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Old 13-06-2014, 15:00   #91
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Re: Thoughts about AIS

I just think you guys are WAY over-thinking this and creating a problem that exists only in your head.

I used AIS on an old and very tiny display in LA-Long beach harbors as well as puget sound which has quite a lot of traffic. And in all honesty I just avoid close CPA's so I guess I just don't understand the whole predicament you are attempting to describe. if the lines cross you got potential problems if not; what's the point in dealing with a problem that may change ten times in the next hour? I keep an eye on developing situations to see what develops but seriously it seems like you are just mind F*Cking the problem to death and that get's you nowhere fast.

Do you really need to know if you're gonna miss that guy by five feet?
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Old 13-06-2014, 15:27   #92
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Re: Thoughts about AIS

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Do you really need to know if you're gonna miss that guy by five feet?
You do need to know that you are going to miss ferry boats and military vessels by the required distances. In our area I've seen Seal Team inflatibles out here to make sure that you do.
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Old 13-06-2014, 16:00   #93
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Re: Thoughts about AIS

The one thing I have been hung up on in this conversation is why the COG predictor lines are not useful? Let's say they are set for yours and all targets at 30 minutes. This means that the predictor line lengths are proportional to the target's speed - they are a vector.

In the above, one has 30 MINUTES when those two vectors meet at any point to judge how the passing will occur and what action one should take.

Speaking personally, 30 minutes is quite reasonable for my attention and reaction times.

Of course, the extended vectors that have been shown here are nice, but - 30 minutes isn't enough?

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Old 14-06-2014, 04:56   #94
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Re: Thoughts about AIS

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Originally Posted by colemj View Post
The one thing I have been hung up on in this conversation is why the COG predictor lines are not useful? Let's say they are set for yours and all targets at 30 minutes. This means that the predictor line lengths are proportional to the target's speed - they are a vector.

In the above, one has 30 MINUTES when those two vectors meet at any point to judge how the passing will occur and what action one should take.

Speaking personally, 30 minutes is quite reasonable for my attention and reaction times.

Of course, the extended vectors that have been shown here are nice, but - 30 minutes isn't enough?

Mark
A good question, but it has a good answer.

Only if your CPA is around 30 minutes, OR if the speed of the vessels is similar, can you tell much about the crossing with 30 minute projected COG lines. If your CPA is around 30 minutes (say 20 to 40 minutes, probably), then the end points of the lines will show you approximately what the relative positions of the vessels will be.

If the speed of the vessels is similar, then you can pick any points along the lines, or imagine the lines projecting out further, and you can also see.

But if neither of these things is true, then you know very little about the crossing, and certainly not whether you will cross ahead or behind, when CPA is small.

That's why the Vesper/OpenCPN system is so brilliant. It marks the position of both vessels at CPA -- brilliant. That's what I want on my plotter.
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Old 14-06-2014, 05:07   #95
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Re: Thoughts about AIS

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Originally Posted by xymotic View Post
I just think you guys are WAY over-thinking this and creating a problem that exists only in your head.

I used AIS on an old and very tiny display in LA-Long beach harbors as well as puget sound which has quite a lot of traffic. And in all honesty I just avoid close CPA's so I guess I just don't understand the whole predicament you are attempting to describe. if the lines cross you got potential problems if not; what's the point in dealing with a problem that may change ten times in the next hour? I keep an eye on developing situations to see what develops but seriously it seems like you are just mind F*Cking the problem to death and that get's you nowhere fast.

Do you really need to know if you're gonna miss that guy by five feet?
Many, probably most sailors, don't really need to know anything except that there is a potential problem from a small CPA. They can just turn and run. This works everywhere except where you have really intense ship traffic.

You can't imagine that predicament because you haven't been in it. Look at an AIS plan of the English Channel and you'll get an idea of what we're talking about. You are crossing and you have to get through a line of ships charging down the lanes at an average of 18 knots a mile or two apart. You are the give-way vessel because say you're crossing an East-going line of ships coming from the North. In order to get through safely, you have to shape your course to bring you fairly close astern of one ship, in order to not pass to closely ahead of another one. The ships may not be in a line; there may be several abreast; you may have to figure out the crossing with four or five ships.

To do this safely, you must know not only the CPA and TCPA, but you must know whether a given course is taking you ahead or behind a given ship. Otherwise you just can't understand the situation. The ships are moving at different speeds, and a ship over there which is presently behind another ship, may well be ahead by the time you get to it. You can get in a terrifically dangerous situation if you steer yourself inadvertently into the middle of a knot of ships and collision courses with all of them -- you then won't know which way to turn, and by maneuvering in close quarters you may put yourself right under the bow of a ship which is unable already to avoid the collision. It can be really f****g terrifying, and the data which is displayed on most of our plotters is not enough to plan it properly.

If you haven't been in such a situation, then naturally you won't understand what we're talking about. It is certainly not a question of planning a crossing plus or minus five feet (!). You need to stay at least a couple of cables behind a fast moving ship, and at least a mile or so ahead, to be reasonably safe. It can be a fantastically complicated computation to achieve those safe distances when several ships are involved, all moving at different speeds, and a really bad situation can develop from nothing in just minutes.
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Old 14-06-2014, 06:40   #96
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Re: Thoughts about AIS

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...

If you haven't been in such a situation, then naturally you won't understand what we're talking about. It is certainly not a question of planning a crossing plus or minus five feet (!). You need to stay at least a couple of cables behind a fast moving ship, and at least a mile or so ahead, to be reasonably safe. It can be a fantastically complicated computation to achieve those safe distances when several ships are involved, all moving at different speeds, and a really bad situation can develop from nothing in just minutes.
This is the money quote... AIS obvious works a charm for very crowded waterways with traffic moving in multiple directions at various speeds. It uses the power of a processor to give the captain reliable collision avoidance info.

So it's certainly a safety feature to consider on your boat and almost now a necessity in certain waters.
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Old 14-06-2014, 06:48   #97
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Re: Thoughts about AIS

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A good question, but it has a good answer.

Only if your CPA is around 30 minutes, OR if the speed of the vessels is similar, can you tell much about the crossing with 30 minute projected COG lines. If your CPA is around 30 minutes (say 20 to 40 minutes, probably), then the end points of the lines will show you approximately what the relative positions of the vessels will be.

If the speed of the vessels is similar, then you can pick any points along the lines, or imagine the lines projecting out further, and you can also see.

But if neither of these things is true, then you know very little about the crossing, and certainly not whether you will cross ahead or behind, when CPA is small.

That's why the Vesper/OpenCPN system is so brilliant. It marks the position of both vessels at CPA -- brilliant. That's what I want on my plotter.
The lengths of the CPA projection lines are directly proportional to the speeds of the vessels. I don't see how having different speeds changes anything. A very fast boat will have a long projected line relative to a slow boat - when the end of those lines are close or touching, you have 30 minutes until CPA.

I guess our difference here is that I consider 30 minutes of warning and reaction time more than sufficient for collision avoidance. A ship 3hrs away with a potential close CPA just doesn't rise up to the top of my list of navigation worries.

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Old 14-06-2014, 06:57   #98
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Re: Thoughts about AIS

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You need to stay at least a couple of cables behind a fast moving ship, and at least a mile or so ahead, to be reasonably safe.
OK, this confirms that our difference in this is due to differences in what we consider "reasonably safe". I certainly do not think I need to maintain a CPA of one mile or more ahead of a ship. Even less than a quarter mile (couple of cables) behind is not a problem for us.

Frankly, the above would be simply impossible for many of the busy shipping areas we have been in - I don't know how you ever get across the English Channel.

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Old 14-06-2014, 07:53   #99
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Re: Thoughts about AIS

wow 1 mile is 2-3 minutes from a collision. Not sure how much closer you would be happy with? Id cut less than a mile if it was anchored or doing under 7 knots if I really had to, but not if its doing 20 knots or so. Also most shipping companies have regulated CPA's so if you cut it that fine you are either going to piss off the captain pretty severely and/or force him to alter course which isn't really cool..
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Old 14-06-2014, 08:06   #100
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Re: Thoughts about AIS

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The lengths of the CPA projection lines are directly proportional to the speeds of the vessels. I don't see how having different speeds changes anything. A very fast boat will have a long projected line relative to a slow boat - when the end of those lines are close or touching, you have 30 minutes until CPA.

I guess our difference here is that I consider 30 minutes of warning and reaction time more than sufficient for collision avoidance. A ship 3hrs away with a potential close CPA just doesn't rise up to the top of my list of navigation worries.

Mark
As I've said before: collision avoidance in busy sealanes in open water with dense concentrations of fast moving ships is very different from collision avoidance in busy harbors or other circumstances. The best way I can describe it is that to do it successfully you have to plan each encounter some miles ahead. Not in any case 3 hours ahead, but by 30 minutes out you need to be well aware of how you're going to cross and you need to plan it. You need to have figured out how you will cross with all the other ships close enough to run you down, and you need to be sure that you have a course which keeps you at a safe distance from all of them. It's not at all like dodging ships in a busy harbor where, I agree, a few cables ahead can be enough. Especially if there are channels and you know for sure where you can dodge to be safe. In open water, you need more margin of error with planning further ahead, and you have to plan further ahead because you have to have solutions for multiple crossings which all work with each other. That's the nature of the beast. A mile ahead can turn into a half mile or into a collision pretty quickly, so not any single Channel sailor I know ever -- ever! -- plans to cross ahead of a fast ship at less than a mile, and most will go to fairly great lengths to avoid crossing ahead at all. And the professional mariners in ships do the same thing. They can cut it closer, because they move faster and don't have the risk of the wind dropping right at the worst moment, but they still will not pass less than half a mile in front of you, when they are the stand-on vessel.

You did not understand my point about COG lines at all. The point was in no way that 30 minutes is not enough. The point is that with a significant difference in speed, the length of the lines is different, and if your CPA is some distance from the end point of the lines, it is hard to extrapolate what your relative positions will be at CPA. You can of course -- yes, as you correctly said, the length is proportional. So if your TCPA is in 15 minutes you can put your finger in the middle of both COG lines and you have a solution. If your TCPA is 20 minutes, you can put your finger at 2/3 of the way down both lines and see. But without putting a ruler to it, it's pretty hard to see it just like that. If you have to do that with three or four ships at once, you just can't form a picture.
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Old 14-06-2014, 08:09   #101
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Re: Thoughts about AIS

Yes, I have no problem with a mile on a crossing - at the CPA, I will be 1/3 of a mile on the other side of it and 1 mile ahead of it.

However it is all situational and you must take into account I also have no problem with a 30 minute warning. If it is open ocean and there is a ship doing 20kts in which we will cross its bow by less than a mile, I will make a degree or two course change within that 30 minutes to go behind it.

If it is a typical issue around shipping ports, etc, the boats are moving much slower and are closer together themselves. One mile is often not possible then.

I have stated in this thread that we never rely on, or make, other vessels change course to avoid us. If we are truly burdened with something that makes our changing course impractical (very rare), we DSC call the ship's bridge and negotiate with lots of time. Whether that pisses off the captain is not my problem.

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Old 14-06-2014, 08:21   #102
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Re: Thoughts about AIS

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You did not understand my point about COG lines at all. The point was in no way that 30 minutes is not enough. The point is that with a significant difference in speed, the length of the lines is different, and if your CPA is some distance from the end point of the lines, it is hard to extrapolate what your relative positions will be at CPA.
I understand you - the difference is only how we see/approach the problem.

If your CPA is more than a mile (pick your comfort level), what difference does it make what your relative positions are at that CPA? You are safely clear. If it is closer than your comfort level, you make adjustments 30 minutes out.

If you have different comfort levels between ahead and astern (and you should!), then I don't see how you cannot easily ascertain that at least 30 minutes before CPA and adjust accordingly.

I'm not arguing that the future projected lines discussed are unnecessary, just that I don't see how your current capabilities limit you in a practical way.

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Old 14-06-2014, 08:32   #103
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Re: Thoughts about AIS

Yep agree with changing course a bit to pass behind and CPA's change from acceptable at one mile inshore. Offshore theres no need to come that close. But crossing the English Channel as you mentioned is definitely a 1 mile plus situation. Id aim for 2 or more if possible. If you take a look at the English channel on https://www.marinetraffic.com/en/ you won't see any boats crossing in under a mile ahead. Actually now that you mention it it makes for a question i don't know the answer to...
CPA is the distance you will be from an AIS vessel when you are nearest to it,
therefore your scenario would be a CPA of 1/3 mile. I had assumed it was the closest you will come before passing ahead or behind and that 1 mile ahead would be a 1 mile CPA. A one mile CPA would actually mean you pass 3 miles ahead of the faster vessel in that case....
Answer...anyone?
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Old 14-06-2014, 09:23   #104
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Re: Thoughts about AIS

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I understand you - the difference is only how we see/approach the problem.

If your CPA is more than a mile (pick your comfort level), what difference does it make what your relative positions are at that CPA? You are safely clear. If it is closer than your comfort level, you make adjustments 30 minutes out.

If you have different comfort levels between ahead and astern (and you should!), then I don't see how you cannot easily ascertain that at least 30 minutes before CPA and adjust accordingly.

I'm not arguing that the future projected lines discussed are unnecessary, just that I don't see how your current capabilities limit you in a practical way.

Mark
Well, that's kind of the point. If your CPA is less than a mile, then how do you adjust your course? You don't know which way if you don't know whether you're passing ahead or behind.

And this becomes much more important when you're trying to avoid several ships at once. Even if your CPA is more than a mile, you will often need to know whether you can turn this way or that without losing your safe CPA.

If you know what the relative positions will be at CPA, then you understand the encounter at a glance, and you see how to turn to increase the CPA, and whether that turn puts you in the way of other ships. You can't see that with our plotters -- COG lines don't do that -- and it is a real disadvantage sailing in such situations.
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Old 14-06-2014, 09:27   #105
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Re: Thoughts about AIS

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Yep agree with changing course a bit to pass behind and CPA's change from acceptable at one mile inshore. Offshore theres no need to come that close. But crossing the English Channel as you mentioned is definitely a 1 mile plus situation. Id aim for 2 or more if possible. If you take a look at the English channel on https://www.marinetraffic.com/en/ you won't see any boats crossing in under a mile ahead. Actually now that you mention it it makes for a question i don't know the answer to...
CPA is the distance you will be from an AIS vessel when you are nearest to it,
therefore your scenario would be a CPA of 1/3 mile. I had assumed it was the closest you will come before passing ahead or behind and that 1 mile ahead would be a 1 mile CPA. A one mile CPA would actually mean you pass 3 miles ahead of the faster vessel in that case....
Answer...anyone?
Yes, this is true! The bigger the difference in speed, the bigger the difference in distance at CPA and at that position which is dead ahead of the ship. But it doesn't matter -- CPA is still the critical number, and that is what the commercial mariners are looking at.
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