Collision Avoidance -- Dealing with Multiple Targets

[Dockhead]

Originally Posted by estarzinger

I dont have any super clever tricks.

But I will suggest that it is different when you have say 4 targets vs when you have very crowded water - say circa +40 targets.

With +40 targets I believe the thought process needs to be changed. Rather than trying to avoid targets, you are trying to slot into and slide between clear zones/lanes. And rather than making 'clear bold' maneuvers you want to be entirely predictable and make small smooth incremental changes.

Yes.

I think the whole system breaks down after a certain number of targets on different courses -- we just can't process that much information. It starts to become like four-dimensional chess.

Fortunately this is rare. You mostly have lines of vessels going in more or less one direction, perhaps following a Deep Water Route or some other kind of shipping lane. Sometimes with ferry routes crossing (thinking of the Dover Straits). Sometimes vessels leaving or entering the lanes at an angle. But this job is mostly two-dimensional.

It gets hairy when you have a really busy place like the North Sea, with lanes criss-crossing each other. In such circumstances, even three or four targets can overwhelm a professional navigator -- see especially the Tricolor/Cariba/Clary collision (we discussed it here: http://www.cruisersforum.com/forums/...ea-184059.html).


I am actually just heading out to sea to cross the North Sea and pass the Dover Straits, so I'll be off line for a couple of days. Later today, I'll be passing right over the spot where the Tricolor sank. I've been through this spot a few times before, and I'm here to tell you that it's hairy, with intense traffic crossing in three different directions -- to and from Rotterdam, to and from the Baltic Sea, in and out of the English Channel, in and out of the Thames Estuary. Throw in huge fleets of fishing boats, wind farms, oil platforms, and shoals, and you've got a real challenge on your hands. This kind of challenge is exactly what got me interested in collision avoidance in the first place. Fortunately today, unlike other times, I am not short handed, and the weather and viz are good. I'll have a very skillful and experienced old shipmate of mine on the helm, another on deck doing nothing but keeping a visual watch, and I'll be at the nav table with radar, AIS, notebook, plotter, and radio to hand, when we are passing through the toughest areas.

[Juho]

Originally Posted by Dockhead

I'll have a very skillful and experienced old shipmate of mine on the helm, another on deck doing nothing but keeping a visual watch, and I'll be at the nav table with radar, AIS, notebook, plotter, and radio to hand, when we are passing through the toughest areas.

... and you have lots of good and bad advice that you should probably ponder less than use your common sense at sea . Have a safe trip.

[Juho]

Originally Posted by Pelagic

Ironically, that was exactly the IMO design brief for ARPA radars.
It was called "PAD" (Predicted Area of Danger)

Raytheon came out with the tracked targets showing a Trapezoid shaped area extending from them that showed on your radar..the area NOT to go into

It was a great system for ML and dense traffic situations but copyright issues prevented it from being generally adopted and is probably only used now by the military
https://trid.trb.org/view.aspx?id=147010

If we have a collision avoidance algorithm that can propose possible changes in course and speed to vessels, we could apply that to all the vessels and show all the possible routes that the vessels might take (with error margin and safe distance). That could protect us from surprise moves and reactions to those surprise moves.

That picture could be a bit messy, so maybe one should read that screen only to see where the safe spaces are (for plan B), and to be alerted become extra careful when there is not much safe safe space left.

[Dockhead]

Originally Posted by Juho

... and you have lots of good and bad advice that you should probably ponder less than use your common sense at sea . Have a safe trip.

Common sense should work in harmony and balance with skill and knowledge, which are no less important than common sense for such a job.

But yes -- the Dutch coast is sinking below the horizon and soon I'll be far away from all this . . .

[Pelagic]

I actually tried the early Raytheon PAD display in simulators.
It was a bit messy at first until you managed the vector lengths and allowed CPA.
Then the trial maneuvers showed you exactly the interaction of many "Danger" boxes as they changed shape and distance to your relative position.

In true motion it was even better as real time changes by all tracked targets displayed the danger interaction with each other and it helped you to know which was the optimum course to stay clear.

[El Pinguino]

Originally Posted by Pelagic

I actually tried the early Raytheon PAD display in simulators.
It was a bit messy at first until you managed the vector lengths and allowed CPA.
Then the trial maneuvers showed you exactly the interaction of many "Danger" boxes as they changed shape and distance to your relative position.

In true motion it was even better as real time changes by all tracked targets displayed the danger interaction with each other and it helped you to know which was the optimum course to stay clear.

I think you will find PADs was a Sperry thing..... only sailed with one in the early 80's.... interesting kit and better than many/most/all of the other ARPA's around at the time....

Some interesting comments on it here.. https://books.google.com.au/books?id...0radar&f=false

The next Sperrys I sailed with had touch screens, more time was spent cleaning the screens than navigating......

[Kenomac]

I really don't understand the need some folks have to contact large ships from many miles out in order to avoid collisions. A sailboat or small powerboat is like a flea on the azz of an elephant in comparison to the freighters. The fleas need to stay out of the way.

I can only think of two times in over eight years that I felt the need to contact larger ships, one time during bad weather and another when we were dead in the water in order to alert them to our status. Cluttering up the airwaves with unnecessary chatter would seem to be nothing more than a nuisance to the larger ships.

[ramblinrod]

Originally Posted by Dockhead

You didn't serve on the Fitzgerald, by any chance? Yeah, it's "very, very easy"


180 feet is not far enough from a ship travelling at sea speed! You need to stop projecting lake and bay-sailing frames of reference onto what happens at sea -- it's totally different out there. The distances are different, the timings are different, it is all different.

Less than one mile is dangerous, and a mile may be too close if passing ahead. Why? Because you can't be exactly sure within a about where that ship travelling at 24 knots will be. A small course error, or variation in set or drift will change the position of a vessel traveling at that speed by hundreds of feet. Besides that, a small boat under sail will vary a lot in course and speed as it goes along. The result is called the "cone of uncertainty", and if you want to be reasonably certain about not getting run down, you have to stay outside of it.

That's exactly why ships you meet at sea will have standing orders to keep usually one mile away from you and any other traffic, and they will set it up from usually 10 miles out, with last maneuvering typically at 3 or 4 miles out. From that point, they are committed to the crossing, and if you make some unpredictable move at a lesser difference, you will mess everything up. This kind of behavior is exactly why they call us WAFIs (wind assisted f**** idiots), and try to steer a wide berth around us.

If you screw up and end up less than a mile from a ship at sea, you may even get the watchstander in trouble, not to mention the risk getting yourself killed. 180 feet or 500 feet is not an acceptable passing distance in open water.

Once again -- this thread is not for these basic, general questions. If you want to discuss this kind of thing, let's please go to: Thread for Basic COLREGS Questions - Cruisers & Sailing Forums

1 mile way or get sucked in? Absolutely preposterous. When the freighters are between Niagara River and the St, Lawrence they are sailing full steam ahead. They produce a bow wave and wake like any other boat. I've never seen a wake
More than 4 ft high. Pushing water is not efficient.

Any boat that could suck a sailboat into it from 180 feet away would kill so much sea life, Green Peace would have shut it down long ago.

[Kenomac]

Originally Posted by ramblinrod

1 mile way or get sucked in? Absolutely preposterous. When the freighters are between Niagara River and the St, Lawrence they are sailing full steam ahead. They produce a bow wave and wake like any other boat. I've never seen a wake
More than 4 ft high. Pushing water is not efficient.

Any boat that could suck a sailboat into it from 180 feet away would kill so much sea life, Green Peace would have shut it down long ago.

100%

I sailed in and around Los Angeles/long Beach/San Pedro harbors for two years on our Hunter 450 and five years on our itty bitty O'Day 20, and don't recall getting "sucked into" any large container ships while transitting the many shipping channels, sometimes less than 100 ft away.

Maybe I've just forgotten. But I do remember getting pushed across a channel by the wash of a tugboat... which was no big deal.

[El Pinguino]

Originally Posted by Kenomac

100%

I sailed in and around Los Angeles/long Beach/San Pedro harbors for two years on our Hunter 450 and five years on our itty bitty O'Day 20, and don't recall getting "sucked into" any large container ships while transitting the many shipping channels, sometimes less than 100 ft away.

Maybe I've just forgotten. But I do remember getting pushed across a channel by the wash of a tugboat... which was no big deal.

Hull design on tugboats leads to big wash at speed as they are designed for low speed grunt..

The Stena HSS's used to operate at over 40 knots... no wash near them but in the shallows quite some miles away they would produce a rolling surf... the speed restrictions applied to them in the approaches to Harwich was one of the reasons they ended up being uneconomic to operate.

[Kenomac]

Originally Posted by El Pinguino

Hull design on tugboats leads to big wash at speed as they are designed for low speed grunt..

The Stena HSS's used to operate at over 40 knots... no wash near them but in the shallows quite some miles away they would produce a rolling surf... the speed restrictions applied to them in the approaches to Harwich was one of the reasons they ended up being uneconomic to operate.

I was referring to the wash produced by a tugboat pushing an enormous container ship into a dock. Pushing a relatively stationary object and producing a wash at a 90 degree angle (across the channel) to those of us transitting the canal.

Like Rod, when I see ridiculous statements written about smaller boats getting sucked into container ships from 180ft away up to a mile away, those folks just need to be called out for making them.

[Pelagic]

Originally Posted by El Pinguino

I think you will find PADs was a Sperry thing..... only sailed with one in the early 80's.... interesting kit and better than many/most/all of the other ARPA's around at the time....

Some interesting comments on it here.. https://books.google.com.au/books?id...0radar&f=false

The next Sperrys I sailed with had touch screens, more time was spent cleaning the screens than navigating......

You are correct...it was Sperry and a long time ago[emoji6]

[ramblinrod]

Originally Posted by Dockhead

You didn't serve on the Fitzgerald, by any chance? Yeah, it's "very, very easy"


180 feet is not far enough from a ship travelling at sea speed! You need to stop projecting lake and bay-sailing frames of reference onto what happens at sea -- it's totally different out there. The distances are different, the timings are different, it is all different.

Less than one mile is dangerous, and a mile may be too close if passing ahead. Why? Because you can't be exactly sure within a about where that ship travelling at 24 knots will be. A small course error, or variation in set or drift will change the position of a vessel traveling at that speed by hundreds of feet. Besides that, a small boat under sail will vary a lot in course and speed as it goes along. The result is called the "cone of uncertainty", and if you want to be reasonably certain about not getting run down, you have to stay outside of it.

That's exactly why ships you meet at sea will have standing orders to keep usually one mile away from you and any other traffic, and they will set it up from usually 10 miles out, with last maneuvering typically at 3 or 4 miles out. From that point, they are committed to the crossing, and if you make some unpredictable move at a lesser difference, you will mess everything up. This kind of behavior is exactly why they call us WAFIs (wind assisted f**** idiots), and try to steer a wide berth around us.

If you screw up and end up less than a mile from a ship at sea, you may even get the watchstander in trouble, not to mention the risk getting yourself killed. 180 feet or 500 feet is not an acceptable passing distance in open water.

Once again -- this thread is not for these basic, general questions. If you want to discuss this kind of thing, let's please go to: Thread for Basic COLREGS Questions - Cruisers & Sailing Forums

No, but my Dad was on the "Fitz" when at the ore docks in Picton, ON.

I watched him and 9 other men (5 swung over the side from the deck of the ship) pull the unloaded boat up to the dock. This was common practice in benign conditions.

Now back to the subject at hand...

In any kind of sea, it is impossible to hold course within 1 degree at any instant. Of course over distance, with many course corrections of various degrees, it is possible to hold a specific course.

Altering course from 5 nm away by 1 degree to pass the stern of a vessel by 180 ft or more is a non-issue.

I agree that when a collision is imminent, any course correction has to be significant, so that intent is clearly visible to other vessel. There is no such obligation to do this from 5 miles out.

Lets be reasonable.

The sailboat may be on a daysail with intention to sail 4 miles further and turn around, one full mile clear of the ship.

Should a ship watch the sailboat? Of course. If the sailboat pulls a bonehead, they need to give 5 short blasts.

If the sailboat passes astern, (by a safe distance far less than 1 nm) no harm no foul.

We've crossed astern of freighters in Lake Ontario on many occasions. They pretty much follow the standard routes and traffic separation north and south of Main Duck Island, but you never know whether they will stay on course or turn off to a loading dock somewhere along the north or south shore.

I agree that in rougher conditions, it is wise to increase the distance, and I would certainly never cross a bow as close as I would astern, but c'mon we have to stay real here. Even between large vessels, 1 nm is a lot of water. Between a large vessel and sailboat, there is absolutely no need (per colregs) to maintain this kind of distance, and in lots of places it isn't possible.

[Dockhead]

Originally Posted by ramblinrod

1 mile way or get sucked in? Absolutely preposterous. When the freighters are between Niagara River and the St, Lawrence they are sailing full steam ahead. They produce a bow wave and wake like any other boat. I've never seen a wake
More than 4 ft high. Pushing water is not efficient.

Any boat that could suck a sailboat into it from 180 feet away would kill so much sea life, Green Peace would have shut it down long ago.

Originally Posted by ramblinrod

. . . In any kind of sea, it is impossible to hold course within 1 degree at any instant. Of course over distance, with many course corrections of various degrees, it is possible to hold a specific course.

Altering course from 5 nm away by 1 degree to pass the stern of a vessel by 180 ft or more is a non-issue.

I agree that when a collision is imminent, any course correction has to be significant, so that intent is clearly visible to other vessel. There is no such obligation to do this from 5 miles out.

Lets be reasonable.

The sailboat may be on a daysail with intention to sail 4 miles further and turn around, one full mile clear of the ship.

Should a ship watch the sailboat? Of course. If the sailboat pulls a bonehead, they need to give 5 short blasts.

If the sailboat passes astern, (by a safe distance far less than 1 nm) no harm no foul.

We've crossed astern of freighters in Lake Ontario on many occasions. They pretty much follow the standard routes and traffic separation north and south of Main Duck Island, but you never know whether they will stay on course or turn off to a loading dock somewhere along the north or south shore.

I agree that in rougher conditions, it is wise to increase the distance, and I would certainly never cross a bow as close as I would astern, but c'mon we have to stay real here. Even between large vessels, 1 nm is a lot of water. Between a large vessel and sailboat, there is absolutely no need (per colregs) to maintain this kind of distance, and in lots of places it isn't possible.

That an obviously intelligent person like you could so totally misunderstand the basics of crossing with fast ships, is evidence that this discussion is really worth having. It might even save your life. I invited you to do the math and see for yourself; you didn't bother. I guess because you are so sure in your own concepts that you don't think you need it. Now I've done the math for you and you will see. But let's discuss it elsewhere, say here:

Collision Avoidance, Cones of Uncertainty, and Appropriate CPA - Cruisers & Sailing Forums

[Kenomac]

Originally Posted by Dockhead

That an obviously intelligent person like you could so totally misunderstand the basics of crossing with fast ships, is evidence that this discussion is really worth having. It might even save your life. I invited you to do the math and see for yourself; you didn't bother. I guess because you are so sure in your own concepts that you don't think you need it. Now I've done the math for you and you will see. But let's discuss it elsewhere, say here:

Collision Avoidance, Cones of Uncertainty, and Appropriate CPA - Cruisers & Sailing Forums

Within ten minutes, I had a 350ft freighter cross ahead of me then parallel pass within 150ft, a car ferry cross ahead of me then cross my stern, basically half circle my boat and pass within 150ft (see video) and a high speed ferry pass very close (I have it on video). At no time did I or the much larger boats feel the need to contact one another, nor do I recall getting "sucked into" the much larger boats.

They see me on AIS and I stay out of their way. There's no way I'd ever hold my course and expect the larger vessels to alter their course, the AIS tells them what I'm doing to give them plenty of room. And no.... they don't maintain a 1 mile separation from other traffic, 150ft seems to be the more common distance/comfort zone. The ferry is actually passing closer than it appears on video, just check out the wake.

https://m.youtube.com/watch?v=3LjCK5dKo5U