Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

[Dockhead]

Originally Posted by ramblinrod

My point exactly. There is no such thing as a cone of uncertainty with 100% certainty. The only thing 100% certain is that no matter what you do, you coculd be ivnvopved in a collision. Any commercial seaman who believes they can or should be able to plot a course from 5 miles away that will be correct should ha e their ticket revoked.

One can only plot a course based on a set of assumptions and the parAmeters could change any which way from Sunday.

They are taught to plot every crossing from 10 miles out and to maneuver BEFORE 5 miles out, in open water, in normal circumstances.

So you're arguing that because you can't be 100% sure -- which is true, I agree with you there -- that you should just sail on towards a collision? That 5 miles is too far to try to maneuver?

This is just silly. Of course you "plot a course based on a set of assumptions" -- that's the way it works. By the time two vessels are 4 or 5 miles apart, the give-way vessel should have already maneuvered so that you can see a one mile CPA. Often this maneuver takes places further out --
even 8 to 10 miles. That's how you do collision avoidance. When the vessels actually cross 15 minutes (or whatever) later, they probably won't be exactly one mile apart -- because of all these uncertainties. But if both vessels hold their course and speed reasonably well, they are almost guaranteed to cross safely because one mile is an adequate margin of error in most cases.

If on the other hand they would just throw up their hands and steam on, and wait until they are one mile apart, and then find themselves with a 0 CPA, then they are in real trouble, with a very significant risk of not being able to unwind the situation. I explained the geometry of this above.

It is done at 4 or 5 miles out, not at 1 mile out, for these reasons.

[ramblinrod]

Originally Posted by Dockhead

You did not understand at all what I was saying.

Obviously, if a much faster vessel decides to turn towards you and run you down, there is little you can do to avoid it. Obviously a significant course or speed change somewhere in the middle of a crossing, will invalidate a collision avoidance solution (and that is exactly why you are obligated to stand on, when you are the stand-on vessel).

But here's the point: even when vessels are holding their course and speed as well as they can, there are variations. These variations do NOT always and automatically average out -- the average will be at the very best, in the case of a ship, plus or minus a couple of degrees, and in the case of a yacht under sail, more. A vessel under sail is, moreover, subject to sometimes very big variations of speed, as the wind varies.

So when you do collision avoidance in open water, and you are trying to maneuver to make a safe pass with another vessel, you must take account of the uncertainties of his and your own position which come from these errors. In order to be safe, you have to maneuver so that the zone of your PROBABLE position at CPA does not intersect with the zone of his PROBABLE position at CPA. These zones are much bigger than the dimensions of the vessels; their size is determined by the cone of uncertainty discussed above. If you do all the geometry based on the dimensions of the vessels ("180 feet is plenty of a safe margin"), without considering these uncertainties, then you fail.

And when we say PROBABLE positions, we mean where we will PROBABLY be IF we are all holding course and speed. This does not take account of what happens, if someone maneuvers. If someone turns or stops in the middle of the crossing, all bets are off, and once again, this is why you are obligated to hold course and speed during certain times, when you are the stand-on vessel.

As a handy rule of thumb, professional sailors are taught to set up crossings with at least one mile CPA, in order to be sure of a safe pass. One mile deals with these zones of uncertainty in almost all cases, provided everyone holds course and speed. On most ships, the bridge standing orders require maintaining minimum 1 mile CPA in all crossings, and require calling the captain in case of a closer encounter. In real life, something less than a mile might be safe enough in some circumstances, but under no circumstances, in open water, should you ever only START maneuvering at one mile -- that's already in extremis -- an imminent collision -- where already no one is standing on any more and everyone is doing whatever desperate thing he can, so that you are taking the risk of simultaneous maneuvering into each other.

Again, you can't have it both ways. If we accept matter what degree of certainty we use for our cone of uncertainty may necessitate a course correction, then we must accept that plotting a course for a 180 foot
miss is acceptable, because no matter what degree of miss we initially solve for, it may be wrong.

This is why I initially mentioned that a 1 degree course change may be an adequate solution. Who knows what is going to happen between now and then; that one degree may have been to little OR TOO MUCH!

[conachair]

Originally Posted by ramblinrod

I see ships 20 miles away all the time, visually by day, by night, and on AIS.
.

Can you really see ships 20nm away?

Must be bigguns......

[ramblinrod]

Originally Posted by Dockhead

They are taught to plot every crossing from 10 miles out and to maneuver BEFORE 5 miles out, in open water, in normal circumstances.

So you're arguing that because you can't be 100% sure -- which is true, I agree with you there -- that you should just sail on towards a collision? That 5 miles is too far to try to maneuver?

This is just silly. Of course you "plot a course based on a set of assumptions" -- that's the way it works. By the time two vessels are 4 or 5 miles apart, the give-way vessel should have already maneuvered so that you can see a one mile CPA. Often this maneuver takes places further out --
even 8 to 10 miles. That's how you do collision avoidance. When the vessels actually cross 15 minutes (or whatever) later, they probably won't be exactly one mile apart -- because of all these uncertainties. But if both vessels hold their course and speed reasonably well, they are almost guaranteed to cross safely because one mile is an adequate margin of error in most cases.

If on the other hand they would just throw up their hands and steam on, and wait until they are one mile apart, and then find themselves with a 0 CPA, then they are in real trouble, with a very significant risk of not being able to unwind the situation. I explained the geometry of this above.

It is done at 4 or 5 miles out, not at 1 mile out, for these reasons.

What I am saying is that one can plot whatever solution they wish from a distance off, monitor the situation, and adjust as conditions dictate. If I have no idea what my course and speed will be, due to wind conditions, then anything I do for distance off, will mostly likely be wrong, with respect to avoiding collision and not devising from original course more than necesssary. As far as Colregs are concerned, if I follow every provision to the letter, and end up avoiding a collision by one hair width, I have complied 100%.

Any amount beyond a hair width is gravy.

[ramblinrod]

Originally Posted by conachair

Can you really see ships 20nm away?

Must be bigguns......

Yes, visually by day, by night, and by AIS. If visibility is clear, it is easy to see a 40 ft motor vessel 5 miles off. A 400 ft ship at 20, no problem.

From our marina we can see the smokestack at the Olcott power plant, which I believe is 45 miles across Lake Ontario.

[transmitterdan]

Assuming there is a risk of collision then the biggest problem with a 1 degree course change by a give way vessel is that it isn't substantial as required by rule 16. This has to be one of the silliest and obviously wrong recommendations ever on CF. like DH this thread is on my (very short) ignore list.

[skipmac]

Originally Posted by ramblinrod

Yes, visually by day, by night, and by AIS. If visibility is clear, it is easy to see a 40 ft motor vessel 5 miles off. A 400 ft ship at 20, no problem.

A 400' ship at 20 miles. I don't think so. The ship would have to be 300' high to see it from sea level at 20 miles. Do the math. Horizon is 1.17 X sq rt of your eye height (or the height of the object you're looking at).

12-15 miles max is the best I ever did at sea.

[Dockhead]

Originally Posted by ramblinrod

What I am saying is that one can plot whatever solution they wish from a distance off, monitor the situation, and adjust as conditions dictate. If I have no idea what my course and speed will be, due to wind conditions, then anything I do for distance off, will mostly likely be wrong, with respect to avoiding collision and not devising from original course more than necesssary. As far as Colregs are concerned, if I follow every provision to the letter, and end up avoiding a collision by one hair width, I have complied 100%.

Any amount beyond a hair width is gravy.

If you go to class to become a professional mariner, and give this answer on a test, you will be failed.

Correct procedure is to plot a solution from far enough away (normally 10 miles, but certainly by 5 miles), which takes account of probable position of both vessels at CPA, then make ONE significant course correction which takes your probable position at CPA far enough away from his probable position at CPA that you pass far enough from each other to deal with expected minor variations in course and speed. And to hold that new course unless you clearly see that something changed which invalidated your solution.

The absolutely wrong way to do it, is to make no course correction at all, because you think all speculation at 5 miles is useless, then "monitor the situation", and make a series of small corrections. As you get closer and closer, your ability to disengage from the other vessel disappears, and if you are the slower vessel, it disappears really fast. At one mile out -- as you have been advocating -- you are pretty much a sitting duck in a small boat making 5 knots, in an encounter with a fast ship. That is far too late to start maneuvering.

[Dockhead]

Originally Posted by ramblinrod

Yes, visually by day, by night, and by AIS. If visibility is clear, it is easy to see a 40 ft motor vessel 5 miles off. A 400 ft ship at 20, no problem.

From our marina we can see the smokestack at the Olcott power plant, which I believe is 45 miles across Lake Ontario.

Rod, have you ever heard of the curvature of the earth?

[Dockhead]

Originally Posted by ramblinrod

Again, you can't have it both ways. If we accept matter what degree of certainty we use for our cone of uncertainty may necessitate a course correction, then we must accept that plotting a course for a 180 foot
miss is acceptable, because no matter what degree of miss we initially solve for, it may be wrong.

This is why I initially mentioned that a 1 degree course change may be an adequate solution. Who knows what is going to happen between now and then; that one degree may have been to little OR TOO MUCH!

Logical fallacy. Does not mean that at all!

For the course correction to be meaningful, it must be big enough to disengage your cone of probable position from his cone of probable position.

The physical dimensions of the vessels are meaningless, because you don't know the probable position of the vessels to that degree of accuracy. But that does NOT mean you know nothing! You DO know within a circle of about half a mile, if you are a competent navigator! You need to draw a circle around both vessels with diameter equal to the degree of uncertainty at CPA, and plot a course which makes these circles NOT intersect.

That is an effective maneuver.

A change of course which might give you 180 feet of separation in case you had perfect knowledge of both vessels position is meaningless. All one degree course changes are meaningless from the very beginning because you can't even hold a course that closely. Forget the 400 foot length of the ship. You can't place that 400 foot length, in order to avoid it. You are rather trying to avoid a zone which is maybe half a mile across, inside which he could be if he holds his course and speed. And forget your 23 foot length (or whatever) of your boat -- you may be anywhere in your own half mile circle, at the moment the crossing occurs.

You must get these circles apart, and you must do it early enough when you still have the power to change the geometry of the crossing.

The distance at which you simply cannot get out of his way anymore, no matter what you do, depends on a few things, but the biggest factor is relative speed. If its 5 knots vs. 20 knots, then it's approximately one mile. It doesn't mean you will definitely collide, it means that after this point, you can no longer meaningfully influence what happens.

[conachair]

Originally Posted by ramblinrod

Yes, visually by day, by night, and by AIS. If visibility is clear, it is easy to see a 40 ft motor vessel 5 miles off. A 400 ft ship at 20, no problem.

From our marina we can see the smokestack at the Olcott power plant, which I believe is 45 miles across Lake Ontario.

From the cockpit of a sailing boat? That's impressive!

[Dockhead]

Originally Posted by skipmac

A 400' ship at 20 miles. I don't think so. The ship would have to be 300' high to see it from sea level at 20 miles. Do the math. Horizon is 1.17 X sq rt of your eye height (or the height of the object you're looking at).

12-15 miles max is the best I ever did at sea.

Of course. This is basic stuff.

If you've spent any amount of time at sea, you know that the horizon is less than 3 miles away from man-height above sea level (sitting in the cockpit will be close to that on small boats). A ship 10 miles off is hull-down on the horizon. Depending on how big the ship is, you might see only part of the superstructure. After about 15 miles off, any ship from a yacht's cockpit, is invisible below the horizon. You have to have combined height of about 75 meters, to see something over the horizon at 15 miles. Few ships have air draft more than 60 meters, so the very tip of the antennas of the very biggest ships will be sinking below the horizon at 14 miles.

Because VHF is mostly line of sight, we will not usually receive AIS from ships 20 miles off, either. So we will usually not even be aware of a ship, that far away.

"It's easy to see a ship 20 miles off -- no problem"

belongs in the same category as

"Just make a 180, one mile off -- no problem"

Both serious miscalculations, based on stubbornly held misconceptions.

I guess we can add also add to this:

"It is easy to see a 40 ft motor vessel 5 miles off."

To see something over the horizon 5 miles off, you need combined height of 7 meters. For a guy sitting in the cockpit of a sailboat, a 40 foot motor cruiser or trawler is going to be below the horizon at 5 miles.

[El Pinguino]

Originally Posted by skipmac

A 400' ship at 20 miles. I don't think so. The ship would have to be 300' high to see it from sea level at 20 miles. Do the math. Horizon is 1.17 X sq rt of your eye height (or plus 1.17 X sq rt the height of the object you're looking at).

12-15 miles max is the best I ever did at sea.

Fixed that for you...

3 metre height of eye and 30 metre high accomodation block = about 15 miles plus any refraction that may be involved.

Masts and funnels would increase that distance..

And also 400 foot isn't very big..... 600 foot and up is the norm for most these days....

[Kenomac]

Originally Posted by ramblinrod

Yes, visually by day, by night, and by AIS. If visibility is clear, it is easy to see a 40 ft motor vessel 5 miles off. A 400 ft ship at 20, no problem.

From our marina we can see the smokestack at the Olcott power plant, which I believe is 45 miles across Lake Ontario.

Please splain how you see around the earth. Climb a tree?

How tall are you?

[StuM]

Originally Posted by barnakiel

My takeaway is that some leisure craft sailors should start using their sails.

A sailing boat is supposedly the one with the right of way (over a steamer). Most of the time.

AAAAhhhhhhhhhh!

Even after all the discussions over the last few months, some still don't grasp the fundamental concept of COLREGs