Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

[StuM]

Originally Posted by Dockhead

Meanwhile you also carefully monitor the ship you are passing ahead of. A mile is a decent margin, IF you have AIS and so relatively precise information about how the crossing is developing. And IF you are maintaining your speed -- if there is a big lull, you can get into trouble pretty fast, and so you need to be ready to get the motor on, or just have it running anyway and put up the motoring cone.

If you have the motor running in case you need it, but it is in neutral, you are still a sailing vessel and shouldn't be displaying the motoring cone.

[Dockhead]

Originally Posted by StuM

If you have the motor running in case you need it, but it is in neutral, you are still a sailing vessel and shouldn't be displaying the motoring cone.

You are right, but I didn't say "in neutral"

I would put it in gear -- but I have a variable pitch prop so in gear at idle is worthwhile in any case.

I have however previously confessed to falsely setting a motoring cone, and it was in a similar situation, where I needed multiple ships to hold course and speed so I could pick my way through them. You can't really count on it being seen and interpreted, so you do need to call also.

I don't think the false signal is so heinous -- it communicates your intention to give way and stay in control of the crossing. I guess wise men may disagree on that, but I think it's a Rule 2 situation.

[conachair]

Originally Posted by Dockhead

Typical Needles-Cherbourg crossing.

Only it's even worse -- the ships aren't in "single file" -- they are on parallel but staggered courses.

And still even much, much worse than that -- they are not moving at the same speed, so the gaps between them are not fixed.

You learn a lot about collision avoidance, sailing in the Channel!

Rubbish! Never seen 50 ships nose to tail at 20Kts in a line anywhere in the world...

Channel isn't that big a deal, often straight across with no course changes needed plus quite often the ships will do a little dogleg to give a 1Nm CPA. Wasn't even that bad before AIS, usually dip behind the 4th ship you can see as you approach the lanes.

[Dockhead]

Originally Posted by conachair

Rubbish! Never seen 50 ships nose to tail at 20Kts in a line anywhere in the world...

Channel isn't that big a deal, often straight across with no course changes needed plus quite often the ships will do a little dogleg to give a 1Nm CPA. Wasn't even that bad before AIS, usually dip behind the 4th ship you can see as you approach the lanes.

Look at MarineTraffic sometime.

There are lulls in the traffic, for sure. I've gotten across from time to time without any maneuvering required.

But it's very common indeed to encounter long lines of ships only a mile or two apart, and scenarios similar to Rod's hypothetical are quite common. Frequently discussed among Channel sailors -- it's a typical challenge.

Now not indeed all at 20 knots. Since the big fuel economy slowdown, more typical speeds are 16 to 18. Passenger liners, ferries, and military vessels are often making making MORE than 20 knots, though.

Situation RIGHT NOW:




400 ships a day transit the Channel (that does not include local traffic). That's 17 per hour, or one every 3 or 4 minutes. That is bloody intense traffic, the most intense of any seaway in the world, and large lines or clusters of ships a mile or two apart is quite typical.

Channels sailors liken crossing to being a squirrel trying to get across a busy motorway. It's not always like that, but often enough.

[conachair]

Originally Posted by Dockhead

Look at MarineTraffic sometime.

There are lulls in the traffic, for sure. I've gotten across from time to time without any maneuvering required.

But it's very common indeed to encounter long lines of ships only a mile or two apart, and scenarios similar to Rod's hypothetical are quite common. Frequently discussed among Channel sailors -- it's a typical challenge.

Now not indeed all at 20 knots. Since the big fuel economy slowdown, more typical speeds are 16 to 18. Passenger liners, ferries, and military vessels are often making making MORE than 20 knots, though.

Situation RIGHT NOW:

Attachment 157998


400 ships a day transit the Channel (that does not include local traffic). That's 17 per hour, or one every 3 or 4 minutes. That is bloody intense traffic, the most intense of any seaway in the world, and large lines or clusters of ships a mile or two apart is quite typical.

Channels sailors liken crossing to being a squirrel trying to get across a busy motorway. It's not always like that, but often enough.

Spent many a moment on marine traffic , nowhere near so many channel crossings as you but a good handful usually single handed, yes, very busy but not too big a deal to get across. Anyway, by the by, point was that Rods mythical 50 ships all in line mile or so apart all doing 20Kts was made up probably to somehow prove CPA of a Nm isn't possible. Across the bows it is.

[Pete7]

Originally Posted by Dockhead

Situation RIGHT NOW:

Attachment 157998

You're being modest, add a stiff South Westerly F5 plus 3 knots of opposing tide into the equation and it really becomes interesting with the crew hanging on for dear life and the waves sweep across the deck.

Oh and the separation lane doesn't hold much respite either because the local fishermen have clicked on that large ships don't use this bit of water so lay there pots there.

Add 3 lots of large wind farms being built on the South Coast and it would be easier to escape from Colditz than cross the channel.

Pete

[Dockhead]

Originally Posted by conachair

Spent many a moment on marine traffic , nowhere near so many channel crossings as you but a good handful usually single handed, yes, very busy but not too big a deal to get across. Anyway, by the by, point was that Rods mythical 50 ships all in line mile or so apart all doing 20Kts was made up probably to somehow prove CPA of a Nm isn't possible. Across the bows it is.

Sure, and that's the point. The "killing zone" is ahead, and that's where you want as much space as possible, so you pass close behind one to give yourself as much space as possible to the one behind.

[David M]

Originally Posted by Dockhead

The MCA is the Maritime and Coastguard Agency of the United Kingdom, the world's preeminent maritime authority, whose guidance notes are studied all over the world. One of the vestiges of Britannia's ruling the waves. We Yanks have nothing to compare with it.

The worlds maritime authority is the IMO, International Maritime Organization. The Brit's opinions do not determine the laws of the seas. Within territorial waters each country sets its own laws relating to maritime safety etc.


https://en.wikipedia.org/wiki/Intern...e_Organization

International Maritime Organization

[conachair]

Originally Posted by Dockhead

Sure, and that's the point. The "killing zone" is ahead, and that's where you want as much space as possible, so you pass close behind one to give yourself as much space as possible to the one behind.

And not *that* difficult now we have AIS

Zooming out a bit, these "what would you do" threads are near impossible to answer with exact actions, there are so many factors for a skipper to take into consideration on passage - weather, seasick crew?, solo? , unreliable engine?, tired? , masses of factors which will only be relevant there and then plus even if you knew them all your reaction would be different sitting in a nice warm house with a laptop on the forums. All we can do is generalise, which is most definitely very helpful, just not definitive.

Keep up with the generalisations!! Always things to learn

Forum rule 2D Anyone asking for *exact* answers to colreg questions should be ignored

[David M]

Originally Posted by Lodesman

You have to follow the rules until it is impossible to do so, while doing what you need to do, to avoid collision.

I hear ship pilots agreeing frequently to pass starboard to starboard in narrow channels...by VHF.

They are not breaking the COLREG"s.

[Dockhead]

Originally Posted by David M

The worlds maritime authority is the IMO, International Maritime Organization. The Brit's opinions do not determine the laws of the seas. Within territorial waters each country sets its own laws relating to maritime safety etc.


https://en.wikipedia.org/wiki/Intern...e_Organization

International Maritime Organization

Of course.

But the IMO doesn't work out the kind of detailed guidance that the MCA does. So even where there is no legal authority, what the MCA publishes is very influential.

[ramblinrod]

Originally Posted by Dockhead

I never promised to be concise

This phrase: ""Colregs do not specify what a "Safe Distance" is. What one considers a 'Safe Distance', depends on the circumstances, and the individuals." is somewhat misleading, if someone thinks it means that "safe distance" is subjective. It is absolutely not subjective. The safe distance is different in different crossings, but it is determined by physics and geometry in every case, not sphincters, not anyone's feelings, not anyone's courage (which is often just lack of knowledge).

As to the scenario you put -- that is an absolutely typical Channel crossing, so something I've done hundreds of time. So I can answer from experience and practice. I would hate to do it at 5 knots; normally I'm on beam reach making 8 to 9 knots, which is significantly easier.

You've designed the scenario in a clever way -- respect! -- because it is impossible to stay a mile clear of everyone. So is it safe? Can it be safe?

The answer, obviously, is that you have to pass as close as possible behind one ship, in order to give as much room as possible to the ship which you are inevitably passing ahead of. Because when you are much slower than the ship, passing behind is far safer than passing ahead. Passing a cable behind might be safer than passing a mile ahead, with this kind of speed difference.

It is actually not that easy, to do this safely, especially without AIS. With AIS, you set up from 2 or 3 miles off, to cross maybe 2 or 3 cables behind one ship, leaving you about a mile to the next one you are passing ahead of.

While you are setting this up, you will have to make maneuvers, probably during the time you should be standing on. Here the VHF call is absolutely invaluable -- you call and tell both ships what you are doing and ask them to hold their course and speed. If you set up a close pass like that from 2 or 3 miles away (and note well that we are talking about 2 or 3 cables, not 180 feet), you will not cross exactly as you expected due to variations in course and speed and the various inaccuracies, so you have to monitor and correct (while avoiding frequent small corrections as per the Rules). Due to the geometry of a crossing behind, it is much harder to end up in the "killing zone" of a ship you are set up to pass behind, so this is reasonably safe. But you can't really eyeball it with that difference in speed -- you will never see his transom until the last seconds.

Meanwhile you also carefully monitor the ship you are passing ahead of. A mile is a decent margin, IF you have AIS and so relatively precise information about how the crossing is developing. And IF you are maintaining your speed -- if there is a big lull, you can get into trouble pretty fast, and so you need to be ready to get the motor on, or just have it running anyway and put up the motoring cone.

Another important factor is the relative speeds of the SHIPS. If the one you are passing ahead of, is moving faster than the other one, then the gap you are trying to get through is closing, and what may look like a decent plan from 3 or 4 miles out may evaporate. And the problem is that this is absolutely impossible to see without AIS or radar until very late. It would take a long time of tracking with a HBC to discern the speed difference, more time than you have at those ranges.

If you see the gap closing, and you see you can't pass a safe distance ahead, then you have to abort, also at a safe distance. Without AIS or radar, you just may not be able to see it until very late. "Safe distance" is not your subjective feelings -- it is far enough away that you can still execute your maneuver and still keep well enough clear that you don't end up in an in extremis situation.

And one really serious danger when passing ahead is what if you find yourself not with a zero CPA to his bow, but you are actually passing somewhat ahead of him, but too close for safety. And this may be happening although the AIS is saying you are passing WELL ahead -- because the AIS is communicating his position as a mathematical point which might be a cable or more behind his bow, so you may not see the problem until you are very close. So what do you do? If you can't increase your speed, then you have to get all the way back in the other direction. At a certain point, this becomes impossible -- you are a sitting duck. I will have some drawings to show the geometry of this over the weekend.

So this is my point.

First, I believe that if the sailboat has the engine on, it is more difficult. Without engine on the sailboat is stand-on, and under Colregs the ships must change course or speed to allow that. If they do, the sailboat need due nothing other than maintain course and speed.

Alternatively, when the sailboat starts the engine, because the ships are approaching from startboard, the ships are stand on, under Colregs they are obligated to stand on and not change course or speed, so if they don't volunteer to, you can't get a bigger gap.

And yes, I set the scenario so that a 1 nm CPA from all is not possible.

And yes, you can call the ships to make room, but they are not obligated to oblige.

And finally, see, "safe distance" is subjective. You elected to cross paths with less than 1 nm CPA from all. You consider this safe (under certain circumstances) based on your Pucker Factor.

Another skipper, may not consider 2.5 cables behind the transom of the ship "safe distance" and may heave to for several hours waiting for the final ship to pass, or change the destination to somewhere else, so that they simply don't have to deal with it.

I may choose to keep more distance than 1 nm crossing the bows of the ship, perhaps even within 180 ft off the transom of the ship ahead of the one I'm crossing.

You may not consider that "safe distance" where I may.

"Safe Distance" is absolutely subjective. If a collision occurs or someone complains and there is a hearing, a ruling will be made after evaluation of the circumstances, and perhaps there will not even be a unanimous decision between "experts".

So there! ;-)

[Exile]

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.

[Exile]

Originally Posted by Dockhead

Look at MarineTraffic sometime.

There are lulls in the traffic, for sure. I've gotten across from time to time without any maneuvering required.

But it's very common indeed to encounter long lines of ships only a mile or two apart, and scenarios similar to Rod's hypothetical are quite common. Frequently discussed among Channel sailors -- it's a typical challenge.

Now not indeed all at 20 knots. Since the big fuel economy slowdown, more typical speeds are 16 to 18. Passenger liners, ferries, and military vessels are often making making MORE than 20 knots, though.

Situation RIGHT NOW:

Attachment 157998


400 ships a day transit the Channel (that does not include local traffic). That's 17 per hour, or one every 3 or 4 minutes. That is bloody intense traffic, the most intense of any seaway in the world, and large lines or clusters of ships a mile or two apart is quite typical.

Channels sailors liken crossing to being a squirrel trying to get across a busy motorway. It's not always like that, but often enough.

Looks like what Nelson was facing at Trafalgar!

One of those blue symbols trying to sneak through the armada must be Dockhead, trying to multi-task his way through, all the while furiously doing his onboard calculations while posting to CF at the same time.

[Dockhead]

Originally Posted by Exile

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.

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.