Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

[Dockhead]

Originally Posted by El Pinguino

Where do people keep finding these 20 knot ships that take half a day to respond to the rudder?

In the real world most ships poke along at about 15 knots and where I sail 8 to 10 is more the norm.

And any ship you encounter shall be somewhere in between....... an infinite number of sizes, aspects, speeds, etc etc which makes fixating on a single scenario a bit... how do you say it?...anal?

So... my recent near death experience and a little bit of karma.

There we were sailing merrily along on a beam reach on passage from Isla Anihue to Hornopiren... glorious vis -you could see all the way to Argentina.
Small ship - same same Pic 1 - appears to the south maybe 10 miles away.... steady bearing about 4 points on stbd bow.

So she stands on and we stands on and she stands and we stands on. We were probably doing 6 knots... she would have been doing 8 or 9.

By 2 or 3 miles her bearing had started to open very slighty but I was still expecting her to alter to port . She didn't and we passed maybe 100 metres ahead of her ie a ship's length.

Now where does the collective CF wisdom stand on this. When should I have altered? Which way ?

In my opinion, you should have altered in time to cross at a safe distance. In my opinion, passing 1/2 cable ahead of a ship is not a safe distance. Because a small variation in his course in the last moments could have caused a collision.

It's much less critical, however, with a small, slow ship travelling close to your own speed, like this. The risk goes up exponentially (using the word in its mathematical sense here, not rhetorically) with difference in speed.

However, it's MORE critical to be dealing with a ship to starboard of you. You need more room in that case. Because his impulse will always be to turn to starboard ("Turn to port, see you in court"). In this case, if he had panicked at the last moment and made any alteration to starboard as you were passing ahead, we might not be talking with you now.

As to "which way to turn" -- definitely to starboard, to pass behind. Even more so since this is the maneuver he was probably expecting, if he saw you at all.


Your waters are very different from ours. In the Channel, it's rare to see ships travelling much under 16 knots. Big box ships used to mostly maintain 20 knots. They have slowed down some in recent years but passenger vessels are mostly still making 20 or 24 knots. There are a lot of fast cat ferries making 35 to 40 knots. But those you don't have to worry about.

[El Pinguino]

All so simple if you say it quickly.....

Altering down off the wind ( ie altering to starboard ) I would lose speed and the whole dynamic would change and if he altered to port as one would have expected very very mucky. Bearing in mind that under 2 miles is the sort of distance you expect these ships to alter for you on a good day.

Put the helm down and bringing her up into the wind? Once again the whole dynamic alters yet again and most likely keeps you on a steady bearing......

The bearing was opening so I stood on.....

And wasn't 1 cable ahead, was half a cable.

I skimmed through that youtube.... apart from taking 21 minutes to describe how to calculate CPA he seriously overcomplicated calculating the time of CPA .... all you need to do is step off the 6 minute distance from last position to CPA and if there are 4 divisions TCPA = 4 x 6 = 24....

And also ... he was using only 2 positions 6 minutes apart which is a big no no.... assuming no visual on the ship here fog or whatever... if the target has made an alteration for any reason within that period you will not be aware of it and your calculation is meaningless. At a minimum you should use three positions - 0,6, and 12 - and carry on plotting every six after that.

21 minutes and a whiteboard full of scribble to explain something so simple?.... that bloke - and others - should realise that brevity is of the essence when explaining complex matters. If you can't be brief then you most probably don't have a proper grasp of the subject yourself.

[daletournier]

Originally Posted by Dockhead

In my opinion, you should have altered in time to cross at a safe distance. In my opinion, passing 1/2 cable ahead of a ship is not a safe distance. Because a small variation in his course in the last moments could have caused a collision.

It's much less critical, however, with a small, slow ship travelling close to your own speed, like this. The risk goes up exponentially (using the word in its mathematical sense here, not rhetorically) with difference in speed.

However, it's MORE critical to be dealing with a ship to starboard of you. You need more room in that case. Because his impulse will always be to turn to starboard ("Turn to port, see you in court"). In this case, if he had panicked at the last moment and made any alteration to starboard as you were passing ahead, we might not be talking with you now.

As to "which way to turn" -- definitely to starboard, to pass behind. Even more so since this is the maneuver he was probably expecting, if he saw you at all.


Your waters are very different from ours. In the Channel, it's rare to see ships travelling much under 16 knots. Big box ships used to mostly maintain 20 knots. They have slowed down some in recent years but passenger vessels are mostly still making 20 or 24 knots. There are a lot of fast cat ferries making 35 to 40 knots. But those you don't have to worry about.

I also wonder about these 20k ships? I genuinely don't think I've ever come across a a ship doing 20k, regularly around the 14-15k.

I also don't get the '' I have the right away '' mentality, regardless of Colregs, as I'm the one that is going to die in any of the big ship hit small boat sceneries.

I come across very few ships that don't have ais, my alarm goes of if cpa is within 1nm and 30min, if a ship is going to come within 1/2nm of me I radio them while they are still 30mins out. I politely ask "have you seen me" if there is the slightest risk of collision I discuss with them what action to take. Mostly they are happy for me to stand on and they adjust their course. Radio and communication!!!!

The rare occasions that a ship isn't on my ais and there is a possibility of a less than comfortable distance coming up, I take action very early and make that action obvious, I make sure, I'm well and truly not going to get hit.

Colregs are great assuming everyone knows them and follows them, and me personally?, I don't particularly care about the right and wrong, I just make sure there will be no accident. The earlier the action the better.

One more thing, I've heard over the radio sailors arrogantly demanding ships to alter course, I have right of way etc, crazy stuff as far as I'm concerned.

[conachair]

Originally Posted by daletournier

I also wonder about these 20k ships? I genuinely don't think I've ever come across a a ship doing 20k, regularly around the 14-15k.

Doesn't seem like there are as many as there used to be but still plenty out there, a few moments on marinetraffic in the channel found a couple..

Colregs are great assuming everyone knows them and follows them, and me personally?, I don't particularly care about the right and wrong, I just make sure there will be no accident.

The IRPCS covers that, everything you need is in there. From sailtrain -

http://www.sailtrain.co.uk/Irpcs/rule2d.htm

Rule 2
This rule makes it clear that mariners can not hide behind behind the rules and use them as an excuse. It is worded so that seamen are able to use their judgment and experience to deal with unusual situations which would be impossible to predict and legislate for.

[Dockhead]

Originally Posted by El Pinguino

All so simple if you say it quickly.....

Altering down off the wind ( ie altering to starboard ) I would lose speed and the whole dynamic would change and if he altered to port as one would have expected very very mucky. Bearing in mind that under 2 miles is the sort of distance you expect these ships to alter for you on a good day.

Put the helm down and bringing her up into the wind? Once again the whole dynamic alters yet again and most likely keeps you on a steady bearing......

The bearing was opening so I stood on.....

And wasn't 1 cable ahead, was half a cable.
. . .

Yes, theory and practice don't always coincide. But you asked, so I answered . . . .

I was in a similar situation last year in the Baltic, except it was a big passenger ship moving at 24 knots. I couldn't make a safe alteration because I was hard on the wind on a stb tack, and the ship was to port. It was moving at 24 knots and it was a bit of a brown shorts moment.

[Kenomac]

Originally Posted by Dockhead

Yes, theory and practice don't always coincide. But you asked, so I answered . . . .

I was in a similar situation last year in the Baltic, except it was a big passenger ship moving at 24 knots. I couldn't make a safe alteration because I was hard on the wind on a stb tack, and the ship was to port. It was moving at 24 knots and it was a bit of a brown shorts moment.

If you find this sort of stuff this so unsettling, just be sure to bring plenty of extra undershorts if and when you visit the Med.

This ferry crossed our stern after passing me at less than one of his boat-lengths off our starboard beam. It was a non-event.... happens here all the time.... no chatter on the radio, no BIG DEAL. We were head on just two minutes or so prior, but at his speed, there's absolutely nothing I can do except for deliberately steering to port when I realized he intended to pass to starboard. Basically, he just swerved around us. Note: The wide angle lens distorts the distance as in "objects are closer than they appear in the mirror (camera)."

"Oh the horror.... the horror." Apocalypse Now

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

[Lodesman]

Originally Posted by El Pinguino

Where do people keep finding these 20 knot ships that take half a day to respond to the rudder?

In the real world most ships poke along at about 15 knots and where I sail 8 to 10 is more the norm.

Ping, you need to get out more. Some of us sail near major shipping ports and most transoceanic stuff runs 18-24 kts. Not saying there aren't slower vessels out there, but they exist and aren't very rare. Nobody said they take a 1/2 day to turn, now did they? Why don't you tell us the handling characteristics of the vessels you've handled?

And any ship you encounter shall be somewhere in between....... an infinite number of sizes, aspects, speeds, etc etc which makes fixating on a single scenario a bit... how do you say it?...anal?

Just working with the particular scenario that was being discussed.

So... my recent near death experience and a little bit of karma.

...By 2 or 3 miles her bearing had started to open very slighty but I was still expecting her to alter to port . She didn't and we passed maybe 100 metres ahead of her ie a ship's length.

Now where does the collective CF wisdom stand on this. When should I have altered? Which way ?

Like you, I am not inclined to turn tail every time I see a ship. Personally, I would have closed to the point where my hard turn to starboard would be inside any turn he could have achieved, and go for the red to red at half a cable. You're just lucky the idiot didn't pull a panic stbd turn at the last moment.

[El Pinguino]

Originally Posted by Lodesman

Like you, I am not inclined to turn tail every time I see a ship. Personally, I would have closed to the point where my hard turn to starboard would be inside any turn he could have achieved, and go for the red to red at half a cable. You're just lucky the idiot didn't pull a panic stbd turn at the last moment.

Last day job 20,000 grt ... 19 knots... twin screw....twin becker rudders....

Box boats, some ferrys and passo ships run at the higher speeds.... the world's tankers, bulk carriers, and all the rest rarely above 15k.... and they are a very substantial percentage of the world fleet....

The 'idiot' wasn't going to panic... he was quite happy with the situation....

[ramblinrod]

Well, it has taken ~ 3 weeks and 300+ posts, but I think we are finally getting somewhere reasonable.

[Dockhead]

Originally Posted by Lodesman

Like you, I am not inclined to turn tail every time I see a ship. Personally, I would have closed to the point where my hard turn to starboard would be inside any turn he could have achieved, and go for the red to red at half a cable. You're just lucky the idiot didn't pull a panic stbd turn at the last moment.

With similar speeds, like in Ping's case, I can see how this would work. In fact, I think I've done something like this myself.

But with a big difference in speed? Do you think this would be a safe maneuver in that case?

With a bigger difference in speed, you will get quite close to his course line by the time you approach so close, so I'm not sure it's even possible to be "inside any turn he can achieve". He can certainly move his bow over a long distance within a couple of cables, if he puts the rudder over hard (or even not so hard), and you won't be able to get out of his way if you are somewhere near his course line.

I realize this is not the scenario you were talking about, but for good order.

[El Pinguino]

Well..... if at 5 miles I had decided to do something I would have just eased sheets and come to stbd.

If at half a mile I had decided the situation wasn't exactly going to my advantage I would have gone hard a port and effectively stopped her.

Altering to starboard at that time could be fatal. This the exception that, how you say, proves the rule about never turning to port.

Re ship speeds.... just have a look at the English Channel on Marinetraffic and spend a spell going through the ships, there are fast ships, slow ships, fat ships, skinny ships.
Off Valpo or San Antonio , yes, the majority of encounters will be with fast box boats - outside any of the northern copper ports its mainly plodding bulk ships....

[Dockhead]

Originally Posted by El Pinguino

Well..... if at 5 miles I had decided to do something I would have just eased sheets and come to stbd.

If at half a mile I had decided the situation wasn't exactly going to my advantage I would have gone hard a port and effectively stopped her.

Altering to starboard at that time could be fatal. This the exception that, how you say, proves the rule about never turning to port.

Re ship speeds.... just have a look at the English Channel on Marinetraffic and spend a spell going through the ships, there are fast ships, slow ships, fat ships, skinny ships.
Off Valpo or San Antonio , yes, the majority of encounters will be with fast box boats - outside any of the northern copper ports its mainly plodding bulk ships....

That's a pretty good argument for doing it at 5 miles, iddn it? Well, 5 miles is too far if you're stand on -- but maybe 2 miles? No drama, no risk. Why wait until you're 5 cables away and have many fewer options?

[Lodesman]

Originally Posted by El Pinguino

Last day job 20,000 grt ... 19 knots... twin screw....twin becker rudders....

Box boats, some ferrys and passo ships run at the higher speeds.... the world's tankers, bulk carriers, and all the rest rarely above 15k.... and they are a very substantial percentage of the world fleet....

The 'idiot' wasn't going to panic... he was quite happy with the situation....

I'm not going to argue ships' speeds - you can have the point. I don't really see the difference; a great big bulker going 14 is still going to take a fair distance to alter course.

Your last sounds like a manoeuvrable vessel (guessing about 500' LOA). Can you educate us with the manoeuvring characteristics - cruising speed, top speed, tactical diameter, advance on 30º turn (standard rudder) and time for rudder to turn hard over?

As to the idiot, you might be right, but how can you be sure - I hadn't got from your post that talked with him?

[El Pinguino]

Originally Posted by Lodesman

I'm not going to argue ships' speeds - you can have the point. I don't really see the difference; a great big bulker going 14 is still going to take a fair distance to alter course.

Your last sounds like a manoeuvrable vessel (guessing about 500' LOA). Can you educate us with the manoeuvring characteristics - cruising speed, top speed, tactical diameter, advance on 30º turn (standard rudder) and time for rudder to turn hard over?

As to the idiot, you might be right, but how can you be sure - I hadn't got from your post that talked with him?

The ship speed thingo.... I don't dispute that there are big fast ships out there... but there are also plenty of not so fast... and to build a thread around encounters only with very big fast ships ignores the realities of life at sea...

184 metres, very manoeuverable.... stuffed if I remember all the fancy stuff ( retired for 11 years now) but at service speed you never applied more than 5* rudder max... typically 3*.... handled like I imagine destroyers handle.... could stop from 15 Knots in a few ship lengths... you don't need to know how I found that out............ pic of her here TASMANIAN ACHIEVER - IMO 9180190 - Callsign VHAF - ShipSpotting.com - Ship Photos and Ship Tracker


'Idiot' was in quotes from your previous....

Talked to him? No... despite what you may have heard about english being the international language of the sea.. it isn't down here.... and for me to enter into a conversation in spanish in such a situation would not have been conducive to a good outcome...

Last conversation I had down here - in Estrecho de Magallanes went thus.... ( in spanish ) ... us running downhill.... her a tug and tow going uphill..

'yacht, tug Elisa'
'Elisa, Westerly Serenade'
'Red Red'
'Red Red'
...
nada mas

[Lodesman]

Originally Posted by El Pinguino

to build a thread around encounters only with very big fast ships ignores the realities of life at sea...

I don't think that was the point of the thread at all - that's just where one particular line of discussion led.
It is kind of ironic that with all your protestations, the last ship you had is quite capable of going 20 kts. And I imagine a full rudder turn would catch any small boater off guard, who was expecting a lumbering slow RoT.
For general info, destroyers (as least those I know) typically use 15 degrees for standard helm and 30 for hard turns. Full rudder is between 30-40. Manoeuvres depend on a number of factors, but they're considered quite nimble so as a rule of thumb it only takes a couple hundred yards for them to complete most turns. As I said before, Cockcroft has a page that gives typical tactical diameters for various classes of ships; not an exhaustive list, but useful nonetheless.