Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

[TJ D]

Originally Posted by Jim Cate

You continually ignore the fact that the process you describe is not feasible to carry out, and then arrogantly state that the case is closed. The only thing closed around this thread is your mind.

Jim

This is also my view. The course of action that's being advocated is reckless at best. Think about it, 180' from a 30,000+ ton ship traveling at speed... It's just nuts. Especially when you consider the geometry of the crossing. Even more nuts.

Just look at the maneuvering board solution!

It's a hard thing to admit you've got something wrong. I understand and sympathise, even- I can be stubborn as hell too. But, for anyone to really think that this kind of close maneuvering with big ships is ok, I don't get it.

I don't think the case is closed at all, frankly. Certainly not in favor of the super-close crossing, it's not.

The discussion sort of swerved into this nonsensical scenario with 50 ships all traveling at the same speed, 1 mile apart. That crossing is basically impossible without coordination with the ships, but remember that this close crossing was advocated for as safe and appropriate LONG before this scenario was introduced. It was being advocated in a crossing with a single ship, too.

[ramblinrod]

Originally Posted by TJ D

This is also my view. The course of action that's being advocated is reckless at best. Think about it, 180' from a 30,000+ ton ship traveling at speed... It's just nuts. Especially when you consider the geometry of the crossing. Even more nuts.

Just look at the maneuvering board solution!

It's a hard thing to admit you've got something wrong. I understand and sympathise, even- I can be stubborn as hell too. But, for anyone to really think that this kind of close maneuvering with big ships is ok, I don't get it.

I don't think the case is closed at all, frankly. Certainly not in favor of the super-close crossing, it's not.

The discussion sort of swerved into this nonsensical scenario with 50 ships all traveling at the same speed, 1 mile apart. That crossing is basically impossible without coordination with the ships, but remember that this close crossing was advocated for as safe and appropriate LONG before this scenario was introduced. It was being advocated in a crossing with a single ship, too.

Gentlemen,

Nobody suggested this crossing would be a walk in the park.

The scenario parameters were purposefully set to be "difficult" requiring one to weigh the circumstances very carefully.

I say again, I would not attempt this crossing unless there was a dire emergency mandating it be attempted, and I would never suggest anyone do this as SOP.

In all probability, the ships could be contacted, and may agree to prepare an opening for the sailboat (as many have correctly suggested).

If that couldn't be arranged, it would be best to wait it out or simply go somewhere else (as I have already posted at least twice).

But if one decided the circumstances (dire emergency for example) mandated they cross the convoy, and communication with the ships could not be achieved (broken sailboat radio equipment for example), the proposed solution of crossing near the transom of the lead ship is the safest, of all the unsafe solutions suggested.

The proposal to cross the stern of the lead ship transom by 2-3 cables was severely flawed and doomed for even greater likelihood of failure.

And yes, one would hope that without any coordinating communication, the ships would initiate maneuvers to prepare a larger opening to make the crossing more safe, while having choice words for the skipper of the sailboat.

I set the scenario as I did, because of the "1 nm minimum CPA under all circumstances" mindset I was getting, and to demonstrate my position that the minimum CPA from the bow of a ship, should be much greater than that from the stern.

Based on the very title of the thread, it should have been clear that the sailboat had to cross very close to the transom of the lead boat.

I hope this exercise helped to elucidate, that one can't apply "rules of thumb" without exercising proper thought, based on the circumstances. This even caught a very experienced navigator, actively participating in this thread.

One thing to consider, is all of this was evaluated and deliberated over several days with ample time to consider all of the circumstances, not in the heat of the moment, when one has only minutes to decide what to do.

Thank you for the debate, everyone.

It was spirited, educational, useful, and hopefully entertaining.

[evm1024]

Wow, I guess that is as close of a consession as we are going to get. I'll take it.

It is a long road from my pucker factor is bigger than yours to I would not attempt this crossing....

In all cases we should use our best judgement and all available information. In the case of the proposed crossing (50 ships, 1.25 nm between them etc) I for the most part only looked at the reality of attempting to cross 180 CPA behind one (the 5th). And how Ill advised it is to follow such a plan.

I have done a few calculations (as always you should check my math) of the total crossing and based the crossing on a desired CPA of 2 cables (1200') to the ship you pass ahead of.

The speed difference gives us a CPA angle of 14 degrees where as in the last calculations I used 45 degrees for ease.

Anyway, using that 2 cables CPA and calculating back I see that the CPA to the ship that we pass astern of is just under 1 cable (600).

This presents a less nerve wracking solution to both the ships. None the less both ships will be sounding 5 blasts if you are not on the radio to them and may take collision avoidance actions that throw the calculations out the window.

Of course decreasing your 600' CPA to the pass astern ship increases your CPA to the pass ahead ship. But with increasing likelyhood that the ship you pass astern will act dramatically thus throwing the entire crossing into chaos.

Also, a 600' CPA gives a much greater allowance for the uncertainties of measurments with your instruments and your eye balls....

[ramblinrod]

Originally Posted by evm1024

Wow, I guess that is as close of a consession as we are going to get. I'll take it.

It is a long road from my pucker factor is bigger than yours to I would not attempt this crossing....

In all cases we should use our best judgement and all available information. In the case of the proposed crossing (50 ships, 1.25 nm between them etc) I for the most part only looked at the reality of attempting to cross 180 CPA behind one (the 5th). And how Ill advised it is to follow such a plan.

I have done a few calculations (as always you should check my math) of the total crossing and based the crossing on a desired CPA of 2 cables (1200') to the ship you pass ahead of.

The speed difference gives us a CPA angle of 14 degrees where as in the last calculations I used 45 degrees for ease.

Anyway, using that 2 cables CPA and calculating back I see that the CPA to the ship that we pass astern of is just under 1 cable (600).

This presents a less nerve wracking solution to both the ships. None the less both ships will be sounding 5 blasts if you are not on the radio to them and may take collision avoidance actions that throw the calculations out the window.

Of course decreasing your 600' CPA to the pass astern ship increases your CPA to the pass ahead ship. But with increasing likelyhood that the ship you pass astern will act dramatically thus throwing the entire crossing into chaos.

Also, a 600' CPA gives a much greater allowance for the uncertainties of measurments with your instruments and your eye balls....

This clearly illustrates how these decisions most definitely are "subjective" at the time they are made.

Remembering that dx is directly proportional to safety:

Me: 180 ft aft (dx increasing rapidly) / 1500 ft ahead (dx decreasing rapidly)

You: 600 ft aft of lead / 1200 ft ahead of following

Dockhead: 1800 ft aft / guaranteed contact with following vessel

As we can see, depending on the skipper, the recommended distances are pretty much anything between the absolute min and max possible for this scenario.

We should all be very thankful that these decisions ARE "subjective", and we are not obliged to follow the "objective" recommendations of other CF posters. ;-)

[Exile]

Originally Posted by ramblinrod

This clearly illustrates how these decisions most definitely are "subjective" at the time they are made.

Remembering that dx is directly proportional to safety:

Me: 180 ft aft (dx increasing rapidly) / 1500 ft ahead (dx decreasing rapidly)

You: 600 ft aft of lead / 1200 ft ahead of following

Dockhead: 1800 ft aft / guaranteed contact with following vessel

As we can see, depending on the skipper, the recommended distances are pretty much anything between the absolute min and max possible for this scenario.

We should all be very thankful that these decisions ARE "subjective", and we are not obliged to follow the "objective" recommendations of other CF posters. ;-)

There are no "objective" recommendations from other posters for the scenario you posed. It was a hypothetical to demonstrate why it could not be Colregs compliant absent arrangements made by radio with the ships. If such arrangements were successfully arranged so as to make it objectively reasonable and therefore safe, then there would be the normal subjective elements of skipper skill and judgement to execute it safely and therefore within Colregs standards. None of this is difficult to grasp, and entirely in line with the basic, common sense judgements most of us make on the water all the time. I think you're too smart and experienced not to understand, so I can only conclude you're continuing to argue because it was Dockhead who also repeatedly made these points, and did so better than I could I should add.

Btw, a "straw man" argument is one in which a person deliberately changes facts or circumstances so as to make it appear they are refuting another person's argument, when in fact that argument was never made. In contrast, examples & hypotheticals are used for illustrative purposes only, and in the case of the Colregs in an effort to elucidate how a set of broadly worded rules are intended to apply in specific circumstances. Sorry Rod, but the only poster I've read using straw man arguments repeatedly and consistently -- and unfortunately quite deliberately -- has been you. And again, the only sensical conclusion to be drawn is some bizarre need to "win" what isn't even a rational or relevant argument.

But as Dockhead alluded to, the repeated debunking of your extreme, objectively and subjectively unreasonable positions have lead to some excellent discussions and quite a bit of learning in the process. For that I thank you.

[Juho]

I drafted one set of parameters that can be used to model the dangers and uncertainties when moving around fast big ships. This is just one approach, there could be many more. Modelling uncertainties is much fuzzier than modelling the ideal cases.

The parameters in the model, and their values in the first drawing are as follows.

size of the ship = 300m x 40m (see the small red area at the origin)
speed ratio = 4:1

Some uncertainty and observation error related parameters:

max error in the observed course of the ship = 5°
max error factor in the speed ratio = 1.2 (up or down)
min distance at which the ship might turn 90° = 4 NM

Some ship specific parameters for our safety:

min CPA = 200m
min allowed distance to approaching ship = 3 NM

The values of the parameters may vary a lot, e.g. if we make visual observations vs by AIS. The estimate on how likely the ship is to turn may depend on its earlier observed behaviour in AIS. If we want to prepare for one quick surprise turn, we could also increase the value of the "max error in the observed course of the ship" parameter.

The "min allowed distance to approaching ship" parameter depends quite a lot on one's personal preferences. It determines the position of the widest point of the "danger zone" that is in front of the ship in the drawing (around the ship and forward all the way to 9.4 NM). The pointy end of the danger zone shows when you should start moving away when the ship approaches you.

The drawings have been made for approaching or escaping the track of the ship in a 90° angle. Drawings for ideal crossing angles are quite similar when the speed difference is high enough.

The two alternative sharp heads of the danger zone are for estimated last minute escape (the shorter one), and the same with the given error margin ("max error factor in the speed ratio").

The three alternative tracks for approaching the track of the ship behind the ship are the estimated track and optimistic and pessimistic tracks (again based on the "max error factor in the speed ratio")

With these parameters you can see that if you are fishing in the middle of a shipping lane, you should move away when the approaching ship is 9.4 NM away. You can return to your fishing spot no later than 3.9 NM behind the ship. With these parameters it is not possible to pass the transom of the ship at the "min CPA" distance (without being inside our danger zone). (In one earlier post I presented a similar example.)

In the second drawing only one parameter has been changed. The "speed ratio" is now 2:1 instead of 4:1. In the drawing you can see that with this faster boat we might be able to pass the ship at the "min CPA" distance if we so wanted.

The ship stays in the origin all the time. Boat's position (along the curves) is shown in relation to the position of the ship.

Is there any feeling of naturalness in these parameters? Are the drawings helpful in visualising the dilemma of staying away from the danger zone and not being able to cross the track right behind the ship? The boats in the given examples do not want to take big risks. Are the parameter values ok for such an attitude, when meeting a large fast ship?

[ramblinrod]

Originally Posted by Exile

There are no "objective" recommendations from other posters for the scenario you posed.

Dude, the debate is over and crowd has gone home.

As usual, all active participants are a little firmer in their original resolve. ;-)

[Lodesman]

Rod,

This is an entertaining discussion and certainly has a lot of attention. I think you can agree that I've been fair and respectful with you? So could I ask of you to do the same and answer some of my questions? Or any questions?

Originally Posted by Lodesman

This brings up a point - how are you measuring your pass? Just eyeballing a bearing change doesn't give the range, and certainly not the CPA. AIS is out the window, and I doubt most radars would give that sort of definition.

How close is too close for you? 120 ft? 80 ft? 0 ft?

This is I think a fair question:

Originally Posted by StuM

For the third time, I ask the as yet un-answered question:

"Have you ever actually sailed close to a large ship travelling at speed?"

Perhaps you could tell us how close you've ever passed a large ship at speed? I accept that you may not have been in dire need to get your wife to medical care, so it may not be as close as what you're advocating here, but it would be nice to know (if) you practice what you preach.

Originally Posted by ramblinrod

I hope this exercise helped to elucidate, that one can't apply "rules of thumb" without exercising proper thought, based on the circumstances. This even caught a very experienced navigator, actively participating in this thread.

To whom are you referring? And can you explain what "rule of thumb" caught out this person?

[Exile]

Originally Posted by ramblinrod

Dude, the debate is over and crowd has gone home.

As usual, all active participants are a little firmer in their original resolve. ;-)

I'm afraid the only one actually "debating" your scenarios was you Rod! Everyone else was trying to explain their absurdity to you, whether they be passing within 180' of a big ship in open water "so as not to interfere with the ship's maneuvering plan," or your frequent reliance on "pucker factor" from sailboat racing as the primary factor in making your "subjective" judgements. While it's now been explained to you with math, diagrams, and remarkably well-written prose, for most I suspect the "debate" never got off the ground.

Fortunately I don't believe you've ever actually executed such reckless maneuvers around big ships since you've never sailed even remotely that close to them. If you had you would have boasted about it when StuM repeatedly asked, and wouldn't have been so dismissive of cautionary remarks about what it's actually like.

I'm not sure about racing sailboats on a lake which I suspect move along at no more than 10 mph, but there's a very old, cautionary quip in motorcycle racing that says "when the green flag drops, the bulls**t stops!" But again, your extreme examples did lead to interesting & educational discussions which probably never would have occurred without your input. And of course a mountain of information for Dockhead's new book!

[robbievardon]

DH in post 583 you again incorrectly quote the Cregs despite the fact that I had already corrected you some time back for your misquote for which you appologised and thanked me. I am not sure why 20 Kn speed is being used as the norm for the merchant vessel for as you have a few posts back stated that due to fuel costs the more realistic speed is around 14Kn. In fact if you look at the performance data for MAN diesel engines for marine application they give max speed as 15 Kn.There is further agreement from within the industry that Slow Steaming is now the norm.

[TJ D]

Oh, there are still plenty of containerships moving at 20 knots. My last 2 crossings involved 20kt ships. I'm currently on the great circle track between Unimak Pass and Seattle, and have encountered a few at 20+. So, it's not uncommon, even these days.

Hanjin, CMA/CGM, Evergreen, and MOL all seem to be booking right along in my neck of the woods.

Average speeds are down, sure, but it's not unusual at all to see guys running this fast.

i wish my boat would go that speed. i would have been home by now!

[carstenb]

Originally Posted by robbievardon

DH in post 583 you again incorrectly quote the Cregs despite the fact that I had already corrected you some time back for your misquote for which you appologised and thanked me. I am not sure why 20 Kn speed is being used as the norm for the merchant vessel for as you have a few posts back stated that due to fuel costs the more realistic speed is around 14Kn. In fact if you look at the performance data for MAN diesel engines for marine application they give max speed as 15 Kn.There is further agreement from within the industry that Slow Steaming is now the norm.

please note that as per my previous post - Emma Maersk was making 19 knots coming down the Cape Finisterre TSS. We find that in open ocean most ships are making 18+ knots

of course - I'm limited in that so far my experience is within the Atlantic/baltic/med.

It might be different in the Pacific..................

[StuM]

Originally Posted by carstenb

please note that as per my previous post - Emma Maersk was making 19 knots coming down the Cape Finisterre TSS. We find that in open ocean most ships are making 18+ knots

of course - I'm limited in that so far my experience is within the Atlantic/baltic/med.

It might be different in the Pacific..................

Not down the east coast of Aus it's not - this is not uncommon.

Here's one that recently came round the end of PNG from China (via Jomard Passage) and is bound for Melbourne.

[robbievardon]

TJ, I am not saying that 20KN vessels do not happen. In fact the container ships come first for potential speed up to 25Kn, followed by Cruise ships at 22 to 24Kn, the reduced running speed is as a result of basically 2 factors. First Environment,ships emit a huge amount of CO2 and sulphur (I think) dioxide,both bad for the planet. Second, the cost of fuel is huge,approx 50% of all costs. 30,000 TD vessel @ 15Kt 239.17 Tons fuel used,whilst 20Kt uses 352.56 Tons based on 6 1/4 days trip. Cruise ships (Newest) run at about 22Kt (fair weather) but can increase to 24 Kt to avoid/make up for bad weather as they obviously have a very critical port arrival time schedule. This at several thousand $ extra cost. Info from MAN and Journal of Ocean Engineering and Science.
In terms of collisions, The Rena 3351 TEU container ship struck a reef at 17 Kt off New Zealand in 2012,cause, poor watchkeeping failure to correctly read Radar, plot course resulting in huge environmental damage. Master and First Mate sentenced to jail term after an attempt to falsify paper chart plot and ships log. In a collision in the Gulf 2 relatively new vessels collided in a heavy glancing blow, but no major damage to either. Statements taken just after the collision by the captains were criticised at the enquiery 2 years later as being embelished in order to deflect blame. The true details were portrayed by the ships "Black boxes" The misunderstanding of one vessels manoeuvre from VHF radio communication due to multiple radios in use added to the problem.
Correct/refresh my memory please, Horizon at sea level = 6NM ,say 40 ft freeboard would make vessel visible at what distance off? Loaded container ship would be considerably more, From this one would expect a sailboat to have plenty of time to plan a course!

[ramblinrod]

Originally Posted by Lodesman

Rod,

This is an entertaining discussion and certainly has a lot of attention.

Cool.

To whom are you referring? And can you explain what "rule of thumb" caught out this person?

Tomorrow will be another day,

With another issue fought and won,

But for me my friends, without a doubt,

This one is pondered, perpended...done.

Suggested take-aways from this thread.

1. Never cross a bow you don't have to.
2. Beware applying an old gestalt to a new situation.
3. The large bolts be in the engine, but the biggest nut, is always behind the wheel.