Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

[IslandHopper]

Originally Posted by ramblinrod

Yes, visually by day, by night, and by AIS. If visibility is clear, it is easy to see a 40 ft motor vessel 5 miles off. A 400 ft ship at 20, no problem.

From our marina we can see the smokestack at the Olcott power plant, which I believe is 45 miles across Lake Ontario.

What model drone do you use?

[StuM]

Originally Posted by ramblinrod

Yes, visually by day, by night, and by AIS. If visibility is clear, it is easy to see a 40 ft motor vessel 5 miles off. A 400 ft ship at 20, no problem.

From our marina we can see the smokestack at the Olcott power plant, which I believe is 45 miles across Lake Ontario.

Assuming that your eye is 10 ft above sea level on the deck of your boat, you won't see any part of that 400 ft ship at 20 nm that is less than 250 ft above the waterline.

Realistically, 10 nm is about the limit of visibility of a ship from the deck of a small craft.

[Jim Cate]

Realistically, 10 nm is about the limit of visibility of a ship from the deck of a small craft.

Not for some folks, unless there is Kryptonite nearby. X-ray vision and all that...

Jim

[Lodesman]

Originally Posted by StuM

Assuming that your eye is 10 ft above sea level on the deck of your boat, you won't see any part of that 400 ft ship at 20 nm that is less than 250 ft above the waterline.

Realistically, 10 nm is about the limit of visibility of a ship from the deck of a small craft.

I assume a Douglas 32 has a height of eye (standing at the helm) of about 6 ft - that gives distance to horizon of roughly 3 miles. For a distance to horizon for the remaining 17 miles, a height of 210 ft is required. (Bowditch table 12)

Not many 400 ft vessels will be 210 ft tall.

[fallingeggs]

Originally Posted by Dockhead

For the course correction to be meaningful, it must be big enough to disengage your cone of probable position from his cone of probable position.

The physical dimensions of the vessels are meaningless, because you don't know the probable position of the vessels to that degree of accuracy. But that does NOT mean you know nothing! You DO know within a circle of about half a mile, if you are a competent navigator! You need to draw a circle around both vessels with diameter equal to the degree of uncertainty at CPA, and plot a course which makes these circles NOT intersect.

That is an effective maneuver.

A change of course which might give you 180 feet of separation in case you had perfect knowledge of both vessels position is meaningless. All one degree course changes are meaningless from the very beginning because you can't even hold a course that closely. Forget the 400 foot length of the ship. You can't place that 400 foot length, in order to avoid it. You are rather trying to avoid a zone which is maybe half a mile across, inside which he could be if he holds his course and speed. And forget your 23 foot length (or whatever) of your boat -- you may be anywhere in your own half mile circle, at the moment the crossing occurs.

You must get these circles apart, and you must do it early enough when you still have the power to change the geometry of the crossing.

DH, you've explained this concept six (or 60?) ways to Sunday on this thread. This last reiteration is the best for my visual understanding: "Disengage your cone of probable position from his cone of probable position."

Cheers!

PS - you have an extreme level of patients. I'm sure it is appreciated by many lurkers here, including myself.

[skipmac]

Originally Posted by El Pinguino

Fixed that for you...

Actually I did think about that point but for the sake of simplicity I ignored that little bit. Wanted to make it very easy to understand for all readers.

Also, the extra 6' or so height of eye from the cockpit of a sailboat contributes very little (I estimate maybe 2-3% or roughly 1/4 - 1/3 nm at 12 miles) to the visible range compared to the superstructure of the ship.

Originally Posted by El Pinguino

And also 400 foot isn't very big..... 600 foot and up is the norm for most these days....

A lot of the big container ships are now over 1000' but the height of the superstructures I think is still in the 200-250' range. I am not so sure about that spec but I'm pretty sure they aren't reaching 300'.

[Dockhead]

Originally Posted by fallingeggs

DH, you've explained this concept six (or 60?) ways to Sunday on this thread. This last reiteration is the best for my visual understanding: "Disengage your cone of probable position from his cone of probable position."

Cheers!

PS - you have an extreme level of patients. I'm sure it is appreciated by many lurkers here, including myself.

Thanks!

It's not "patience" -- I am working on and trying to improve my explanations, and I owe a big debt of gratitude to those who challenge and argue with me. I'm writing a book on this, and I really hope it will be the definitive guide to collision avoidance for recreational sailors. So I need to be able to explain all this stuff to absolutely anyone -- it's not at all a simple job, to formulate a really clear and easy to understand explanation of some of these concepts.

I also need to test all of my procedures and concepts for weaknesses, and exposing them to argument on here is a really efficient way to do that. We have a lot of really smart, really knowledgeable people on Cruisers Forum, including a number of people with experience as masters of ships (like Lodesman, Nigel, and TJ), and if I make a mistake, it takes about three milliseconds to get called on it! So believe me -- I'm getting more out of these discussions than probably anyone else!

[Quebramar]

Originally Posted by Dockhead

Thanks!

It's not "patience" -- (...)

I also need to test all of my procedures and concepts for weaknesses, and exposing them to argument on here is a really efficient way to do that. We have a lot of really smart, really knowledgeable people on Cruisers Forum, including a number of people with experience as masters of ships (like Lodesman, Nigel, and TJ), and if I make a mistake, it takes about three milliseconds to get called on it! So believe me -- I'm getting more out of these discussions than probably anyone else!

Good seamanship demands a lot of humility, I think. Your last post illustrates it nicely.

In the last threads DH started over ColRegs, there have been a few inaccurate answers or inappropriate assumptions or even misunderstandings of the regs... pity that some participants continuously argue even when facing the obvious. I guess that's what happens in real life as well, but then I would not call it good seamanship.

One of my key learnings over time could be summarised in the popular "better safe than sorry". This can apply to almost anything.

In dealing with others, especially at sea, the other expression that comes to my mind is one I learnt from a previous boss who was quite straight in his talk...: "when I assume I make an ass of you and me (ass-u-me)" -- pls excuse my French [emoji18]

[ramblinrod]

Originally Posted by ramblinrod

Yes, visually by day, by night, and by AIS. If visibility is clear, it is easy to see a 40 ft motor vessel 5 miles off. A 400 ft ship at 20, no problem.

From our marina we can see the smokestack at the Olcott power plant, which I believe is 45 miles across Lake Ontario.

Whoops, sorry guys, just double checked my estimates for seeing distance off, based on a few posts correctly indicating my figures were incorrect:

1. Lake freighter (actually ~ 700 ft long): Visual = ~12 miles, AIS = ~20 miles
2. 40 ft motor vessel: Visual = ~ 9 miles, AIS = ~14 miles
3. 40 ft Sailing vessel: Visual = ~12 miles, AIS = ~ 20 miles

Notwithstanding, pretty much any "ship" can be seen visually (on a clear flat day) better than 10 miles, perhaps a little further in waves (albeit perhaps only for brief periods).

But the main point is, making a course adjustment this far out cannot be declared "too soon" (there is no such thing as taking steps to avoid collision too soon), but is not "essential to avoid collision" at this point, and the solution could (and most likely will) prove invalid, due to changes in course and speed of the sailing vessel, before the intersect point is reached.

So lets ask ourselves a question.

Question: If a sailboat is travelling at 5 knots, what is its speed going to be 30 seconds from now?

Fellas, we don't control the wind, we just trim our sails.

Answer: "We don't really know, it depends on the wind."

Question 2: Then how the heck would we know what the speed will be over the next 15 minutes?

Answer: We don't. We can plot a course to steer now, but it is most likely wrong.

Lets look at a specific scenario between a 40 ft sailboat and 700 ft lake freighter.

The sailboat is traveling 5 knots 0 degrees (N) and intends to cross the ships course at 90 degrees, with the ship travelling 20 knots at 270 degrees (W).

Presently, if everything continues as it is, there will be a collision (bow to stern) in 15 minutes, if the nearest point of the ships transom is 5 miles from the intersect point and the sailboat bow to the intersect point is 1.25 miles.

But wait a minute, the ship is likely to maintain 20 knots, the sailboat, well, we have no idea.

Lets say the sailboat speed drops to 4 knots (it could fall into a lull and drop to 0, but lets say just to 4 knots).

At 4 knots, instead of most certainly hitting the ship, the sailboat will fall 1/4 mile short of the intersect point, by the time the ship gets there.

By the time the sailboat does get to the original intersect point, 3.75 minutes after the original scenario, the ship has travelled 1.25 miles past the intersect point.

Yes, by the time the sailboat gets to the original point of impact, there isn't going to be one, and they are looking at the ships transom 1.25 miles away.

They didn't change course at all.

So even though the sailboat speed is relatively slow in the scenario, that speed has a huge influence on the outcome, because the ship is going so fast in relation. By the time the sailboat gets there, the ship could be long gone.

As we can see in this scenario, we can suggest there is risk of collision at the onset, but really, there is only risk of collision if everything stay as is, which is actually highly unlikely. Using this argument, one could claim there was a risk of collision, when an ocean freighter is in the Pacific Ocean, and a sailboat is in the Atlantic. (If one anticipates the ships and sailboats courses correctly they could predict a collision 1000s of miles apart.

So the real question is, when does a "risk of collision" really exist.

My answer is, "when the closing angle remains constant, and the speed and course of both vessels is likely to remain constant."

Back to the scenario, if the sailboat is in 12 knots +/-5, their speed could vary from say 4 to 9 knots, depending on wind speed and angle.

This is why a sailing vessel should never plan to cross a ships bow.

When crossing astern, if calcs were wrong or scenario changes as it unfolds, one can easily turn off, to ensure the vessel is taken astern.

How far stern, depends on ones pucker factor and how willing they are to slow down, or go off course.

It is true, that as distance off shortens, the course and/or speed alterations have to become more significant, to avoid collision.

So if winds were quite steady, when the sailboat was back at 1.25 miles from the intersect point, they could have intentionally eased a sheet, to reduce speed from 5 knots to 4, and would never have come closer than 1/4 mile to the freighter.

Yes, at some point the freighter could unexpectedly turn onto a new course, that would then create a more drastic scenario. Remember that the entire solution is based on assumptions that everything remains constant.

Now one can establish a "Cone of Uncertainty". What degree of certainty is this cone going to be arbitrarily set at, 10%, 50%, 90%?

Obviously the greater the degree of certainty we feel we need, increases the distance off we must stay from the vessel.

All we know is that with 100% certainty, whatever we do, the ship could alter course to run us down. (Apparently that solution could have been plotted about 10 miles out, we are just unaware of it.)

So, if we have no reason whatsoever to expect the ship is going to alter course or speed, I suggest there is no real reason to stay 1 mile away from the ship. We have all the solution we need by reducing speed or changing course.

So when crossing at 90 degrees, if the closing angle remains constant until 1/4 mile away, the sailboat can change course to 90 E, for a period of time, until they can turn back to 0 N, (actually it now would need some west component to a course back to the original destination), to ensure they clear the stern of the vessel by say 1/4 mile. (Yes 180 ft would be close, possibly too close for comfort, but well within the requirements of colregs.)

So my point is, at 1.25 miles from the intersect point, we have no idea which way we should turn, or by how much, as we don't know what our speed will be for the next 15 minutes. We may not be able to maintain speed at all (as colregs requires for the stand on vessel) but we can always slow down to control the crossing (unless we are stopped already).

According to colregs, if we are stand-on, we should maintain course and speed, and make the freighter move.

IMHO, if the freighter is on a known shipping route, and they have not altered course 5 miles away to avoid us, we can just carry on our merry way, until we see the real scenario unfold, and take action as required to avoid collision with the freighter.

I have seen this type of scenario unfold, many, many times; where the freighter is quite sure they will pass ahead of the sailboat, so press on, even though the sailboat could speed up, and foil the solution. (One could argue that according to colregs the sailboat should maintain speed, but then again, the wind could be gusting and lulling like crazy making this totally impractical. What is the solution? Have a plan, be aware of the situation as it unfolds. As far as plan execution, one may start as early as they please, knowing full well that their actions are likely wrong, and will require further alteration. Or, they may wait, to see if the variables change and there is really a risk of collision. Perhaps the original course and speed, WAS the best solution, and premature change actually puts the vessel in harms way.

All this being said, if there are other vessels in the mix, to which the sailboat is the stand-on vessel, the sailboat should do best to hold course and speed (or altered course and speed), so that the other vessel solution(s) remain valid.

So here's a question, how many sailors (under sail) automatically strike the iron genny to maintain constant speed when a ship is spotted on a collision course 5 miles away?

(This is rhetorical, I know what the answer is.)

[TeddyDiver]

Originally Posted by ramblinrod

So lets ask ourselves a question.

Question: If a sailboat is travelling at 5 knots, what is its speed going to be 30 seconds from now?

Fellas, we don't control the wind, we just trim our sails.

Answer: "We don't really know, it depends on the wind."

Question 2: Then how the heck would we know what the speed will be over the next 15 minutes?

Answer: We don't. We can plot a course to steer now, but it is most likely wrong.

Lets look at a specific scenario between a 40 ft sailboat and 700 ft lake freighter.

The sailboat is traveling 5 knots 0 degrees (N) and intends to cross the ships course at 90 degrees, with the ship travelling 20 knots at 270 degrees (W).......................

You made a very good point to stand on

[Dockhead]

Originally Posted by ramblinrod

Whoops, sorry guys, just double checked my estimates for seeing distance off, based on a few posts correctly indicating my figures were incorrect:

1. Lake freighter (actually ~ 700 ft long): Visual = ~12 miles, AIS = ~20 miles
2. 40 ft motor vessel: Visual = ~ 9 miles, AIS = ~14 miles
3. 40 ft Sailing vessel: Visual = ~12 miles, AIS = ~ 20 miles

Notwithstanding, pretty much any "ship" can be seen visually (on a clear flat day) better than 10 miles, perhaps a little further in waves (albeit perhaps only for brief periods).

But the main point is, making a course adjustment this far out cannot be declared "too soon" (there is no such thing as taking steps to avoid collision too soon), but is not "essential to avoid collision" at this point, and the solution could (and most likely will) prove invalid, due to changes in course and speed of the sailing vessel, before the intersect point is reached.

So lets ask ourselves a question.

Question: If a sailboat is travelling at 5 knots, what is its speed going to be 30 seconds from now?

Fellas, we don't control the wind, we just trim our sails.

Answer: "We don't really know, it depends on the wind."

Question 2: Then how the heck would we know what the speed will be over the next 15 minutes?

Answer: We don't. We can plot a course to steer now, but it is most likely wrong.

Lets look at a specific scenario between a 40 ft sailboat and 700 ft lake freighter.

The sailboat is traveling 5 knots 0 degrees (N) and intends to cross the ships course at 90 degrees, with the ship travelling 20 knots at 270 degrees (W).

Presently, if everything continues as it is, there will be a collision (bow to stern) in 15 minutes, if the nearest point of the ships transom is 5 miles from the intersect point and the sailboat bow to the intersect point is 1.25 miles.

But wait a minute, the ship is likely to maintain 20 knots, the sailboat, well, we have no idea.

Lets say the sailboat speed drops to 4 knots (it could fall into a lull and drop to 0, but lets say just to 4 knots).

At 4 knots, instead of most certainly hitting the ship, the sailboat will fall 1/4 mile short of the intersect point, by the time the ship gets there.

By the time the sailboat does get to the original intersect point, 3.75 minutes after the original scenario, the ship has travelled 1.25 miles past the intersect point.

Yes, by the time the sailboat gets to the original point of impact, there isn't going to be one, and they are looking at the ships transom 1.25 miles away.

They didn't change course at all.

So even though the sailboat speed is relatively slow in the scenario, that speed has a huge influence on the outcome, because the ship is going so fast in relation. By the time the sailboat gets there, the ship could be long gone.

As we can see in this scenario, we can suggest there is risk of collision at the onset, but really, there is only risk of collision if everything stay as is, which is actually highly unlikely. Using this argument, one could claim there was a risk of collision, when an ocean freighter is in the Pacific Ocean, and a sailboat is in the Atlantic. (If one anticipates the ships and sailboats courses correctly they could predict a collision 1000s of miles apart.

So the real question is, when does a "risk of collision" really exist.

My answer is, "when the closing angle remains constant, and the speed and course of both vessels is likely to remain constant."

Back to the scenario, if the sailboat is in 12 knots +/-5, their speed could vary from say 4 to 9 knots, depending on wind speed and angle.

This is why a sailing vessel should never plan to cross a ships bow.

When crossing astern, if calcs were wrong or scenario changes as it unfolds, one can easily turn off, to ensure the vessel is taken astern.

How far stern, depends on ones pucker factor and how willing they are to slow down, or go off course.

It is true, that as distance off shortens, the course and/or speed alterations have to become more significant, to avoid collision.

So if winds were quite steady, when the sailboat was back at 1.25 miles from the intersect point, they could have intentionally eased a sheet, to reduce speed from 5 knots to 4, and would never have come closer than 1/4 mile to the freighter.

Yes, at some point the freighter could unexpectedly turn onto a new course, that would then create a more drastic scenario. Remember that the entire solution is based on assumptions that everything remains constant.

Now one can establish a "Cone of Uncertainty". What degree of certainty is this cone going to be arbitrarily set at, 10%, 50%, 90%?

Obviously the greater the degree of certainty we feel we need, increases the distance off we must stay from the vessel.

All we know is that with 100% certainty, whatever we do, the ship could alter course to run us down. (Apparently that solution could have been plotted about 10 miles out, we are just unaware of it.)

So, if we have no reason whatsoever to expect the ship is going to alter course or speed, I suggest there is no real reason to stay 1 mile away from the ship. We have all the solution we need by reducing speed or changing course.

So when crossing at 90 degrees, if the closing angle remains constant until 1/4 mile away, the sailboat can change course to 90 E, for a period of time, until they can turn back to 0 N, (actually it now would need some west component to a course back to the original destination), to ensure they clear the stern of the vessel by say 1/4 mile. (Yes 180 ft would be close, possibly too close for comfort, but well within the requirements of colregs.)

So my point is, at 1.25 miles from the intersect point, we have no idea which way we should turn, or by how much, as we don't know what our speed will be for the next 15 minutes. We may not be able to maintain speed at all (as colregs requires for the stand on vessel) but we can always slow down to control the crossing (unless we are stopped already).

According to colregs, if we are stand-on, we should maintain course and speed, and make the freighter move.

IMHO, if the freighter is on a known shipping route, and they have not altered course 5 miles away to avoid us, we can just carry on our merry way, until we see the real scenario unfold, and take action as required to avoid collision with the freighter.

I have seen this type of scenario unfold, many, many times; where the freighter is quite sure they will pass ahead of the sailboat, so press on, even though the sailboat could speed up, and foil the solution. (One could argue that according to colregs the sailboat should maintain speed, but then again, the wind could be gusting and lulling like crazy making this totally impractical. What is the solution? Have a plan, be aware of the situation as it unfolds. As far as plan execution, one may start as early as they please, knowing full well that their actions are likely wrong, and will require further alteration. Or, they may wait, to see if the variables change and there is really a risk of collision. Perhaps the original course and speed, WAS the best solution, and premature change actually puts the vessel in harms way.

All this being said, if there are other vessels in the mix, to which the sailboat is the stand-on vessel, the sailboat should do best to hold course and speed (or altered course and speed), so that the other vessel solution(s) remain valid.

So here's a question, how many sailors (under sail) automatically strike the iron genny to maintain constant speed when a ship is spotted on a collision course 5 miles away?

(This is rhetorical, I know what the answer is.)

OK, now we're getting somewhere! This is already a more reasonable conversation.

One thing we can agree on is the fairly obvious point that the greater the variation of our speed, the less knowledge we have about where we will cross. And at some point, in really gusty wind say, normal collision avoidance procedure simply doesn't work. In such a case, Rod has correctly stated that passing ahead of a ship is really dangerous and must be avoided. But in most cases, particularly if this is the ocean and not a lake, particularly if the sailboat is larger, the variation of speed with the wind does not mean we have no idea where we will be in 15 minutes, so normal collision avoidance procedure applies.

Where we don't agree is what to do with this. If the wind is gusty and our speed varies a lot, that does NOT mean we just sail on blithely into close quarters with a ship. On the contrary, the less steady or speed and course, the further we have to stay away. The fundamental concept is that we need to prevent intersection of the zone where ship is likely to be at CPA, from the zone where we are likely to be at CPA, and if we are give-way, this maneuver must be done in one large, visible course change which makes it obvious to the other vessel that we have taken control of the crossing and have resolved it. Early enough that the other vessel will still have a chance to maneuver itself in case it is not satisfied with what we have done.

If we are give-way, then we can maneuver prior to the risk of collision arising, and that's a good idea if we can manage it. But we can't just make up our own ideas about when the risk of collision exists.

Court cases say that a "risk of collision" exists at different ranges, depending on the circumstances, but there are some cases which say that at the very least, it exists at 5 miles. Other cases say that the give-way vessel should maneuver at no less than 3 miles off. What they teach professionals is, as I mentioned before, detection of dangerous targets should be completed by 10 miles off, and maneuvering normally by 4 miles off (but perhaps less if circumstances of multiple targets etc. require).

It's crucially important to coordinate your maneuvers, with what ships you encounter are doing. So if you are stand-on, and at 4 miles off you still have a small CPA (less than 5 cables is per se too small in open sea, and as I mentioned, one mile is the usual minimum standard), then you need to be getting ready to maneuver yourself. You have to keep in mind that your ability to resolve a risk of collision situation is proportional to your speed. So you must not leave it until too late. 4 miles is probably not too early to take action yourself in most cases in open sea, if you are stand on, and certainly by 3, or at the very least 2, you need to act.


You have recommended just turning 90 degrees, 1/4 miles out, if you still have a problem. I'm sorry I can't think of a gentler way to say this, but that is simply nuts! I really don't understand why this is not obvious to you. At 1/4 mile off, you are already invisible below the bows of a medium sized ship, so now only you can do anything. But you can't do anything! 1/4 mile off is less than 44 seconds from impact! 5 knots is only 2.57 meters per second. Even if you could turn instantaneously, there is no way to get out his way -- that is, out of where he might be in 44 seconds -- which you cannot know +/- 100 meters. His beam might be 50 meters!! If you wait until 1/4 mile off and find yourself there with a 0 CPA, you will live or die only by sheer luck.




Good practice in open sea is to keep 2 miles away from other vessels, at least, from fast moving ships. One mile is pretty much minimum, although you might pass a bit closer behind once you can see his transom. The slower you are, the less you can afford to cut these distances.

[carstenb]

Originally Posted by Dockhead

Yes, that's the way I figure it out, on my system.

BUT -- the question is a really good one! I agree that this is one of the basic things, which we should be able to know at a glance. Sometimes the bearing is not changing quickly enough to see this very soon.

OpenCPN (and I believe Vesper separate AIS displays) solve this problem by graphically displaying the crossing geometry.

This is so valuable that in really complicated situations, where you're dealing with multiple targets and really need to see this at a glance, I always use OpenCPN at the nav table while someone else is at the helm and keeping a visual watch.

Our AIS shows the crossing geometry correctly with the arrows. If the arrows cross in front of the apparoaching target - that is where the CPA wil be, if they are behind the target so will the CPA be.

on another note - when we are sailing passage (or anytime on open sea), we have our AIS set to pick up and show the targets at 14nm . As soon as we spot it on the chartplotter, we check the CPA (if the arrows aren't showing it yet.)

We usually make a course adjustment at 8-10nm. We also call the ship on our VHF and discuss who is going to do what if the situation is unclear. We've found that the big ships almost invariably prefer to give way to us, unless we already have made a significant course correction that shows we are going to fulfill our colreg obligations.

I agree with DH - thinking you can safely make a correction at <5nm from a collision point is cutting very fine indeed.

[ramblinrod]

Originally Posted by Dockhead

1/4 mile off is less than 44 seconds from impact!

This is incorrect.

For the scenario under discussion, if the ship holds course and speed, (as you would have observed for quite some time now) and you turn 90 degrees (parallel with it's course), you will not be closer than 1/4 mile (less a few feet for your turning radius).

Additionally, I don't care what the beam of the ship is; I plot a solution to miss the nearest part of the ship, not for where the AIS transmitter antenna might be.

If one only has AIS (poor visibility and no radar) then of course they would allow more room.

Sometimes it's wise to get your head out of your electronics.

IMHO, an AIS alarm should trigger one to take a look outside the cockpit.

Frankly, IMHO one's obligation to keep watch goes way beyond staring at screens.

[ramblinrod]

Originally Posted by Lodesman

I assume a Douglas 32 has a height of eye (standing at the helm) of about 6 ft - that gives distance to horizon of roughly 3 miles. For a distance to horizon for the remaining 17 miles, a height of 210 ft is required. (Bowditch table 12)

Not many 400 ft vessels will be 210 ft tall.

Note that one does not need to know the colour of the waterline stripe.

Try again.

Standing at the helm, in flat water, while sailing flat, my eye will be about 7 feet off the water.

Sitting on the high side, when heeled 20 degrees, it will be about 9 feet.

(Though I often helm from the low side, when someone else is on lookout.)

Sitting on the high side, when heeled 20 degrees and in 6 foot waves, will be about 12 feet (on the crests).

From this vantage, something large enough, assuming clear conditions, can be seen from 10 miles if it is more than 12 feet off the water.

Reminds of the day my son (about 10 at the time) was looking up ahead through binoculars, and advised there were a bunch of people "standing in the water".

Knowing that the water ahead was deep ahead, I asked to have a look through the binoculars.

I explained that due to the curvature of the earth, and his vantage point, we could not see the topsides of the aluminium fishing boats they were standing in. (They were about 3 miles away.)

He now keeps his sailboat at the same marina we keep ours.

I had to tease him last year, returning from a cruise with our respective boats (and spouses), when we sailed near a freighter that was a couple miles off-shore.

After, we landed, he advised that he would have liked to get as close as we did, but was afraid of being run down.

I chuckled as I let him know the ship was at anchor.

My point?

Some people have a much tighter pucker factor than others.

Somehow, the old man has managed to stay alive all these years.

Amazing!

Just luck, I'm sure. ;-)

[Dockhead]

Originally Posted by ramblinrod

This is incorrect.

For the scenario under discussion, if the ship holds course and speed, (as you would have observed for quite some time now) and you turn 90 degrees (parallel with it's course), you will not be closer than 1/4 mile (less a few feet for your turning radius).

Additionally, I don't care what the beam of the ship is; I plot a solution to miss the nearest part of the ship, not for where the AIS transmitter antenna might be.

If one only has AIS (poor visibility and no radar) then of course they would allow more room.

Sometimes it's wise to get your head out of your electronics.

IMHO, an AIS alarm should trigger one to take a look outside the cockpit.

Frankly, IMHO one's obligation to keep watch goes way beyond staring at screens.

How do you "plot a solution to miss the nearest part of the ship"? You are describing something which is impossible. What kind of "plotting" are you talking about? You have no way of knowing where the "nearest part of the ship" will be at CPA. We've discussed this; I'm surprised you still haven't caught on.

As to not getting any closer, if you merely turn 90 degrees -- this is utter nonsense. The only thing I can guess, as to how you could think something like this, is that you are imaging a highway, and you stop at the edge of. But in open sea, there is no highway and no edge to stop at -- there is just the ship towering over you, with no change in bearing you can see, 40 seconds or so from impact. You cannot see what's going to happen. He is barrelling right at you and getting closer by the second, even if there actually is a positive CPA. Will he pass you by, or run you down? You can't know this -- at 2 1/2 cables out, you are a pure sitting duck. The best information you have is the CPA indicated on your AIS, but even this is not accurate enough to tell you whether you will get run down or not, at only 2 1/2 cables away. At 5 knots, you can only move 100 meters or so in the time it will take him to run you down. You should never get that close to a ship in open water!

As I said -- forget the dimension of the ship, its "closest part", and all that. Draw a cone based on your reasonable knowledge of where he will be at CPA. You are not safe unless you are outside of that cone, and you're not completely safe even outside of it. You cannot do collision avoidance without margins for these things you can't know.