Thoughts about AIS

[Dockhead]

Originally Posted by monte

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..

Yes, that's all exactly correct. One mile ahead is 3 minutes from collision with a ship making 20 knots. If your engine fails or the wind drops (or God forbid, both), you might really be dead before the OOW figures out to put the rudder over. Less than a mile in open water ahead is really almost unthinkable.

And typical standing orders on the bridge of a commercial vessel are to avoid getting within one mile of any other vessel in open water. This is typical:

OOW should note that when collision avoidance action is to be taken it should be done well in time & the action should be such as to be readily apparent to another vessel observing visually or by radar as a rule an aleration of 30 to 40 degrees must be made. In general in open sea, if there is sufficient sea room & if circumstances permit, action must be taken at a distance of at least 4-5 miles and a minimum cpa of 1 nautical mile must be maintained.

http://www.siamluckymarine.com/MAROP...NG%20ORDER.pdf

A sailboat cutting half a mile ahead in open water is a real pain for the bridge of a commercial vessel. The OOW will curse the idiot WAFI who did it, because now he has to record it in the log and call the master, if not actually make hard maneuvers to prevent it.

Cutting in less than a mile behind is theoretically the same thing (ships' standing orders don't generally distinguish ahead from behind), but in places like the Channel you often have no choice but to cut in closer than a mile behind, because otherwise you will be too closer to the next ship. The ships' bridges seem to understand that and don't complain about it.

[colemj]

Originally Posted by Dockhead

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.

You know this 30 minutes before CPA. That was my point - it is enough time for me, but maybe not for others.

For ships a long way off, you can easily turn a degree or two and see how the CPA changes. That tells you pretty much everything from miles away.

Just curious - you seem to have a sweet spot you are looking for. You said 3hrs was too long, but 30 minutes too short.

What is your sweet spot there?

Mark

[Dockhead]

Originally Posted by colemj

You know this 30 minutes before CPA. That was my point - it is enough time for me, but maybe not for others.

For ships a long way off, you can easily turn a degree or two and see how the CPA changes. That tells you pretty much everything from miles away.

Just curious - you seem to have a sweet spot you are looking for. You said 3hrs was too long, but 30 minutes too short.

What is your sweet spot there?

Mark

I never said 30 minutes was too short. That's about right. Typical decision point is about 30 minutes ahead, which is usually 7 to 10 miles depending on the speed of the ship, with last maneuvering at about 5 miles, just like in the standing orders I quoted above.

Yes, of course, you can turn and see what happens to CPA. Sometimes I do this. But it is a fairly idiotic way of getting information which should be displayed on the plotter, and it also doesn't tell you about relative positions other than ahead or behind.

By the way, another reason why it is particularly bad for sailing vessels to cut it too close in encounters with ships is that our courses are very often not very straight. Sailing hard on the wind, our courses will necessarily vary, sometimes quite a lot. Different course will mean different CPA, and the planned CPA must be able to absorb any course changes.

[monte]

hold up a sec, in that situation, with a 90 degree crossing and a ship doing 20 knots to my vessel doing 7, and a CPA of 1 mile, the crossing would actually be 3 miles ahead, with the 1M CPA when the ship is dead astern (if I am following correctly) So Mark, your example was actually for a 1/3 M CPA not 1M CPA?

[colemj]

Originally Posted by monte

hold up a sec, in that situation, with a 90 degree crossing and a ship doing 20 knots to my vessel doing 7, and a CPA of 1 mile, the crossing would actually be 3 miles ahead, with the 1M CPA when the ship is dead astern (if I am following correctly) So Mark, your example was actually for a 1/3 M CPA not 1M CPA?

Yes, that is correct - I meant to write that I would end up 1/3 mile to the side of it (at CPA) when crossing 1 mile ahead of it. I wrote "and" instead of "at", but even then it isn't clearly written.

On our plotter, it would be clear that I would be passing ahead of it and that its position at CPA would be behind us.

If the CPA was 1 mile, then you are clear in all directions by a... erm… mile.

Mark

[colemj]

Originally Posted by Dockhead

I never said 30 minutes was too short. That's about right.

Sorry, I misunderstood your complaints on the limits of your chartplotter's CPA projection and the need for longer lines as meaning 30 minutes was too short for you.

Mark

[Dockhead]

Originally Posted by colemj

Sorry, I misunderstood your complaints on the limits of your chartplotter's CPA projection and the need for longer lines as meaning 30 minutes was too short for you.

Mark

I never complained about the length of the lines. I can set them at different lengths. My complaint was that they don't tell me the relative positions at CPA unless either the vessels are going at similar speeds, or the TCPA happens to correspond to the time horizon of the projected COG lines.

The bottom line is that projected COG lines are not very useful for collision avoidance with a moving vessel, except in isolated circumstances. They are supremely useful for avoiding fixed objects, and can even be life-savingly useful, when for example trying to claw up against a strong wind and head sea, such that your COG is very different from your heading, and you need to avoid getting thrown back on some rocks, or running into a buoy. Which is exactly what happened to me in May getting out of Borkum against a F7, sometimes making just 1 knot SOG at 3800 RPM. I don't know what I would have done without the projected COG line, so my remarks should not be interpreted as my not appreciating them.

[colemj]

Originally Posted by Dockhead

The bottom line is that projected COG lines are not very useful for collision avoidance with a moving vessel, except in isolated circumstances.

I just described how I find them useful and don't have any problems using them. So yours is not a general or factual statement.

Mark

[Pelagic]

Originally Posted by colemj

.......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.
Mark

This brings up an interesting part of collision avoidance management with AIS.

Years back in Marine College in the Radar Simulator, we were taught to break down bridge management as such:
12-9nm...... Detection
9-6nm..........Analysis
6-3nm.........Action

+"Any Action (change of course)....should be sufficient to be readily apparent from the opposing vessel"... Course changes being favoured over change in speed....

Has AIS changed that?

[monte]

Originally Posted by Pelagic

This brings up an interesting part of collision avoidance management with AIS.

Years back in Marine College in the Radar Simulator, we were taught to break down bridge management as such:
12-9nm...... Detection
9-6nm..........Analysis
6-3nm.........Action

+"Any Action (change of course)....should be sufficient to be readily apparent from the opposing vessel"... Course changes being favoured over change in speed....

Has AIS changed that?

That sounds about right to me for inshore, although I would say with AIS you would get a headstart on those distances if offshore.
I would prefer
15-20nm...... Detection
12-15nm..........Analysis
10-12nm.........Action
based on a CPA of less than 2 miles

[Pelagic]

So "offshore" under sail but acting as the give way v/l you would make minor course changes from 10nm away?

What if the other v/l does the same in a way to cancel out your avoidance action?

[colemj]

Originally Posted by Pelagic

This brings up an interesting part of collision avoidance management with AIS.

Years back in Marine College in the Radar Simulator, we were taught to break down bridge management as such:
12-9nm...... Detection
9-6nm..........Analysis
6-3nm.........Action

+"Any Action (change of course)....should be sufficient to be readily apparent from the opposing vessel"... Course changes being favoured over change in speed....

Has AIS changed that?

I don't think so. Remember, though, that AIS provides a readily apparent change of course to the opposing vessel even if your change is a seemingly small one that would not be apparent otherwise.

For example, at 8 miles out, your 2 degree change will show up on his bridge as a CPA jumping from (say) 0.2nm to 2nm. Or from passing in front of him to behind him.

Mark

[colemj]

Originally Posted by Pelagic

What if the other v/l does the same in a way to cancel out your avoidance action?

If it did come to a continuing resonance situation (very unlikely), that is what DSC radio is all about - ring him up directly and talk to him.

Mark

[Pelagic]

Mark...It would be great if we could rely on radio com to resolve confusion but that doesn't happen in the real world with language barriers

Especially if a young WK on a ship is recognising you as the stand on sailing v/l and your own small actions are confusing him at such a long distance.

The seductive nature of AIS allows us as sailors to trim slightly from far away to avoid a situation... (which 80-90% of the time works great)..... but are we inducing new bad practices of not making our intentions very clear?

[Pelagic]

The situation I am envisioning is closing where the other vessel is seen approaching at low relative speed from about122° on your port side.