Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

[Exile]

Originally Posted by Lodesman

Yes, but during stage 1 you would be free to manoeuvre, which given slow speeds for both vessels could be assumed to be about >5 miles (IMO). Inside that, the rules require you to stand on. That said, you can make an arrangement over VHF, to act contrary to the rules - it happens all the time, with "head-ons" agreeing to pass green to green, starboard offering a port turn and other such actions. If you call the tug on 13 and offer to stay out of the way, it would do a lot to alleviate the master's stress level. I assume these guys so frequently see recreational sailors fail to stand on, that they wait until later than normal to take action.

OK got it. Thank you.

[conachair]

Originally Posted by Exile

But can only be a RAM if showing proper symbols (and/or transmitting such on AIS).

If over 12m. 27 g.

[daletournier]

Originally Posted by Dockhead

Yes, the MCA note certainly has the danger of communicating with the wrong vessel, front and center. That danger is much less now.

But still -- you have to agree with the basic point -- why call and make up an ad hoc crossing arrangement, if everyone could just follow the rules?

The point is that yakking on the radio shouldn't be a SUBSTITUTE for knowing the rules.

But in case of doubt or something not clear or typical -- I'm all in favor of it.

Making the assumption that everyone knows the rules could get you killed, the vhf takes assumption out if the equation. A vhf at the helm is a great safety device.

[conachair]

Originally Posted by daletournier

Making the assumption that everyone knows the rules could get you killed, the vhf takes assumption out if the equation. A vhf at the helm is a great safety device.

The rules don't assume that everyone knows the rules

[Dockhead]

Originally Posted by conachair

The rules don't assume that everyone knows the rules

Indeed, but I don't really disagree with him. In case of doubt, do call. Unnecessary VHF calls are distracting and not really good process, but if you do have a doubt after actually analyzing and thinking about what the other vessel is doing -- then I don't think the call is unnecessary.

And what you're saying is also true and very important. If for example you are stand on, and the give-way vessel is clearly not going to make a move, then you do it yourself, with a large obvious maneuver in good time. Normally that shouldn't require a call. But if you have any doubt about whether he might still maneuver -- then the call could be a really good idea.

[Exile]

Originally Posted by conachair

The rules don't assume that everyone knows the rules

Important point! And one which trips many up I suspect. The Rules are rather brilliantly designed in the respect that their deliberate lack of precision reflects the myriad amount of variables unique to navigating on the water. Couldn't imagine how it could work in any other type of medium.

The relatively few times I've felt the need to get on the radio, and the even fewer times I've been hailed by big ships, have all been positive, friendly, and resolved any uncertainty.

[conachair]

Originally Posted by Dockhead

Indeed, but I don't really disagree with him. In case of doubt, do call. Unnecessary VHF calls are distracting and not really good process, but if you do have a doubt after actually analyzing and thinking about what the other vessel is doing -- then I don't think the call is unnecessary.

Likewise don't disagree with that, but I suspect quite a lot of VHF calls from recreational sailing boats to ships are in fact unnecessary.

[hasse_A]

A very informative thread indeed.

I want to describe a close encounter east of the Brazilian coast we had a half year ago.

We were under sail from the Cape Verde Island to Recife, Brazil heading WSW in daylight.
This was 20-30 Nm of the coast so definitely in open water.
From the south comes 2 merchant vessel appearing on our port bow.
We use OpenCPN with it's magnificent way of displaying AIS targets graphically and showing CPA. At about 10 Nm distance I set "show target CPA" for the two targets. The CPA for both of them are well below 1 Nm.
The 2 ships are running on parallel course with one closer to me and ahead of the other one. Both are doing 12-15 knots. We are doing 6 knots.

When the distance to the first one is about 5 Nm I see that he changes course slightly to port so that the CPA opens up well above 1 Nm. I can't remember what it turned out to be but I was comfortable with it.

The second ship does not seem to make any attempt to change course and when we are 2 Nm apart I call him on the VHF. He claims that he is not able to give way because he can't see what my destination port is.
I then explain to him that we have a B transponder and it does not give that info. He then says he cant see me on the AIS at all.
I then tell him that obviously he is not going to oblige the demand on him to give way and I tell him to keep his course and speed and that I will go astern of him. We pass astern of him at a distance of about 0.5 Nm.
After we passed him I call him again and ask if he had not seen us visually. I only got some mumbo jumbo about him not seeing us.
I then gave up trying to figure out why he behaved the way he did.
I realize that if I had not taken action we would have been run over and the captain did not seem to care.

To confirm that there was nothing wrong with our AIS-transponder I called the first ship and asked if he saw me fine and got a confirmative answer.
We where then 8-10 Nm apart. I am sure he had listen to my conversation with the second ship as he laughed when he told me I was seen very well.

The second ship was a Brazilian registered one. The first one, the one I asked if our AIS was ok, I can't remember the nationality of.

[ramblinrod]

Originally Posted by Dockhead

There is no magic circle one mile around a ship, outside of which you are in perfect safety, and inside of which you are in mortal danger. Every crossing is different -- it depends on the geometry, relative speeds, and size of the vessels concerned.

But one mile is a widely accepted rule of thumb for a minimum safe CPA -- outside of that, you are still generally pretty safe in almost every situation, but inside of that the risks start to pile up rapidly. We use rules of thumb here like in many cases in order to avoid having to do a deep analysis of every case -- where there is likely not to even be time to analyze carefully.

In many cases, inside one mile may be so dangerous as to be an "in extremis" situation where both vessels are obligated to maneuver. If you get into an "in extremis" situation which could have been avoided, that is a violation of the COLREGS and a violation of good seamanship, because it's concretely dangerous, it's dangerous by definition -- because at the "in extremis" stage, vessels are now maneuvering uncoordinated, and there is no longer any time to recover from a mistake or miscalculation. One of the main goals of the COLREGS is to prevent uncoordinated maneuvering, to reduce the risk of vessels turning into each other.

I'm not quite sure from your question whether you doubt that uncoordinated collision avoidance maneuvers are dangerous or not altogether, or only within one mile. If you do doubt this, then I suggest you read some of the basic literature. It's a key cause of collision -- vessels turning into each other.

Either maneuver by itself might have been safe, but the two maneuvers turn out to be in conflict with each other, and a collision results. At a certain distance, and in many cases that's already one mile -- maybe just a few minutes TCPA -- both vessels are obligated to maneuver, and if the maneuvers turn out to be in conflict with each other, then there is no time for correction and the collision becomes unavoidable.

Remember in all of this also that a large ship turning doesn't turn like a car. Large ships can be surprisingly maneuverable -- able to achieve very high rates of turn in short period of time -- but they don't turn like on rails -- the stern swings out, and there is displacement -- that is, the ship moves with sideways component.So if you are approaching a large ship traveling at sea speed, and you get within a mile and it puts the rudder over to avoid you, you may be facing the whole side of the ship coming at you at 20 knots. Some ships are two cables (!) long. This is why talking about 180 foot CPAs in relation to a fast moving, large ship, is -- well, hard to think of another term for it, but -- pure madness. You do not want to be there, not anywhere near that close, to a large ship, which is moving fast. There is a large piece of ocean where it might be a few seconds from now, much bigger than even the very large size of some of these ships, and to be safe you must be outside that whole piece of ocean.

OK, so that was not concise (20 words or less, at all).

I think I can summarize with, "Colregs do not specify what a "Safe Distance" is. What one considers a "Safe Distance", depends on the circumstances, and the individuals.

So I ask you this,

There is a convoy of 50 commercial ships in open sea heading 0 degrees north in single file. They are all travelling at 20 knots and are 1-1/4 miles apart, bow to stern.

You are travelling in a sailboat 90 degrees east at 5 knots and need to cross the course of the convoy. Winds are steady, seas are calm, visibility is 20 miles. How would you do it?

[ramblinrod]

Originally Posted by ramblinrod

OK, so that was not concise (20 words or less, at all).

I think I can summarize with, "Colregs do not specify what a "Safe Distance" is. What one considers a "Safe Distance", depends on the circumstances, and the individuals.

So I ask you this,

There is a convoy of 50 commercial ships in open sea heading 0 degrees north in single file. They are all travelling at 20 knots and are 1-1/4 miles apart, bow to stern.

You are travelling in a sailboat 90 degrees east at 5 knots and need to cross the course of the convoy. Winds are steady, seas are calm, visibility is 20 miles. How would you do it?

Whoops! Your iron genny is on. ;-) (The ships are stand on, and you are give way.)

[conachair]

Originally Posted by ramblinrod

OK, so that was not concise (20 words or less, at all).

I think I can summarize with, "Colregs do not specify what a "Safe Distance" is. What one considers a "Safe Distance", depends on the circumstances, and the individuals.

So I ask you this,

There is a convoy of 50 commercial ships in open sea heading 0 degrees north in single file. They are all travelling at 20 knots and are 1-1/4 miles apart, bow to stern.

You are travelling in a sailboat 90 degrees east at 5 knots and need to cross the course of the convoy. Winds are steady, seas are calm, visibility is 20 miles. How would you do it?

Where in the world is this?

[Dockhead]

Originally Posted by ramblinrod

OK, so that was not concise (20 words or less, at all).

I think I can summarize with, "Colregs do not specify what a "Safe Distance" is. What one considers a "Safe Distance", depends on the circumstances, and the individuals.

So I ask you this,

There is a convoy of 50 commercial ships in open sea heading 0 degrees north in single file. They are all travelling at 20 knots and are 1-1/4 miles apart, bow to stern.

You are travelling in a sailboat 90 degrees east at 5 knots and need to cross the course of the convoy. Winds are steady, seas are calm, visibility is 20 miles. How would you do it?

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.

[Dockhead]

Originally Posted by ramblinrod

Whoops! Your iron genny is on. ;-) (The ships are stand on, and you are give way.)

It's much, much easier that way. Less risk of a lull killing your speed, and less risk that a ship will maneuver to give way and mess up your solution. You can set up your pass without violating the obligation to stand on. You'll want your motoring cone, if you have any sails up.

It's much harder if you are stand on. In that case you really just about have to use the VHF. Before AIS you couldn't call if you couldn't make out the name somehow. Before AIS, I would sometimes heave to and wait for a bigger gap to appear.

[Pete7]

Originally Posted by ramblinrod

You are travelling in a sailboat 90 degrees east at 5 knots and need to cross the course of the convoy. Winds are steady, seas are calm, visibility is 20 miles. How would you do it?

Well certainly not the French way, but then sailing to the French isn't a past time or hobby it's a religion and they are fanatical about it.

Perhaps the rule of tonnage doesn't apply to French yachts:

Solo skipper in collision with cargo ship

[Dockhead]

Originally Posted by conachair

Where in the world is this?

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!