Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 

In the next week or two I intend to have a simulator of sorts up and running, available to share. My plan is to capture the worldwide AIS data for, say, a 24-hour period (I can get this from aishub.com) with embedded timestamps. I will provide a software tool that lets you put a "fence" around the AIS traffic so as not to overload OpenCPN or whatever your plotter of choice is. The tool will let you place your virtual boat in the middle of the selected region and give you course/speed/heading controls, using generated NMEA RMC sentences. OCPN or NavMonPc can be your dashboard while you dart in and out of shipping traffic. Sorry, there won't be any "bang, crunch" sound effects.

OK, perhaps this isn't a plan just yet, but I am seriously considering it. If I don't get distracted.

Oh look! A bunny!

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 

You might get away with this in a conversation, but the forum gives us a permanent chronological record - let us review and see who is confused:

You started this ridiculous scenario with post 459:

Quote:

Originally Posted by ramblinrod (Post 2502284) 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?

In post 477 you clearly haven't realized how close you will come to the ship astern - even stating you'll keep more than 1 mile ahead of it, by even going closer than 180' off the transom of the ship ahead:

Quote:

Originally Posted by ramblinrod (Post 2502520) And yes, I set the scenario so that a 1 nm CPA from all is not possible. 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".

Then in post #490, displaying your complete misunderstanding of the situation you talk about "turning to port to maintain maximum distance from the following boat." Hilarious - even if you knew that it would be about a 15º turn to port that would maximize that CPA, you clearly have no clue that it would only be increased by a couple of yards and are still blithely unaware of how close it will actually be:

Quote:

Originally Posted by ramblinrod (Post 2502621) That there is human decision involved at the time the "Safe Distance" decision is made is clearly subjective. I'm not saying that I would pass The transom within 180 ft, but I guarantee that as I approach the crossing, I will subjectively determine the minimum safe distance I need to give the lead boat transom port corner, and if that distance is determined to be 180 ft by me, that s how close I will come. I will also turn to port as soon as I clear that corner, by the degree required to get no closer to the lead boat, and maintain maximum distance from the following boat, until I am the hell out of its path. To me, in this example, that is the "safe distance" and it s completely subjective. Handle it.

If there was any doubt that you had no appreciation of this situation, it would soon be confirmed by your post 512:

Quote:

Originally Posted by ramblinrod (Post 2503007) Of course that would be madness. Of course if you turn 90 degrees to port, directly in the path, you will be run down by the following ship. I didn't say to turn 90 degrees to port. I said, "Turn to port as required to maintain the maximum distance from the following boat". This will obviously be considerably less than 90 degrees.

Backtracking a bit, in post 494, evm1024 posts the calculations that should have made it clear that you didn't know what you thought you knew. And you missed it. But at post 513 StuM tried to give you a nudge by telling you how fast the following ship was approaching you.


In post 516, you actually did answer "yes" to the question of whether you've passed close to a fast moving ship - my apologies for repeating the question, but you never answered my follow-up questions "how fast and how close?" You still don't seem to have caught on by post 517 where you still think your vague port turn would have any effect:

Quote:

Originally Posted by ramblinrod (Post 2503048) I don't know where you are getting this from but it is incorrect. If I am crossing at 90 degrees, to be 180 ft off (nearest distance) his port transom corner, at any time before reaching this point, I am further away from him. Yes, as required to provide "safe distance" from this vessel and the bow of the following ship, which requires greater distance to be "safe distance". As I have declared in this thread several times, I repeat my answer, "Yes". Yes. There is a wake, as there is with any boat, especially a 40 ft motor yacht not yet on plane. As I said, during the approach, I would evaluate those conditions. And as I said, the nearest I "may" be to the boat, depending on those conditions, is 180 ft (over 50 m).

Quote:

Originally Posted by ramblinrod (Post 2503058) Exactly. This is why one should keep maximum distance from the bow of the following boat in the scenario I posted. In the scenario I painted, if the ship is 100 ft wide, and I was 180 ft from his port corner, if I kept my original course (90 degrees), by the time I passed the course of his starboard stern corner, I would be 400 ft from it. If I turned to port to some degree, I could reduce that gap with the lead boat, while maintaining maximum distance from the following vessel by the time they pass my crossing course.

Even by post 558, you still don't realize just how close you're gonna get to that ship astern:

Quote:

Originally Posted by ramblinrod (Post 2504756) So, to clear something up, I did make an error previously, describing a crossing situation. Keeping a 180 ft minimum distance from the ship, and greatest distance from the following ship... On the approach to the lead ship, on a 90 degree course, the give way sailboat would be 180 ft from the ships port transom corner. The instant the sailboat reaches this point, proximity to the lead ship will be every expanding and distance from the following ship will be every diminishing. This is why it is so important to be close to the lead ship for the crossing. If one doesn't feel comfy with the 180ft minimum, they can choose something greater. If one is comfy being a bit closer, they can do that to. Of course large, possibly heavily laiden ships, at speed, are not on rails, but as has been described so much in this thread, they have a lot of momentum, and it takes a whole helluva lot to get one to suddenly jump sideways at you. If one maintains a minimum 2.5 cables (1500 ft) from the lead ship they will be crossing the bow of the following ship more closely, which is a more dangerous maneuver, than a 180 ft minimum distance, under these circumstances.

No closer to the truth by 570:

Quote:

Originally Posted by ramblinrod (Post 2504907) The scenario that I prepared has a degree of risk associated, no matter how one attempts to cross. Therefore, I believe passing closer to the stern of the lead ship may be safer, than your proposed 1/4 nm CPA, which my proposed solution would keep you further ahead of the bows of the following ship you are crossing. If the lead ship has standing orders for 1nm CPA min, they are already going to be concerned with your approach, so it makes more sense to stay closer to their stern, and further ahead of the following ship.

In post 577, I inform you of how close you'll come to the following ship:

Quote:

Originally Posted by Lodesman (Post 2505066) Your scenario is entirely contrived, so I can't imagine making a crossing between ships 1 1/4 miles apart, going 4 times my speed. But let's say I have to be somewhere and can't wait, I certainly wouldn't make this crossing without having talked to both ships by VHF.

In the very next post, you still state that you'll be crossing the bow of a 20 kt ship 1nm away; you're right that it's not very bright, but it still hasn't clicked...

Quote:

Originally Posted by ramblinrod (Post 2505107) Crossing the bow of a 20 knot ship 1 nm away, may not be bright, but crossing even closer, just for the purpose of being further away from a ship that you have no risk of collision with, because at that moment they are passed you and nothing they can do short of instantly stopping and backing up can hurt you, just isn't wise.

Post 579, it's starting to sink in:

Quote:

Originally Posted by ramblinrod (Post 2505112) Even worse if one maintained a minimum 1/4 nm CPA from the lead ship port transom, which is exactly my point.

580, you still haven't quite got there. "Starboard side course"??? Really?!?!

Quote:

Originally Posted by ramblinrod (Post 2505157) Your proposed solution was to maintain a minimum 1/4 nm CPA from the lead vessel (which by the way, would put you closer than 1nm from the following ship, by the time you cross in front of it's starboard side course. Bad idea. Dumb.

Posts 581-5 were all yours, and still you haven't figured it out, but in post 587 I tell you again:


Dockhead realized the mistake in post 588:

Quote:

Originally Posted by Dockhead (Post 2505239) Yes, and I failed to fully visualize this myself. I've done a lot of threading through lines of ships in real life. But I am usually making 8 or 9 knots, and they are usually more like 2 miles apart, so it's MUCH safer than this hypothetical. So if you get reasonably close to the ship ahead you can pass reasonably comfortably ahead of the second one, despite the real life variations like the ships aren't exactly in line, the crossing is not exactly perpendicular, etc. As the difference in speed increases, the Relative Motion Line gets steeper and steeper, and this kind of crossing rapidly becomes more and more dangerous. The hypothetical case COULD be done I think, but it's really dangerous. You'd have to nail the crossing close behind and you would absolutely have to agree the whole thing with both vessels, as you suggested.

Dingdingdingding, you finally got it by post 591, 4 hours after my second post telling you this:

Quote:

Originally Posted by ramblinrod (Post 2505337) So in summary, from being crushed by the following ship: a) Ramblin Rods proposal clears the path of the following 20 knot ship by 1555 ft. b) Dockheads proposal clears the path of the following 20 knot ship by 240 ft. Everyone is free to make their own decisions (and live (or not) with the consequences) but I would far sooner be a 180 ft from the lead ship and 1555 ft from the following ship, than 1500 ft from the lead ship and 240 ft from the following ship.

Then the next umpteen posts by you boiled down to "I'm right, Dockhead's dead, case closed" giving the impression that your fingers were stuffed in your ears while you yelled "lalalalalala, I can't hear you..."


Dockhead gives his mea culpa in post 620, but let's not forget, you had no clue about this until I pointed it out - twice. The reality, if DH had been on a suicide mission, doing a close shave a la Rod, do you think he would have ignored the steady bearing, relative vector over own ship, or AIS alarm for the ship 1 mile to the right of the one he's setting up to pass? Of course not, so don't be so smug.


You still have absolutely no clue why it would never work as you say. Stu Jackson even put your method to the test, and got the results all the rest of us expected - the big ship simply won't let you get that close, and with a fourfold speed advantage, there ain't nuthin' you gonna do about it!


And if you did, by some miracle find yourself crossing between two ships going 20 kts, 1 1/4 miles apart, and managed a 180' cpa on the first, you would probably learn firsthand what pressure and suction zones on a big ship are like, and hitting the pressure zone would stall your forward momentum enough that you would never be able to clear the second ship - as you like to say: BANG! GLUG! GLUG! GLUG!


For anyone else who is interested, the video posted earlier has a great example of the effect of pressure zones. Watch the pink-spinnakered boat in relation to the hills/buildings in the background. At 0:26 you will start to see him pushed rapidly to our left, his starboard. If you pause at 0:28 you can see the water washing up his hull on the portside. It started from at least 100' in front of the ship and got him going about 5 kts sideways, which (good news) meant a gentle dismasting vice an explosive disassembly.

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 

All I hope you learned is to be more respectful to others and knock off the insulting, back-handed comments toward anyone who posts a differing opinion.

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 

Reminds me of Hitchikers Guide....

Only Zaphod Beeblebrox is reported to have survived the Vortex unscathed (and then to have eaten the small piece of fairy cake). When it showed him the "You Are Here" marker, Zaphod correctly interpreted the Vortex as simply telling him that he was the most important being in the universe.

As a character, Zaphod is hedonistic and irresponsible, narcissistic almost to the point of solipsism, and often extremely insensitive to the feelings of those around him.

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 

Wow, that must have taken quite some time.

I think you are confused.

The 180 ft CPA from the lead ship I proposed does keep the sailboat 1 nm ahead of the following ship.

As opposed to the 2-3 cable CPA proposed by another poster. As I advised, 2 cables would be perilously close, and 3 cables would be into it. (They clearly didn't calculate how close their solution would place them to the following ship, which they have since admitted.)

As for the optimum angle to turn to port, my estimate based on speeds, (I didn't bother to perform the actual calcs) was around 15 degrees.

I knew this would not increase distance from the following ship much (but a small distance more ahead of a fast moving ship, with no down side, is better than none). However, because I knew the distance away gained was minimal, I didn't bother to perform the calcs, and include that in my analysis.

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 

RamblinRod sofware

010 Make a wild statement
020 Ignore feedback
030 Repeat the statement
040 Accuse responders being wrong
050 Repeat the statement
060 Twist things a bit
070 Ignore feedback
080 Repeat the statement
090 Responders are wrong
100 Goto 020

:biggrin:

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 

Teddy Diver Software

rem owa tajer kiam

010 contribute nothing useful
020 goto 10
030 end

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 

guys,

let's please move on to something a bit more constructive than putting labels on each other.

We can still debate the actual scenario or move on to a different scenario.

Personally, I have to wonder why anyone would want to attempt this in open sea conditions. I mean, why?

Even assuming the ship(s) would let you get that close - why? You just giving your self more problems than you need to.

Rule 2 sort of sums this up:

Nothing in these Rules shall exonerate any vessel, or the owner, master or crew thereof, from the consequences of
any neglect to comply with these Rules or of the neglect of any precaution which may be required by the ordinary
practice of seamen, or by the special circumstances of the case.

Good seamanship preclude attempting what is described in this thread. Noone excercising good seamanship would ever attempt this.



But let's move on - i think the idea of a simulator is a good one. Although nothing beats the actual experience on the water

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 

When it was pointed out to you that you weren't calculating this right, you unfortunately took it as a personal insult, and didn't follow up.

More than one of us actually took some trouble to point you towards the right way to do it, and this was intended in a friendly, constructive way. If you will read back through the thread, you can find those posts.

There are two ways to do it which will give you correct results, but the much easier of the two is plotting it.

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 

It's very usefull. Once I get it patented internet trolls must pay royalties for me :wink:
Might give you a share, got the idea from you :thumb:

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 

Only two ways to figure it out....

....Reeeeeally?

I hate to rattle your world but...

The sailboat starts out, with bow perpendicular to, and 180 ft away from the port stern corner of the lead ship.

This places the sailboat 1.25 nm forward of the following ship.

At 90 degrees from the ships course, the sailboat has to travel 320 ft, to cross the original 180 ft to the port course line, the 100 ft beam of the ship, and clear the starboard course line with it's 40 ft length, in advance of the following ship.

At 5 knots this takes 0.1017 hours.

In that time the ships will have advanced .213 nm.

Take the original distance the sailboat is forward of the following ship, and subtract the distance the following ship travels in the time for the sailboat's transom to cross completely, and we have 1.25 nm - 0.213 nm = 1.037 nm

This calculation does not take into account:

1. The shape of vessels (for simplicity and due to variation in vessel design)

2. The beam of the sailboat, (negligible influence, and just makes crossing close to the lead ship even more important).

3. The sailboat turning to port slightly to maximize distance from the following ship. (Limited benefit for the 180 ft CPA but increasingly important for the longer CPAs proposed).

Are you kidding me, after all of this, none of you have this figured out yet?

A guy sailing a small sailboat on Lake Ontario with no formal navigation training, using simple D = S x T calcs, figures this out in seconds, and outwits the self-proclaimed armchair navigation pros all over the world, now having weeks to figure it out using plotting boards and then professing to need simulators (yet to be created) to be able to "visualize" it?

Only 2 ways to figure it out, you say?

Too funny!

Consider yourself educated!

:biggrin::biggrin::biggrin::biggrin::biggrin::bigg rin::biggrin::biggrin::biggrin::biggrin::biggrin:: biggrin::biggrin::biggrin::biggrin::biggrin:

PS, I think I'll stick with my way thanks. You know, the one where I didn't run into the following ship, vs using your methods where you did.

hee, hee, hee, hah, hah, hah, ho, ho, ho.

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 

rod, you forgot the step where Scotty beams your boat into this position 180 feet away from the ship. I don't think folks are saying that the rest of your proposal is incorrect, it's just that it isn't feasible to arrive at your initial spot reliably without a very high risk of collision.

And mate, you might back off on the self-proclaimed victories. They are not becoming to you.

Jim

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 

Rod, you are not going to rattle anyone's world with your home-made methods and made-up terminology. They don't work. I'm not going to waste my time on this, but if you can find someone else to argue with, you can have a go - in another thread. I'll start it, for convenience.

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 

In Rod's perfect world where he sets the rules, boats go at a constant speed, courses are straight lines, he is always right and no one should even consider changing the rules except Lord Rod.

But for the rest of us Rod (who did bother to do the calculations many many postings ago) Boats do not do 5.000 kts, Ships do not do 20.000 kts, courses are often +-15 degrees. And we do not ignore those Cones of Uncertainty as you do to stroke your ego.

Many here have significant formal training, have held or currently hold professional licenses (my self included) would never ever want to be in a crossing situation with you.

Your arrogance and pride in your ignorance are profound.

PS you never did answer my questions. But then again that is how you win arguments.

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 

PS Check your private messages