were analysed. This sample comprised 225 respondents whose views on the distance at which to start the avoidance action and on the CPA were analysed

The first scenario presented to respondents was the following: two power-propelled vessels with the LOA 200 m and the speed of 15 knots are crossing on the open sea with the risk of collision. You are aboard the vessel that gives way. At what distance will you start the action to avoid the collision? The results are presented in Figure 3

The graph shows that the majority of respondents answered that the distance would be 5.1-8 M, and a slightly smaller number of respondents said the distance would be 3.1-5 M.


The second scenario was the following: What do you think is the safe CPA for two power-driven vessels, LOA 200 m, when they meet on the open sea. The results are presented in Figure 4.


The obtained results lead to the conclusion that the majority of respondents consider the safest CPA to vary from 1.6 to 2.5 M.

[El Pinguino]

Originally Posted by Dockhead

I'm ready -- actually eager -- to be educated on what is considered a safe CPA by pros, in different situations.......

Oh, that's an easy one......

It varies....

As shown in the graph.... on age and experience of the watchkeeper.

And also on searoom , relative and absolute size of both vessels, relative and absolute speed of both, aspect... you will make a broader alteration in a crossing situation than end on, night or day, weather, visibility, other traffic....

As I say .... it varies ...

'I'm also interested to know if anyone thinks that safe passing distances should be less or more between ships and small vessels as opposed to ships and other ships.'

I would expect bigger the ships greater the distance... avoiding small stuff less... and if I alter on my yacht 0.1 is fine as long as she can see what I am doing.. but I would expect a ship altering for me to give me more room if possible.

If in a bog standard crossing situation ( on my yacht) and I am motoring/give way I just show them red and then keep aiming for their stern... they are happy, I am happy, and we miss.... works for all sizes.

'but where I sail I see all the time ships maneuvering at 5 to 10 miles out, to get exactly 1.0 mile CPA when crossing with me. '

How do they see whether or not you are flying a motoring cone at 10 miles?

Indeed how do they even see you at 10 miles? And -possibly- having class A AIS doesn't count ... I've seen 3 big fishing boats in the last few weeks that were 'under way - sailing' so that is not to be relied on.

[conachair]

Originally Posted by El Pinguino

'but where I sail I see all the time ships maneuvering at 5 to 10 miles out, to get exactly 1.0 mile CPA when crossing with me. '

How do they see whether or not you are flying a motoring cone at 10 miles?

Indeed how do they even see you at 10 miles? And -possibly- having class A AIS doesn't count ... I've seen 3 big fishing boats in the last few weeks that were 'under way - sailing' so that is not to be relied on.

Mid ocean they pick up my steel boat 15 - 20 miles and often manoeuvre 5 or so miles out to give more searoom.

Just what happens.

[Lodesman]

Originally Posted by Dockhead

According to various things I've read, treatises and articles, 2 miles minimum is taught as a recommendation when there is sea room; maybe less but at least 1 mile in traffic.

I was never taught a minimum, let alone a specific 2 mile min cpa. But, maybe pertinence or coincidence, 2 miles has almost always been the criteria for calling the captain. I know many junior oow's that would use their discretionary manoeuvring to avoid calling the old man.

[El Pinguino]

Originally Posted by conachair

Mid ocean they pick up my steel boat 15 - 20 miles and often manoeuvre 5 or so miles out to give more searoom.

Just what happens.

Just what I would expect to happen.... classic case would be start taking an active interest in you at 10... watch range and bearing for 12 minutes by which time - if you are stationary and she is doing the standard 15 knots - range has reduced to 7 miles.... watchkeeper has a quiet scratch and a think and then alters at 5 miles.

Once again so many variables .... on opposite headings rangle closing at maybe 23 knots... things happen quickly.

Overtaking and range closing at only 7 knots... it will be an hour and a half before she is abeam....

And every variation in between...

[Dockhead]

Originally Posted by El Pinguino

.. .

How do they see whether or not you are flying a motoring cone at 10 miles?

Indeed how do they even see you at 10 miles? And -possibly- having class A AIS doesn't count ... I've seen 3 big fishing boats in the last few weeks that were 'under way - sailing' so that is not to be relied on.

Good question, but I've observed it millions of times. From talking to them, I think they don't care about the motoring cone. They don't want to let us near them, and take early action to prevent it.

When possible, of course. Often they have bigger fish to fry.

[Exile]

Originally Posted by Dockhead

According to various things I've read, treatises and articles, 2 miles minimum is taught as a recommendation when there is sea room; maybe less but at least 1 mile in traffic. Obviously you can't set a rigid rule about it, which is why none exists.

Originally Posted by El Pinguino

Oh, that's an easy one......

It varies....

As shown in the graph.... on age and experience of the watchkeeper.

And also on searoom , relative and absolute size of both vessels, relative and absolute speed of both, aspect... you will make a broader alteration in a crossing situation than end on, night or day, weather, visibility, other traffic....

As I say .... it varies ...

'I'm also interested to know if anyone thinks that safe passing distances should be less or more between ships and small vessels as opposed to ships and other ships.'

I would expect bigger the ships greater the distance... avoiding small stuff less... and if I alter on my yacht 0.1 is fine as long as she can see what I am doing.. but I would expect a ship altering for me to give me more room if possible.

If in a bog standard crossing situation ( on my yacht) and I am motoring/give way I just show them red and then keep aiming for their stern... they are happy, I am happy, and we miss.... works for all sizes.

'but where I sail I see all the time ships maneuvering at 5 to 10 miles out, to get exactly 1.0 mile CPA when crossing with me. '

How do they see whether or not you are flying a motoring cone at 10 miles?

Indeed how do they even see you at 10 miles? And -possibly- having class A AIS doesn't count ... I've seen 3 big fishing boats in the last few weeks that were 'under way - sailing' so that is not to be relied on.

Originally Posted by Lodesman

I was never taught a minimum, let alone a specific 2 mile min cpa. But, maybe pertinence or coincidence, 2 miles has almost always been the criteria for calling the captain. I know many junior oow's that would use their discretionary manoeuvring to avoid calling the old man.

This sounds all too reasonable, and if it's reasonable it's Colregs-compliant, and if it's Colregs-compliant it will generally result in collision avoidance. For most situations, this is the rough guideline some of us less experienced recreational sailors probably need to know. After that, it probably comes down to experience, especially perhaps in places like the English Channel.

[evm1024]

Originally Posted by Dockhead

Yes. This is all about "what is CPA". The place you are aiming to be at the time you are closest to the ship (that's what CPA is) needs to a safe distance away from him -- a place where he won't be with minor variations of everyone's course and speed or inability to measure them.

This is really good lesson in this, because with relative motion only 14 degrees from the ship's course line, the point at which you might theoretically be crossing his course line is meaningless. You've still got a long ways to go before CPA.

It's a lot easier to see this if you plot it, the way you suggested some time earlier.

As to when the crossing is complete -- that's a very relevant and interesting question. I'm not sure it can be answered the way you did, because a much faster ship can always turn around and run you down. I also don't really know what the answer is, but I don't think it's earlier than CPA. Certainly hasn't happened just because you've crossed his course line.

I answered from the prospective of the Ship. The crossing is far from complete from the prospective of the boat.

A non answer might be "the crossing is complete when changes in course or speed could results in a new crossing situation". But that is not helpful....

[Dockhead]

Originally Posted by evm1024

I answered from the prospective of the Ship. The crossing is far from complete from the prospective of the boat.

A non answer might be "the crossing is complete when changes in course or speed could results in a new crossing situation". But that is not helpful....

I see what you mean. But I think that this is in fact what some pros are thinking -- how to keep us far enough away that we can't get under their bows.

I think achieving "can't" may be difficult without a big difference in speed, but a safe CPA at least gives room and time to make another correction in case the WAFI does something dumb.

My sense is that a mile is a really good rule of thumb because if you're set up to pass with a mile CPA from several miles off, then with both vessels holding course and speed in the final approach, both vessels get to observe and be sure that it's going to plan, with time and space to correct in case something goes wrong -- like for example someone changes course or a yacht under sail speeds up or slows down because of a change of the wind.

That's another reason for setting up a crossing from a decent distance away, rather than trying to resolve it with some kind of last-minute maneuver. Interesting that I can't seem to find anything written about this anywhere. But it's what's intended with the Rule about EARLY action, isn't it? Maybe it's considered to be obvious and common sense and so not worth discussion.

[Dockhead]

Originally Posted by El Pinguino

Oh, that's an easy one......

It varies....

As shown in the graph.... on age and experience of the watchkeeper.

And also on searoom , relative and absolute size of both vessels, relative and absolute speed of both, aspect... you will make a broader alteration in a crossing situation than end on, night or day, weather, visibility, other traffic....

As I say .... it varies ...

'I'm also interested to know if anyone thinks that safe passing distances should be less or more between ships and small vessels as opposed to ships and other ships.'

I would expect bigger the ships greater the distance... avoiding small stuff less... and if I alter on my yacht 0.1 is fine as long as she can see what I am doing.. but I would expect a ship altering for me to give me more room if possible.

If in a bog standard crossing situation ( on my yacht) and I am motoring/give way I just show them red and then keep aiming for their stern... they are happy, I am happy, and we miss.... works for all sizes.

'but where I sail I see all the time ships maneuvering at 5 to 10 miles out, to get exactly 1.0 mile CPA when crossing with me. '
.

Thanks; very useful.


Concerning "aiming for the stern" -- I've read criticism of this technique, but I don't understand the problem -- seems foolproof to me. My Dad taught me this when I was a wee chappy -- first collision avoidance technique I ever knew. As long as you have a bit of side aspect and can actually see the stern, and the ship is moving faster than you are, what's wrong with this technique? Am I missing something?

I use it even now in the Solent or other crowded places -- no need to mess with the AIS.