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Dockhead 23-08-2017 08:31

Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Recreational sailors frequently have a fundamental misunderstanding of how to cross safely with ships. They don't understand the critical decision point in a crossing, misjudging it by miles, and don't understand what safe passing distances are. We recently had this discussion in another thread:


Quote:

Originally Posted by CF poster (Post 2461564)
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.

Quote:

Originally Posted by CF poster (Post 2461734)
. . . 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.


The poster believes that a mile CPA is "preposterously" much for crossing with a ship -- "180 feet" is plenty. Five miles is way too far away to get worried.

He has expressed these common misconceptions very well, and I hope he'll let me use some of his posts in the book I'm working on.

I gave him a clue, which he unfortunately did not take up. I suggested that he just do the math, but he is so sure in his visualization of how he crosses with ships that wouldn't bother. The math is not at all complicated (Trigonometry 101), and I did it myself in about 5 minutes. I hope that other boaters who suffer from the same misconceptions will learn something useful.

The popular misconception runs something like this – “My boat is highly maneuverable; I don’t need no math; all I have to do is keep a good lookout and dart out of the way if I see something scary. 5 miles is a long, long way, plenty of time to deal with it, and a one degree course change at that distance will easily get me well out of danger. 180 feet is plenty of room. 5 miles away is hull down on the horizon – we hardly even notice ships at that distance; surely a ship can’t already be dangerous so far away.”



Every sentence of this is completely wrong, and reflects a potentially fatal misconception.


Let’s say we encounter a large container ship moving at 24 knots (the latest box boats are usually somewhat slower than this, but plenty of passenger ferries travel at this and higher speeds) and having 60 meters of beam. It is 5 miles away and we are on course for a head-on collision. We are travelling at 6 knots and our boat has 4 meters of beam. We are keeping a fantastic watch, and recognize the problem and work out and execute our maneuver in two minutes. We make a one degree correction to “get me well out of danger.” What happens?


Combined speed is 30 knots, so if no one alters course, we will get run down and crushed to smithereens in 10 minutes, during which the ship will travel four miles and we will travel one mile. The place of our death will be one mile from our starting position.


But what about our one-degree course correction?


Trig tells us that an instantaneous one-degree course correction (we generously assume that our nimble little sailboat has an infinite ROT), executed two minutes after we spot the ship, will change our position 8 minutes and 8 cables later, by 0.0175 miles or 32 meters. So if the ship continues perfectly along its course, we will not move more than half his beam plus half of our own beam, and so we will be crushed. RIP, Rod.


OK, well, how about 10 degrees? Surely that will do it? An instantaneous 10 degree course change will move your position at the moment of potential collision by 0.14 miles or 261 meters. So we’re safe, right? Not so fast! IF we had perfect information, IF the ship’s GPS is exactly on the centerline, IF the ship (and we) perfectly hold course and speed as we approach each other – then yes – we will pass a little less than one cable from his side – a very close call, but not a collision.


But none of those “IF’s” is realistic – not one of them. The ship’s position at the point of potential collision is subject to a so-called cone of error – defined by adding up all of the potential errors and projecting them over time. Do we know where his GPS is? It’s included in the static AIS data, but not even displayed by recreational plotters. So the GPS receiver might be anywhere – and he has a 60 meter (200 foot) beam. So there’s plus or minus 30 meters right there. What’s the position error of his GPS? The new ones are better, but a position error of 10 meters is not unusual even with a modern set. How accurate is our data on his course? Surely not better than plus or minus a couple of degrees. A couple of degrees in the given scenario will change his position (since he is travelling four times faster than we are) by more than a cable in either direction – 0.11 miles or 207 meters. And how well is he keeping his course? Very often fast-moving ships will wander a bit, just like we do. Even one degree plus or minus of error in course keeping will add another error of plus or minus 103 meters. These are not all of the possible errors in our prediction of his position! But add just these up, and we have plus or minus 340 meters, which is nearly two cables. Your 10 degree course correction will simply enter you in a lottery, where the stakes are your life – how will the compounded errors add up? Do you feel lucky?


OK, so what if we turn 90 degrees and high tail it out of his way? This maneuver, of course, provides the best chance of a happy outcome, provided of course you are absolutely sure you are turning the right way (you will need the AIS for that, and you might need some time to discern the change of bearing – most recreational AIS displays do not tell you which way you are crossing). At 6 knots, you will get 8 cables – likely to get you out of trouble IF he doesn’t change course himself and IF you are sharp enough to realize the problem while the ship is still hull down and execute your maneuver in two minutes.


I rarely meet sailors who are that sharp. And the “I don’t need no math; collision avoidance is easy” type of sailor doesn’t even notice ships that far away.


On top of all of that -- can you assume that he will not change course? What if he is avoiding another vessel? What if he is trying to avoid you, and turns the same way you do? What if he has a turn? What if he doesn’t see you? THIS is why competent sailors set up their passes in open water to stay at least a mile, and if the waters are not congested, two miles away from passing ships, and to be reasonably safe, you need to do that, too. And why good sailors stay alert to course changes and changes in CPA, during encounters with ships, until they are safely past.


Concerning the myth of sailboats’ supposed greater maneuverability – let’s lay it to rest once and for all.


It is true that our small boats have a higher rate of turn (ROT) than big ships. So we can just “dart out of the way”, right? Well, no. If you are headed towards a collision, and you want to maneuver to get yourself into a place other than where that collision would happen, your power to do so depends not only on ROT, but speed. At very short distances (like in bays and harbors) ROT may be relatively more important, but in open water, dealing with fast ships traveling at sea speed, speed is the key factor and ROT is relatively meaningless. That is because even a fully loaded VLCC can change its course by 10 degrees in less than two ship-lengths, and two ship-lengths even of a VLCC is nothing at 5 miles out -- it's just a few seconds.



With respect to the fast box ship in the scenario above, the effect of his course change is radically different than the effect of ours, and at four miles out, will result in:


1 degree Ship 129 meters Yacht 26 meters
10 degrees Ship 1 306 meters Yacht 261 meters

So who is more maneuverable? It’s not us – and it is a gigantic misconception to think so. When encountering fast ships in open water, our slow boats are almost like sitting ducks, and more and more so, the greater the difference in speed. We can’t just “dart out of the way” at all -- that's a fantasy. So on the contrary, we have to detect potential collisions from far away and take early action. If we get in trouble, we have much less power to deal with it, than a fast-moving ship. And they don’t always see us.


Many recreational sailors feel confident in their ability to avoid collisions because they have never actually found themselves on a collision course with a ship, and so don’t think it could ever happen to them. But that is usually because ships maneuver far earlier than we typically do, and have usually maneuvered to avoid us before we are even aware that they are there. Commercial mariners usually follow the principle that all targets must be analyzed by 10 miles out, and when they see WAFIs like us, they typically take early action, because they know they can’t rely on us to know what to do. They like to give us such a wide berth that no stupid thing we could do, could cause a collision. And so your typical recreational sailor thinks collision avoidance is no big deal – because it’s been done for him all his life. Do you want to continue to depend on ships avoiding you? Or do you want to play a meaningful role yourself in avoiding collisions? It's up to you.



Like many recreational sailors, the poster above thinks that 180 feet is plenty of space, and 5 miles out is way too soon to get worried. They need to get acquainted with the math, and understand what is a "cone of uncertainty" concerning the ship's position at the point of impact. It could save lives.

ramblinrod 23-08-2017 12:35

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

Originally Posted by Dockhead (Post 2462454)
Recreational sailors frequently have a fundamental misunderstanding of how to cross safely with ships. They don't understand the critical decision point in a crossing, misjudging it by miles, and don't understand what safe passing distances are. We recently had this discussion in another thread:







The poster believes that a mile CPA is "preposterously" much for crossing with a ship -- "180 feet" is plenty. Five miles is way too far away to get worried.

He has expressed these common misconceptions very well, and I hope he'll let me use some of his posts in the book I'm working on.

I gave him a clue, which he unfortunately did not take up. I suggested that he just do the math, but he is so sure in his visualization of how he crosses with ships that wouldn't bother. The math is not at all complicated (Trigonometry 101), and I did it myself in about 5 minutes. I hope that other boaters who suffer from the same misconceptions will learn something useful.

The popular misconception runs something like this – “My boat is highly maneuverable; I don’t need no math; all I have to do is keep a good lookout and dart out of the way if I see something scary. 5 miles is a long, long way, plenty of time to deal with it, and a one degree course change at that distance will easily get me well out of danger. 180 feet is plenty of room. 5 miles away is hull down on the horizon – we hardly even notice ships at that distance; surely a ship can’t already be dangerous so far away.”



Every sentence of this is completely wrong, and reflects a potentially fatal misconception.


Let’s say we encounter a large container ship moving at 24 knots (the latest box boats are usually somewhat slower than this, but plenty of passenger ferries travel at this and higher speeds) and having 60 meters of beam. It is 5 miles away and we are on course for a head-on collision. We are travelling at 6 knots and our boat has 4 meters of beam. We are keeping a fantastic watch, and recognize the problem and work out and execute our maneuver in two minutes. We make a one degree correction to “get me well out of danger.” What happens?


Combined speed is 30 knots, so if no one alters course, we will get run down and crushed to smithereens in 10 minutes, during which the ship will travel four miles and we will travel one mile. The place of our death will be one mile from our starting position.


But what about our one-degree course correction?


Trig tells us that an instantaneous one-degree course correction (we generously assume that our nimble little sailboat has an infinite ROT), executed two minutes after we spot the ship, will change our position 8 minutes and 8 cables later, by 0.0175 miles or 32 meters. So if the ship continues perfectly along its course, we will not move more than half his beam plus half of our own beam, and so we will be crushed. RIP, Rod.


OK, well, how about 10 degrees? Surely that will do it? An instantaneous 10 degree course change will move your position at the moment of potential collision by 0.14 miles or 261 meters. So we’re safe, right? Not so fast! IF we had perfect information, IF the ship’s GPS is exactly on the centerline, IF the ship (and we) perfectly hold course and speed as we approach each other – then yes – we will pass a little less than one cable from his side – a very close call, but not a collision.


But none of those “IF’s” is realistic – not one of them. The ship’s position at the point of potential collision is subject to a so-called cone of error – defined by adding up all of the potential errors and projecting them over time. Do we know where his GPS is? It’s included in the static AIS data, but not even displayed by recreational plotters. So the GPS receiver might be anywhere – and he has a 60 meter (200 foot) beam. So there’s plus or minus 30 meters right there. What’s the position error of his GPS? The new ones are better, but a position error of 10 meters is not unusual even with a modern set. How accurate is our data on his course? Surely not better than plus or minus a couple of degrees. A couple of degrees in the given scenario will change his position (since he is travelling four times faster than we are) by more than a cable in either direction – 0.11 miles or 207 meters. And how well is he keeping his course? Very often fast-moving ships will wander a bit, just like we do. Even one degree plus or minus of error in course keeping will add another error of plus or minus 103 meters. These are not all of the possible errors in our prediction of his position! But add just these up, and we have plus or minus 340 meters, which is nearly two cables. Your 10 degree course correction will simply enter you in a lottery, where the stakes are your life – how will the compounded errors add up? Do you feel lucky?


OK, so what if we turn 90 degrees and high tail it out of his way? This maneuver, of course, provides the best chance of a happy outcome, provided of course you are absolutely sure you are turning the right way (you will need the AIS for that, and you might need some time to discern the change of bearing – most recreational AIS displays do not tell you which way you are crossing). At 6 knots, you will get 8 cables – likely to get you out of trouble IF he doesn’t change course himself and IF you are sharp enough to realize the problem while the ship is still hull down and execute your maneuver in two minutes.


I rarely meet sailors who are that sharp. And the “I don’t need no math; collision avoidance is easy” type of sailor doesn’t even notice ships that far away.


On top of all of that -- can you assume that he will not change course? What if he is avoiding another vessel? What if he is trying to avoid you, and turns the same way you do? What if he has a turn? What if he doesn’t see you? THIS is why competent sailors set up their passes in open water to stay at least a mile, and if the waters are not congested, two miles away from passing ships, and to be reasonably safe, you need to do that, too. And why good sailors stay alert to course changes and changes in CPA, during encounters with ships, until they are safely past.


Concerning the myth of sailboats’ supposed greater maneuverability – let’s lay it to rest once and for all.


It is true that our small boats have a higher rate of turn (ROT) than big ships. So we can just “dart out of the way”, right? Well, no. If you are headed towards a collision, and you want to maneuver to get yourself into a place other than where that collision would happen, your power to do so depends not only on ROT, but speed. At very short distances (like in bays and harbors) ROT may be relatively more important, but in open water, dealing with fast ships traveling at sea speed, speed is the key factor and ROT is relatively meaningless. That is because even a fully loaded VLCC can change its course by 10 degrees in less than two ship-lengths, and two ship-lengths even of a VLCC is nothing at 5 miles out -- it's just a few seconds.



With respect to the fast box ship in the scenario above, the effect of his course change is radically different than the effect of ours, and at four miles out, will result in:


1 degree Ship 129 meters Yacht 26 meters
10 degrees Ship 1 306 meters Yacht 261 meters

So who is more maneuverable? It’s not us – and it is a gigantic misconception to think so. When encountering fast ships in open water, our slow boats are almost like sitting ducks, and more and more so, the greater the difference in speed. We can’t just “dart out of the way” at all -- that's a fantasy. So on the contrary, we have to detect potential collisions from far away and take early action. If we get in trouble, we have much less power to deal with it, than a fast-moving ship. And they don’t always see us.


Many recreational sailors feel confident in their ability to avoid collisions because they have never actually found themselves on a collision course with a ship, and so don’t think it could ever happen to them. But that is usually because ships maneuver far earlier than we typically do, and have usually maneuvered to avoid us before we are even aware that they are there. Commercial mariners usually follow the principle that all targets must be analyzed by 10 miles out, and when they see WAFIs like us, they typically take early action, because they know they can’t rely on us to know what to do. They like to give us such a wide berth that no stupid thing we could do, could cause a collision. And so your typical recreational sailor thinks collision avoidance is no big deal – because it’s been done for him all his life. Do you want to continue to depend on ships avoiding you? Or do you want to play a meaningful role yourself in avoiding collisions? It's up to you.m



Like many recreational sailors, the poster above thinks that 180 feet is plenty of space, and 5 miles out is way too soon to get worried. They need to get acquainted with the math, and understand what is a "cone of uncertainty" concerning the ship's position at the point of impact. It could save lives.

Well, you've posted a great deal in support of your misperception of my position.

When I mentioned, the 1 degree course change, it was with respect to a situation that was not dire, where this is all that is necessary to get out of a collision situation. Of course if circumstances demand a sharper turn, a sharper turn is required. To put this in proper context, it was in repsonse to those suggesting it be necessary to stay more than 1nm away at all times.

I was responding to how that is nonsense and not even possible in lots of cases.

If a 1 degree course change 5 nm or more away, gets you to a stand-on position, that is all that is required. You do not have to make a 30 degree course change (for example) to get clear by a mile.

Pelagic 23-08-2017 12:41

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
I'd hate to face you in Court Dockhead [emoji4]
Well done!

JPA Cate 23-08-2017 13:05

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

Trig may be basic math to you, but I never had anything beyond high school algebra. Now, my guess is that many, maybe even most of the men here have that knowledge, but unless you propose a Trig for Dummies class here on CF, can you show us maths ignoramuses another way to do this, please?

I do know about closing speeds, and I do know to make large course changes, which have worked so far ;-), both in terms of crowded (SF Bay) and uncrowded (shipping lanes in Oz), but for the arithmetically or mathematically challenged, is there a short cut?

Thanks.

Ann

Dockhead 23-08-2017 13:10

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

Originally Posted by JPA Cate (Post 2462618)
Dockhead,

Trig may be basic math to you, but I never had anything beyond high school algebra. Now, my guess is that many, maybe even most of the men here have that knowledge, but unless you propose a Trig for Dummies class here on CF, can you show us maths ignoramuses another way to do this, please?

I do know about closing speeds, and I do know to make large course changes, which have worked so far ;-), both in terms of crowded (SF Bay) and uncrowded (shipping lanes in Oz), but for the arithmetically or mathematically challenged, is there a short cut?

Thanks.

Ann

To avoid the trig -- just use a triangle solver. Millions of them available online.

These will give you all three angles and all three sides of any triangle, if you give any three. So for any problem such as this -- make a right triangle with one defined angle 90 degrees, the other defined angle whatever variable you have -- like your 1 degree or 10 degree course change. Define one side as the distance to the collision point. Then the solver will give you the two missing sides -- how far away you got from the collision point being the shorter of the two new sides.

ramblinrod 23-08-2017 13:37

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

Originally Posted by Pelagic (Post 2462603)
I'd hate to face you in Court Dockhead [emoji4]
Well done!

Actually, straw man arguments don't work in court.

boatman61 23-08-2017 13:47

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

Originally Posted by JPA Cate (Post 2462618)
Dockhead,

Trig may be basic math to you, but I never had anything beyond high school algebra. Now, my guess is that many, maybe even most of the men here have that knowledge, but unless you propose a Trig for Dummies class here on CF, can you show us maths ignoramuses another way to do this, please?

I do know about closing speeds, and I do know to make large course changes, which have worked so far ;-), both in terms of crowded (SF Bay) and uncrowded (shipping lanes in Oz), but for the arithmetically or mathematically challenged, is there a short cut?

Thanks.

Ann

Likewise Ann.. however during many miles back, forth and along the English Channel I've never come close to being run down and my method is simple.. line the approaching/crossing vessel with a stanchions and it'll let you know if the ship will pass the bow or stern.. if it holds steady alter course.
Only thing that's ever hit me was a French yacht.. they hate Brits. :biggrin:

ramblinrod 23-08-2017 14:05

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

Originally Posted by boatman61 (Post 2462644)
Likewise Ann.. however during many miles back, forth and along the English Channel I've never come close to being run down and my method is simple.. line the approaching/crossing vessel with a stanchions and it'll let you know if the ship will pass the bow or stern.. if it holds steady alter course.
Only thing that's ever hit me was a French yacht.. they hate Brits. :biggrin:

Precisely.

Some here seem to be taking themselves way too seriously.

If one is crossing shipping lanes in a small boat, all that is required, is a changing angels to pass astern, and it most certainly doesn't have to be by a mile. In fact, if there is only 2 miles between ships in the lane, 1 mile astern the first is not desireable at all.

JPA Cate 23-08-2017 14:32

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

Originally Posted by boatman61 (Post 2462644)
Likewise Ann.. however during many miles back, forth and along the English Channel I've never come close to being run down and my method is simple.. line the approaching/crossing vessel with a stanchions and it'll let you know if the ship will pass the bow or stern.. if it holds steady alter course.
Only thing that's ever hit me was a French yacht.. they hate Brits. :biggrin:

Thanks for that, Boatie, I understand that method. Mostly use a hand bearing compass for rate of change on their course, also use radar, and peek at the AIS. I think I'm safe enough. I actually think you are making a right angle triangle with your boat as the shortest leg, but I may be mis-conceptualizing.

I appreciate your method for its dead simplicity. Even exhausted, you could make it work. I probably couldn't add 2 + 2 when I'm really exhausted. :wink:

Ann

barnakiel 23-08-2017 14:34

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

Be seen (AIS). Be heard (vhf). Sail.

Imho 1 Nm is a close encounter and when the CPA prediction gets down to 0.5 I get itchy all over.

But it may be just me. Maybe other sailors are happy with a cable or so.

Cheers,
b.

Cheechako 23-08-2017 14:46

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Not only recreational but navy watchmen also!
The basic rule: "if a boat's bearing to your boat doesn't change as you move along... you are on a collision course..."

Seaslug Caravan 23-08-2017 14:54

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

Originally Posted by boatman61 (Post 2462644)

I've never come close to being run down and my method is simple.. line the approaching/crossing vessel with a stanchions and it'll let you know if the ship will pass the bow or stern.. if it holds steady alter course.

:biggrin:[/SIZE][/FONT]

Yep , youv'e nailed it. end of thread.

StuM 23-08-2017 17:24

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

Originally Posted by barnakiel (Post 2462677)

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

.

Oh-oh, here we go again :popcorn:

StuM 23-08-2017 17:29

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

Originally Posted by Cheechako (Post 2462686)
Not only recreational but navy watchmen also!
The basic rule: "if a boat's bearing to your boat doesn't change as you move along... you are on a collision course..."

It's actually set out in COLREGs.

Rule 7 - Risk of Collision
...such risk shall be deemed to exist if the compass bearing of an approaching vessel does not appreciably change


El Pinguino 23-08-2017 19:29

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

Originally Posted by boatman61 (Post 2462644)
Likewise Ann.. however during many miles back, forth and along the English Channel I've never come close to being run down and my method is simple.. line the approaching/crossing vessel with a stanchions and it'll let you know if the ship will pass the bow or stern.. if it holds steady alter course.
Only thing that's ever hit me was a French yacht.. they hate Brits. :biggrin:

What he said ^^^^^ :thumb:

El Pinguino 23-08-2017 19:34

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

Originally Posted by barnakiel (Post 2462677)
....
A sailing boat is supposedly the one with the right of way (over a steamer). Most of the time.
.........

Cheers,
b.

Given that the only 'steamers' you are going to see these days are merkin aircraft carriers and SSNs ( and you won't be seeing them ) I wouldn't be pushing my luck :biggrin:

ramblinrod 23-08-2017 19:35

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

Originally Posted by StuM (Post 2462762)
It's actually set out in COLREGs.

Rule 7 - Risk of Collision
...such risk shall be deemed to exist if the compass bearing of an approaching vessel does not appreciably change


Yup, when you operate a small boat, collision avoidance is not that difficult if one simply keeps watch, stands on, gives way, and applies some common sense as required.

The is no math to do, no cones of silence required. Just don't f' around in the shipping lanes and you'll be fine.

Most intelligent sailors don't run up the centre of a shipping lane, route, or course head on. When one is crossing a ships course, at right angles (as they should), turning off a little bit from 5 nm away is more than ample. If in the event one miscalculates, just luff up until you can safely pass astern. And yes, there is absolutely no reason to pass a full mile astern. You can if you wish, and it is safe to do so, but a full mile certainly isn't necessary.

David M 23-08-2017 20:59

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
This is why the Rules say "early" and "apparent" action to avoid a collision. You don't even have to know second-grade addition in order to comply with early and apparent.

If you get to the point of an imminent collision, you have already broken at least one rule.

Apparent means very obvious to the other vessel. One degree of course change....bad. 30 degrees of course change....good.

Jim Cate 23-08-2017 23:22

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

Most intelligent sailors don't run up the centre of a shipping lane, route, or course head on.
True, of course, but there are places where meeting ships on near reciprocal courses is fairly common, and that do not meet your classification.

EG, up and down the east coast of Australia. Not as busy as the English Channel, but with significant ship traffic, some N bound, some S bound and some peeling off for destinations to the East. There are no designated "shipping lanes", no TSS spots, and one finds ships at varying distances from the coast, going both ways. And it isn't always a matter of avoiding just one, nor of the conflicting ships all being going the same way. When you are caught in the middle of a pair of N and a pair of S bound ships, it isn't all that simple to calculate how to dodge them all, so standing on as required by COLREGS seems a good practice, especially if you are broadcasting AIS info.

Life here has been much better since I installed AIS 1

Jim

conachair 23-08-2017 23:39

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

Originally Posted by ramblinrod (Post 2462823)
Yup, when you operate a small boat, collision avoidance is not that difficult if one simply keeps watch, stands on, gives way, and applies some common sense as required.

The is no math to do, no cones of silence required. Just don't f' around in the shipping lanes and you'll be fine.

Most intelligent sailors don't run up the centre of a shipping lane, route, or course head on. When one is crossing a ships course, at right angles (as they should), turning off a little bit from 5 nm away is more than ample. If in the event one miscalculates, just luff up until you can safely pass astern. And yes, there is absolutely no reason to pass a full mile astern. You can if you wish, and it is safe to do so, but a full mile certainly isn't necessary.

easy peasy ;)

https://youtu.be/pzJwXxUY3MM

BTW, ships will often have standing orders for no CPA less than 1 mile so you being getting battered by the wash tucked in to their stern will freak them out a bit, not a very nice thing to do. In the channel anyway, with the miracle of ais it's common to have them manoeuvre to give you exactly 1nm CPA, which is you crossing thier bows maybe 3nm ahead.




What happened to the add video button?

bobgarrett 24-08-2017 01:18

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Some excellent thoughts on this topic, and we all must bear in mind that ship collisions are rare.
But why is that? Because of the colregs and the professional application of them.
Like others I will look at handheld compass bearings as well as AIS. However, those who think they have avoided a low CPA by a minor course correction might learn otherwise if they had also watched their AIS and seen a ROT figure for the ship a few miles away. I would contend that it may well have been the ship that avoided the collision not you.

StuM 24-08-2017 01:20

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

Originally Posted by Jim Cate (Post 2462910)
True, of course, but there are places where meeting ships on near reciprocal courses is fairly common, and that do not meet your classification.

EG, up and down the east coast of Australia. Not as busy as the English Channel, but with significant ship traffic, some N bound, some S bound and some peeling off for destinations to the East. There are no designated "shipping lanes", no TSS spots, and one finds ships at varying distances from the coast, going both ways. And it isn't always a matter of avoiding just one, nor of the conflicting ships all being going the same way. When you are caught in the middle of a pair of N and a pair of S bound ships, it isn't all that simple to calculate how to dodge them all, so standing on as required by COLREGS seems a good practice, especially if you are broadcasting AIS info.

Life here has been much better since I installed AIS 1

Jim

Generally, I find the problems occur when they peel off to destinations to the West (i.e they are heading in to Gladstone, Rockhampton, Mackay,Townsville etc while I'm transiting North or South and holding just inshore of the two way routes) :)

four winds 24-08-2017 01:39

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
I've crossed with big ships offshore maybe about a dozen times only. Two in the last week while coming back up to AL from FL.

On a good night I can see the glow off the deck lights before the ship appears. And begin noticing the bearing then. A few times I have been able to notice thier course change from a constant bearing. How did they know I was under sail, maybe my speed. Maybe they eventually see my lights showing sailing. I don't know.

But my hat's off to the professional mariners out there. I feel comfortable saying that each time they passed one or two miles ahead or astern.

I do my part, I stand on. And they appreciate it. Once, a skipper hailed and said, "Hey sailboat, thanks for being a stand-up stand on sailor". I called back, "Your welcome cap, ..... wait, there's a boat out there! ..(slight pause, big laugh)... gotcha!, have a good night".

I've never had to alter course for a big ship offshore.

Alan Mighty 24-08-2017 01:56

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

Originally Posted by StuM (Post 2462928)
Generally, I find the problems occur when they peel off to destinations to the West (i.e they are heading in to Gladstone, Rockhampton, Mackay,Townsville etc while I'm transiting North or South and holding just inshore of the two way routes) :)

I've found that Gladstone Harbour and Brisbane Harbour give excellent service to yachties. Gladstone Harbour requires (or used to) yachties to log in with them by VHF. In Moreton Bay, Brisbane Harbour on VHF12 gives clear info on traffic and in the past has delivered good service disciplining both commercial and recreational traffic.

See the AIS heat map from marinetraffic dot com: E Aus Coast heat map.gif

Alan Mighty 24-08-2017 02:02

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
The AIS heat map around your waters, StuM, looks pretty impressive too.

rramsey 24-08-2017 03:00

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
There's another thread on the recent collision of a USN vessel (US Navy Destroyer Collision Again!!!) that might be of interest to this discussion.

As I understand it, USN vessels often run "dark", e.g. no way to pinpoint them. No radar, I presume, no AIS. I am not sure if that means no receiving AIS because that too might be detectable. This means that the sole responsibility for avoiding collisions rests on the USN vessel. We probably should assume that the crew is highly trained and very professional and yet collisions seem to happen. Assuming they run dark, the other vessel involved has no way of avoiding the collision as it is not aware in any way that there is a USN vessel crossing their bow.

That (maybe) being the case it shows that avoiding collisions is best done by all vessels involved and that it is also up to us, as WAFI's, to do our part. Being slow and small, I think the best we can do is, to be abundantly clear about our intentions (e.g. don't suddenly change course if not absolutely necessary) and radio in. I have been told by captains of large vessels that they anticipate well in advance and change course if required. They hate WAFI's then changing course (in the false belief that that avoids collision) without need or communication, thereby bringing themselves back into the path that leads to recreational antifouling getting a sun tan.

Abundantantly clear, to me, means anticipate well in advance, 5nm is probably reasonable (as it is 10 to 15 minutes to CPA for large vessels), make your move and stick to it. If the large vessel changes course and brings you in danger, get on the radio immediately to coördinate a safe solution.You have only minutes to avoid a collision.

Maybe a silly remark, but still: if in doubt, aim for the other vessel. By the time you get there it will be long gone (unless it is anchored of course, in which case you're gonna look as silly as a baboon for sinking your yacht). Don't keep aiming for it, of course, aim once and stick to your course.

ps. Have also seen 30ft yachts trying to sink a 300mtr container ship. Unsung heroes of our messed-up gene pool :trash:

rabbi 24-08-2017 03:01

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
AIS is a wonderful thing. not always.
Standing on is a wonderful thing. not always.

AIS plus standing on can be dangerous.


When we first added an active AIS to our boat I expected that this would make us more visible and give clear info on our course to all big ships out there. So standing on would be more prudent than altering course to avoid close encounters, which we did before.

Didn't really work out well: On our first night passage after adidng AIS we were sailing between Italy and greece, with lights on, broadcasting our AIS position. We have been passed very close by a huge container ship and a cruise ship.

CPA was around 300m but felt much closer (no clue if the antenna position was part of the calculation). Not a problem for them but frightening for us.
I bet without AIS they would have stayed clear, but AIS told them we are safe so they just kept course.

If huge ships are passing us with 30kn there is not much we can do, especially not if we stand on until the last moment. Now we alter course early and significantly enough for them to notice, and/or we hail them on VHF.
Just what we did before we had AIS.

Dockhead 24-08-2017 03:28

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

Originally Posted by ramblinrod (Post 2462823)
Yup, when you operate a small boat, collision avoidance is not that difficult if one simply keeps watch, stands on, gives way, and applies some common sense as required.

The is no math to do, no cones of silence required. Just don't f' around in the shipping lanes and you'll be fine.

Most intelligent sailors don't run up the centre of a shipping lane, route, or course head on. When one is crossing a ships course, at right angles (as they should), turning off a little bit from 5 nm away is more than ample. If in the event one miscalculates, just luff up until you can safely pass astern. And yes, there is absolutely no reason to pass a full mile astern. You can if you wish, and it is safe to do so, but a full mile certainly isn't necessary.

And this, on the other hand, I agree with completely.

I've written it many times on here -- if you are sailing in harbors or bays or approaches to harbors, then you have a huge advantage compared to sailing in open water -- you know where the ship is going to be. All you have to do is stay out of the channel or fairway until he is safely past. This makes all other collision avoidance techniques unnecessary.

And yes, passing behind by a couple of cables can be fine, when you're crossing a fairway or channel and it is clear you will actually pass behind.


The only thing wrong is projecting this formula onto an entirely different situation -- a crossing in open water. The problem is that in open water, unlike in pilotage waters where the commercial traffic is following defined fairways or channels, you cannot know exactly where he will be 10 minutes from now. It was said in another thread, something like "You don't have to be a mile away, you just have to avoid being in the exact same place as he is -- it's easy." The problem is -- what is this "exact same place"? You don't know and can't know. "Just alter course by one degree at five miles and you'll be fine -- 180 feet is enough space." I hope it's now clear why this doesn't work.

rramsey 24-08-2017 03:29

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

Originally Posted by rabbi (Post 2462945)
AIS is a wonderful thing. not always.
Standing on is a wonderful thing. not always.

AIS plus standing on can be dangerous.


When we first added an active AIS to our boat I expected that this would make us more visible and give clear info on our course to all big ships out there. So standing on would be more prudent than altering course to avoid close encounters, which we did before.

Didn't really work out well: On our first night passage after adidng AIS we were sailing between Italy and greece, with lights on, broadcasting our AIS position. We have been passed very close by a huge container ship and a cruise ship.

CPA was around 300m but felt much closer (no clue if the antenna position was part of the calculation). Not a problem for them but frightening for us.
I bet without AIS they would have stayed clear, but AIS told them we are safe so they just kept course.

If huge ships are passing us with 30kn there is not much we can do, especially not if we stand on until the last moment. Now we alter course early and significantly enough for them to notice, and/or we hail them on VHF.
Just what we did before we had AIS.

You're probably right ... AIS should not change our behavior. It just means you and they see things sooner.

Pelagic 24-08-2017 03:36

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
Many here are over thinking the issue of how much for course change ....and trying to create a one fits all guideline.

Firstly COLREGS recommends a course change that is sufficient and READILY APPARENT TO THE OTHER VESSELS.

That makes sense and a simple technicque I employ at night when making a decision to alter course is to OVEREMPHASIZE the course change at fist.

That often results in a change in the running lights they see from me, which I hold for about 5 minutes if sea room allows, so they know my intentions.

I also do that during daylight

Then I bring back course to an appropriate CPA.

Appropriate CPA is really dependant on sea room.

If you have lots of sea room, what is to be gained from being inside 1nm CPA. ...??

Masters give standing orders of calling them to the bridge inside 1 nm CPA.... to make sure they are not disturbed with stupid stuff.

So why not in a passing situation simultaneously adjust so that both captains can get some rest?

Obviously in a limiting channel 1nm is not practical and the captain should be assisting the watch keeper.

Hope all that makes sense.

rramsey 24-08-2017 03:36

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

Originally Posted by Dockhead (Post 2462953)
And this, on the other hand, I agree with completely.

I've written it many times on here -- if you are sailing in harbors or bays or approaches to harbors, then you have a huge advantage compared to sailing in open water -- you know where the ship is going to be. All you have to do is stay out of the channel or fairway until he is safely past. This makes all other collision avoidance techniques unnecessary.

And yes, passing behind by a couple of cables can be fine, when you're crossing a fairway or channel and it is clear you will actually pass behind.


The only thing wrong is projecting this formula onto an entirely different situation -- a crossing in open water. The problem is that in open water, unlike in pilotage waters where the commercial traffic is following defined fairways or channels, you cannot know exactly where he will be 10 minutes from now. It was said in another thread, something like "You don't have to be a mile away, you just have to avoid being in the exact same place as he is -- it's easy." The problem is -- what is this "exact same place"? You don't know and can't know. "Just alter course by one degree at five miles and you'll be fine -- 180 feet is enough space." I hope it's now clear why this doesn't work.

Apart from that, the wind that is propelling you rapidly and safely across the bow of the containership might, and probably will (according to Murphy), suddenly disappear, changing the equation entirely.

rramsey 24-08-2017 03:39

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

Originally Posted by Pelagic (Post 2462956)
That often results in a change in the running lights they see from me, which I hold for about 5 minutes if sea room allows, so they know my intentions

Then I bring back course to an appropriate CPA.

Would it not scare the hell out of the other vessel if you seem to change to a safe course and then change course again?

El Pinguino 24-08-2017 03:49

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

Originally Posted by rramsey (Post 2462959)
Would it not scare the hell out of the other vessel if you seem to change to a safe course and then change course again?

I would have thought so....

I will, if altering to stb.... show them red and keep showing red while describing an arc under their stern...

Pelagic 24-08-2017 03:50

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

Originally Posted by rramsey (Post 2462959)
Would it not scare the hell out of the other vessel if you seem to change to a safe course and then change course again?

Not at all and COLREGS actually warns against making a series of small course alterations

By changing your 'Aspect' dramatically for 5 minutes, or so, it allows the WK on other boat, who may not have a good radar to understand your intentions.

First they see two running lights, then only the red for 5 minutes, then the red and flickering green as you slew around but by then, they know what you are doing until they pass clear at 1nm

The problem with many mariners is that they haven't done the longhand plotting to appreciate what "ASPECT" means in navigation and depend on their AIS or visibility to give them the decision making info.

This video demonstrates what you should be able to do mentality and why Aspect is important
https://youtu.be/ZOVwM-mFpXE

Dockhead 24-08-2017 03:57

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

Originally Posted by rabbi (Post 2462945)
AIS is a wonderful thing. not always.
Standing on is a wonderful thing. not always.

AIS plus standing on can be dangerous.


When we first added an active AIS to our boat I expected that this would make us more visible and give clear info on our course to all big ships out there. So standing on would be more prudent than altering course to avoid close encounters, which we did before.

Didn't really work out well: On our first night passage after adidng AIS we were sailing between Italy and greece, with lights on, broadcasting our AIS position. We have been passed very close by a huge container ship and a cruise ship.

CPA was around 300m but felt much closer (no clue if the antenna position was part of the calculation). Not a problem for them but frightening for us.
I bet without AIS they would have stayed clear, but AIS told them we are safe so they just kept course.

If huge ships are passing us with 30kn there is not much we can do, especially not if we stand on until the last moment. Now we alter course early and significantly enough for them to notice, and/or we hail them on VHF.
Just what we did before we had AIS.


My advice about this:

First of all, don't stand on "until the last moment", ever. The bigger the difference in speed between you and the target, the less power you have to alter the crossing. So any effective move has to be made early.

The only purpose of standing on is to give the give-way vessel the right to set up the crossing they way he wants to. If you have reason to believe he's not going to claim that right, and is not going to alter course, then there is no point in standing on, and you are legally free to act yourself. In open water, a ship will almost never maneuver at less than 3 miles out, unless he's dealing with multiple situations or there is some obstacle he has to get around before turning, and when they meet a WAFI, the maneuver is often made at as much as 10 miles out. So if you've been watching him (as you should have been) and he hasn't made a course alteration by 4 miles out, and you are not comfortable with the CPA (and 300 meters is far too close!), then it's time for you to get ready to make your own move.

The other thing I would say is that I have never in my life seen a ship intentionally pass within 300 meters of another vessel, in open water, without some really good reason. Standing orders will be to maintain 1 mile -- if not more. I would say in the situation you described -- he didn't see you. AIS greatly increases your changes of being seen, but it's not magic -- and not all commercial ships' bridges are manned by competent (or awake) people.

I just yesterday arrived in Dover after crossing the North Sea for the eighth time. We had risk of collision crossings with at least 100 ships, not counting the fishing vessels and yachts (although there are very few yachts in the middle of the North Sea). Including the Queen Elizabeth herself, who made an impressive sight at night! Who passed us in a TSS. It went very smoothly, and despite a few multiple-target situations, not a single radio call was required (but I listened to dozens of passing arrangements being made between other vessels). Every ship we crossed with, without exception, gave way when required, and stood on, when required, and we did the same, and so everything went smoothly. We kept a motoring cone up while motorsailing to claim our give-way status with regard to ships coming from starboard. Ships in the North Sea may behave differently from those in the Med -- the traffic here is so intense that no one can get away with a slack watch.

Dockhead 24-08-2017 04:13

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

Originally Posted by rramsey (Post 2462958)
Apart from that, the wind that is propelling you rapidly and safely across the bow of the containership might, and probably will (according to Murphy), suddenly disappear, changing the equation entirely.

Absolutely right. That's a bloody terrifying prospect, too.

To stay safe, you have to cross with other vessels with enough distance to deal with possible variations in course and speed of both vessels. You can't set up a close pass and expect to correct it at the last moment, if something changes. As you get closer and closer to a fast-moving ship, your ability to change the meeting point with that ship disappears fast, and at some point, you are just a sitting duck. So you need to set up the crossing from the beginning, to give you enough margin for error to deal with this kind of thing.

I, for one, never ever cross as close as one mile in front of a fast-moving ship, under sail. I might do it with the engine running, if I have to, but what if there is a sudden lull in the wind? I much prefer crossing behind in any case, because although different things may cause us to go slower than we planned, there is little risk that a large ship will slow down. So if Murphy happens, and we're crossing behind, the CPA will only increase so there's no problem. Whereas, if you are crossing ahead, any loss of speed will reduce the CPA.

I had a discussion about this, by the way, with some commercial skippers on GCaptain, and they were surprised to hear that I preferred for them to cross ahead of us. They did understand, though, when I explained the above.


I was crossing the Gulf of Finland a couple of years ago, sailing fast and hard in a strong W wind, and encountered a Russian tanker sailing West out of St. Petersburg. He had seen me and had set up a 1 mile CPA -- but I was passing ahead, and his solution was based on my current speed of over 9 knots, which I could not be certain of maintaining over the 20 minutes or so until the crossing. It could have gotten very dangerous, very fast, if the wind dropped a bit. So what did I do? I should have probably hove-to and waited for him to get past, but I hated to interrupt this beautiful fast sail, so I called him up on the VHF. I explained the situation and he was quite happy to alter to starboard a bit to give me more room.

Quebramar 24-08-2017 05:03

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

Originally Posted by Jim Cate (Post 2462910)
True, of course, but there are places where meeting ships on near reciprocal courses is fairly common, and that do not meet your classification.

EG, up and down the east coast of Australia. Not as busy as the English Channel, but with significant ship traffic, some N bound, some S bound and some peeling off for destinations to the East. There are no designated "shipping lanes", no TSS spots, and one finds ships at varying distances from the coast, going both ways. And it isn't always a matter of avoiding just one, nor of the conflicting ships all being going the same way. When you are caught in the middle of a pair of N and a pair of S bound ships, it isn't all that simple to calculate how to dodge them all, so standing on as required by COLREGS seems a good practice, especially if you are broadcasting AIS info.

Life here has been much better since I installed AIS 1

Jim



I'm regularly crossing the Channel close to its busiest/scariest area North of Sandettie where TSS's from Antwerp, Rotterdam and Scotland merge to get to Dover Straight. It is a very busy area and what makes it scary is that when you've crossed one lane, you cannot expect to immediately cross the other one in opposite direction, making the 'look left... look right' method very insufficient.

AIS is clearly my friend in such a case, and clearly at a 10NM distance from me so I can assess (not especially take action immediately). That is the area where it happened to me to heave-to, waiting for the complex multi-target situation to clarify and possibly resolve.

One issue I'm facing is how to get to clarify where Vessels (mine and the individual target identified) will be at CPA. My B class transponder does not provide this info and I am wondering whether a simple (...) trig formula could help figure out. That would help assess my next steps, for I'm fine crossing 3 cables astern but not less than a full NM ahead.

For those in doubt of manoeuvrability of sailboats in this kind of situation, may I point to the video of the collision of R. Wilson on YouTube : with full sail (genny) the boat seemed to have (surprisingly as per the skipper) stalled right in front of the tanker...

In such place I mentioned above, I'm always ready to crank the engine if need be in order to shorten the time in the TSS's... safety first.

Happy to hear from more experienced sailors how to interpret where vessels will cross at CPA

Jhhastie 24-08-2017 08:21

Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA
 
One of the best threads I have ever read.


Sent from my iPad using Cruisers Sailing Forum

S/V Alchemy 24-08-2017 08:34

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

Originally Posted by boatman61 (Post 2462644)
Likewise Ann.. however during many miles back, forth and along the English Channel I've never come close to being run down and my method is simple.. line the approaching/crossing vessel with a stanchions and it'll let you know if the ship will pass the bow or stern.. if it holds steady alter course.
Only thing that's ever hit me was a French yacht.. they hate Brits. :biggrin:

That's exactly how I do it, save that now I have AIS to tell me CPA and TCPA...and I confirm the CPA by watching those stanchions. In fact, a big freighter steaming SW on Lake Ontario crossed us, motoring NNW, just a few days ago, and, oddly I thought, going slower at 6 knots than us motoring at 6.8 knots (very little wind about). I picked it up at six NM range, confirmed its rough course and speed by binoculars and a hand-bearing compass, and determined it would never get closer than 0.90 NM...which was fine by me. I saw another boat motoring even faster for some reason on a parallel course on its port side. I thought this unnecessary as these ships either typically continue to Hamilton (hence SW) or turn SSE into the Welland Canal to go uplake. The second course would potentially bugger the sailboat if it wasn't doing the same thing.

My point is "avoid big ships". COLREGS may favour you, but physics won't.

wjhutchings 24-08-2017 08:39

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

Originally Posted by boatman61 (Post 2462644)
Likewise Ann.. however during many miles back, forth and along the English Channel I've never come close to being run down and my method is simple.. line the approaching/crossing vessel with a stanchions and it'll let you know if the ship will pass the bow or stern.. if it holds steady alter course.
Only thing that's ever hit me was a French yacht.. they hate Brits. :biggrin:

Agreed it is a good thread, however Boatman61 has it in one, I spent 30 odd years on those things yachties love to hate, merchant ships, from deck cadet to Master, and the best bit of anti collision information I was given was pick your spot on the bridge and stay there, line vessels up with a window frame whatever, stanchion is just as good, if the other vessel move off the bearing towards the bow it is crossing, if moves aft you are crossing if stays on the bearing you are on collision course. Once you have ascertained this then get serious and look at CPA's etc. Remember this you can cross astern of a vessel in perfect safety once you see its stern it's separation distance is increasing, but never never try cutting across the bow of a ship, one because they loose sight of you very easily under the bow and if something goes wrong whilst attempting this manoeuvre then you will be run down


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