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Old 16-06-2014, 09:53   #181
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Re: Thoughts about AIS

Dockhead in regards the Wakhuna incident its important to realise the following was the conclusion of the MAIB

1. The skipper of the Wakhuna, fundamentally mis-read his radar screen, believing the Nedloyd was going to pass 1.5 away, he slowed to increase that CPA.

Thats was the key error that was made.

2. The master of the ship, relied too heavily on the ARPA information and allowed a close CPA to develop.


Fundementally its was Wakhunas incorrect radar assessment that precipitated the incident. It was then compounded by the lack of action my the ship.


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Old 16-06-2014, 10:21   #182
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Re: Thoughts about AIS

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Originally Posted by poiu View Post
That sounds a really useful way to control OpenCpn on a cockpit screen - how is it done?
I use a wireless mouse--most of the OpenCpn controls are point and click.
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Old 16-06-2014, 10:39   #183
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Re: Thoughts about AIS

Quote:
Originally Posted by goboatingnow View Post
Dockhead in regards the Wakhuna incident its important to realise the following was the conclusion of the MAIB

1. The skipper of the Wakhuna, fundamentally mis-read his radar screen, believing the Nedloyd was going to pass 1.5 away, he slowed to increase that CPA.

Thats was the key error that was made.

2. The master of the ship, relied too heavily on the ARPA information and allowed a close CPA to develop.


Fundementally its was Wakhunas incorrect radar assessment that precipitated the incident. It was then compounded by the lack of action my the ship.


dave
Of course -- and that doesn't contradict anything I said.

The skipper of the Wahkuna was not able to determine whether the container ship was passing ahead or behind. Which is actually the whole topic of the original post of this thread!! He erroneously concluded that the container ship was passing ahead, and took his engine out of gear to let the container ship go past, as he thought.

The Wahkuna disaster really shows that knowledge of whether a target is passing ahead or behind can be matter of life and death, literally. Very much on point to the original post.

And it shows also that a highly experienced, professional master (the master himself had the con) can be sucked into taking the CPA numbers from his electronic navigation at face value, without realizing that he is dealing with a specific situation -- big difference in speeds, close crossing, target which cannot or does not hold a very steady course or speed -- which increases the "cone of error" of the system.

Tremendously useful lessons for all of us, in my opinion.
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Old 16-06-2014, 10:44   #184
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Re: Thoughts about AIS

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The skipper of the Wahkuna was not able to determine whether the container ship was passing ahead or behind. Which is actually the whole topic of the original post of this thread!! He erroneously concluded that the container ship was passing ahead, and took his engine out of gear to let the container ship go past, as he thought.
No , that not correct, he simply mis-read his radar,

Quote:
the radar contact of P&O Nedlloyd Vespucci would pass 1.5 miles ahead as a result of his action. This assessment, however, was based on only a visual interpretation of the radar display because none of the crew knew how to use the radar's automatic plotting facilities, and a manual plot was not undertaken.
He was not actually in the cone of error ( well he could have been , but he'd never realise anyway) , he merely just couldn't read a radar screen.

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Old 16-06-2014, 10:47   #185
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Re: Thoughts about AIS

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Originally Posted by goboatingnow View Post
I think those people that don't have to deal with some of the unique situations typical of high density traffic and more importantly high speed traffic , these areas are typical in certain pinch points around the Northern European coast.

My own personal experiences, mostly predate having AIS

couple of things

The Wakhuna accident , was in FOG. the running lights weren't visible and the whole process was done by radar. In was hindered by having the radar only visible below and hence details had to be relayed by the radar watch keeper.

The primary issue was that Wakhuna had entered the " cone of error", i.e. the radar plot of the approaching vessel simply was not accurate enough to determine whether he was already across or yet to cross the oncoming vessel. This is a very common issue and anyone with a predicated COG vector ship on ( or MARPA ) vector, will readily see that in a seaway on a small yacht they can be almost useless.

The failings on the bridge of the Nedloyd were much smaller, ground referenced ARPA is actually a common setting.

the only saving grace with AIS, is at least the bigger ships prediction is based on his data not yours.

Today , I use COG predictive vectors. I usually set them to max and equal time.

I then use the following logic

If they don't cross and my vector end before their vectors, i will pass astern, a simple rough proportional calculation will give you roughly where you will pass.

if they cross, again I do a simple proportional calculation ( by eye usually), if I'm and he's then we have a problem houston, if the proportions are skewed I can usually determine quickly if Im across or not at CPA.

Often I turn off all the alarms, cause otherwise its info overload. MARPA , I don't use at all in these situations, I simply don't trust it.

I agree crossing the TSS in the channel, is a butt puckering exercise, AT night its even worse and in fog, I just pray a lot. personally I try to cross the channel , outside the TSS areas.

you get the same problem sometime off Ushant, land ends TSS and approaches to Gib. not as bad though.

I agree with Pelagic, if you transmit AIS data that you are a sailing vessel, big ships are like elephants and yachts, mice.

Calling up bridges in these situations is just laughable.

dave
I've thought about your COG lines method, and about the fact that you extend the lines to maximum (hadn't thought about that and no one had suggested it).

I still don't think I would feel confident that this method was giving me much certainty about how I'm crossing with another vessel where there's a big difference in speed and a close crossing.

Until I have a better display of this information like Vesper and OpenCPN do it, I think I will continue writing down bearings, which works pretty well, and which is anyway a habit from pre-AIS days. It's a little awkward because you have to have a pencil and paper and you have to observe over some time, but I think when you're in a potential situation with several vessels you need to be concentrating on it anyway and not hoping to figure it out at a glance.

Maybe I'll get OpenCPN working on a tablet someday and will use that.
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Old 16-06-2014, 10:57   #186
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Re: Thoughts about AIS

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Originally Posted by goboatingnow View Post
No , that not correct, he simply mis-read his radar,



He was not actually in the cone of error ( well he could have been , but he'd never realise anyway) , he merely just couldn't read a radar screen.

dave
I don't really understand your point, since you haven't said anything which contradict anything I said. Yes, he mis-read his radar screen. And the specific information which he mis-interpreted from his radar screen was whether the container ship was passing ahead or behind. He read the screen -- based on using his bare eyeballs, since he did not know how to use MARPA (but MARPA would not have told him anything on this anyway) -- to say that the ship was passing ahead when it was not. And so he slowed down (which reduced CPA to 0) instead of speeding up (which would have increased CPA).

So why do you say "that's not correct"? What exactly was not correct? I don't think you are disagreeing with me.

And the cone of error discussion concerns the master of the ship, not the Wahkuna. He saw a few cables of CPA and relaxed, not realizing that he was in a situation where the errors in the system, including variability of the course and speed of the sailboat, were greater than the passing distance calculated.

I don't think you disagree with this, either.
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Old 16-06-2014, 11:01   #187
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Re: Thoughts about AIS

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Originally Posted by Pelagic View Post
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?
This is one of the advantages of a small boat with Class B AIS--you get to see the big boys and the developing situation before they recognize that you are there. During the time between when you see them and when they see you, you can make your course changes so that when you pop up on their display you are not a problem and they don't have to change course.

For example, a few of times I have been in near head-on situations with what was a comfortable 1 mile CPA to pass green-to-green, and the ship made a major course change to pass port-to-port. Now I know to either set up a port-to-port pass or get the starboard-starboard CPA out near 2 miles so I don't have to deal with a tense situation where the other vessel is changing course.


Don't ever think that if you change course 2 degrees that the ship is going to see the change and understand your intentions. AFAIK, the Class B transponders still transmit your unsmoothed COG, so what the ship display receives can jump around 5-10 degrees every time you update. Its a really dumb system, and easy to fix by using smoothing techniques.
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Old 16-06-2014, 11:04   #188
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Re: Thoughts about AIS

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I still don't think I would feel confident that this method was giving me much certainty about how I'm crossing with another vessel where there's a big difference in speed and a close crossing.

Where its useful , is in a multi-ship situation, i.e. several parallel ships at right angles to you offset from each other.

Actaully where there is greater speed on the ships , it makes the situation easier, because typically for the same time , they are either ahead, too close or you are well ahead. Th greater disparity in the length of the COG vectors, makes it easier to work out.

what it lets you do is see that you intended course, is safe, or not safe, it not a method to finally decide to "risk it".

My favourite senario is my COG vector falls short of the crossing ships COG vectors, then I am always clear and will cross by the stern. my CPA will tell me how close hence I can slow up to open that .

Equally if my COG vector end point is crossing , but well clear by a good percentage , then I know I will adequately clear their bows, again my CPA readings tell me by how far.

The tricky bit is when is not so clear, i.e. half and half, or both vectors short short of the crossing point ( i.e. you are trying to determine this from several miles away) .

Personally , I think being buried in a chart table is the very wrong thing to do. situational awareness is key. Hence I need a technique that I can run from the wheel.

its why I think radar and plotters that can ONLY be used below are utterly ridiculous and potentially unsafe.


personally I never sail across busy TSS's, I motor or motor sail.

dave
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Old 16-06-2014, 11:11   #189
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Re: Thoughts about AIS

Quote:
Originally Posted by goboatingnow View Post
Where its useful , is in a multi-ship situation, i.e. several parallel ships at right angles to you offset from each other.

Actaully where there is greater speed on the ships , it makes the situation easier, because typically for the same time , they are either ahead, too close or you are well ahead. Th greater disparity in the length of the COG vectors, makes it easier to work out.

what it lets you do is see that you intended course, is safe, or not safe, it not a method to finely decide to "risk it".

My favourite senario is my COG vector falls short of the crossing ships COG vectors, then I am always clear and will cows by the stern. my CPA will tell me how close hence I can slow up to open that .

Equally if my COG vector end point is crossing , but well clear by a good percentage , then I know I will adequately clear their bows, again my CPA readings tell me by how far.

The tricky bit is when is not so clear, i.e. half and half, or both vectors short short of the crossing point ( i.e. you are trying to determine this from several miles away) .

Personally , I think being buried in a chart table is the very wrong thing to do. situational awareness is key. Hence I need a technique that I can run from the wheel.

its why I think radar and plotters that can ONLY be used below are utterly ridiculous and potentially unsafe.


personally I never sail across busy TSS's, I motor or motor sail.

dave
Yes, it may be that with some practice I would feel more confident with that method, but I'm not sure. We'll see. It seems to me that it might be good to pick out the clear situations early and eliminate them, but not good at all for the hard cases, which are the dangerous ones, inherently.


I agree with you about instruments at the helm -- I think they're essential, especially chart plotter. But trying to work out a crossing with several different ships, I personally (YMMV) find it easier to really concentrate and understand the situation, at the chart table, with pencil and paper at hand. There is usually nothing you get from visual observation that adds anything to your understanding of this specific situation. Usually I have a crewman at the helm who is doing his own observation and thinking, and we shout back and forth to each other, "he's passing ahead, after all", or "what about the car carrier over there", and so forth, which I find really helpful, two heads being better than one.
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Old 16-06-2014, 11:17   #190
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Re: Thoughts about AIS

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I've thought about your COG lines method, and about the fact that you extend the lines to maximum (hadn't thought about that and no one had suggested it).
Sorry, I had been talking about 30min because that has been sufficient time for us to use them to navigate safely with AIS. My earlier confusion around why using COG vectors was a problem assumed that one would make them as long of a time prediction as one required for comfort.

For example, I can extend them into tomorrow on Coastal Explorer and OCPN if I wanted. I do think our Furuno chartplotter limits it to 30 minutes. What does the Zeus allow?

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Old 16-06-2014, 11:18   #191
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Re: Thoughts about AIS

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Sorry, I had been talking about 30min because that has been sufficient time for us to use them to navigate safely with AIS. My earlier confusion around why using COG vectors was a problem assumed that one would make them as long of a time prediction as one required for comfort.

For example, I can extend them into tomorrow on Coastal Explorer and OCPN if I wanted. I do think our Furuno chartplotter limits it to 30 minutes. What does the Zeus allow?

Mark
Up to two hours.
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Old 16-06-2014, 11:24   #192
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Re: Thoughts about AIS

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Yes, it may be that with some practice I would feel more confident with that method, but I'm not sure. We'll see. It seems to me that it might be good to pick out the clear situations early and eliminate them, but not good at all for the hard cases, which are the dangerous ones, inherently.
not trying to convince you, but consider this

(a) at least with AIS, you have more( I stress only "more") reliable CPA's, CPAs are the biggest issue

(b) One ignores all CPAs,that fall outside what you regard as dangerous


(c) The key with channel crossings as you said is that you want asymmetric CPAs. close once you are astern and greater if you are crossing ahead.

(d) Where you have a close call, I don't see any method being better then another, because personally I distrust my own COG vector ( and hence CPA) more then anything else, This is not fixed by using either Vesper or Open CPN. ( it was awful using MARPA)

(e) Personally I have not ever tried to run the TSS with marginal CPAs, too date I have found there is "adequate" windows once you don't try and just run across. The great thing about AIS is I can see the "bunches" of ships and try and time my crossing to slip into the "gaps", that was never really possible before.

(f) So the issue boils down to evaluating the CPAs that are inside ones own "danger zone", I take simple decision, vectors look to close in time, thats not a crossing I will risk. Usually the CPAs are screaming this anyway.

The other thing I find is with the transponder , I see ships in the TSS opening the CPAs themselves anyway.

I don't see what help plotting bearings is, all you are really doing is re-constructing something the electronics are doing for you anyway.
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Old 16-06-2014, 11:25   #193
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Re: Thoughts about AIS

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think I will continue writing down bearings, which works pretty well, and which is anyway a habit from pre-AIS days. It's a little awkward because you have to have a pencil and paper and you have to observe over some time, but I think when you're in a potential situation with several vessels you need to be concentrating on it anyway and not hoping to figure it out at a glance.
Do you really run up and down the companionway taking constant hand bearings and transposing them to paper down on a chart table, work out a solution, and then run up and get a new bearing, etc?

To me, the potential for errors in either transcription, calculation or timing seem very high.

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Old 16-06-2014, 11:30   #194
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Re: Thoughts about AIS

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Calling a ship to negotiate a crossing should only be done if one finds themselves in a impractical situation where maneuvering is difficult and one is the stand-on vessel. And only then if it is apparent that the burdened ship has somehow misread the situation. This has occurred to us only once in the past 6 years.

Mark
When a situation like this is developing, you should consider doing what 3rd world fishermen do--whistle into the mike on Channel 16. This may wake up a somnolent WK, and the clarity of your transmission lets him know that someone is close by.
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Old 16-06-2014, 11:34   #195
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Re: Thoughts about AIS

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Originally Posted by colemj View Post
Do you really run up and down the companionway taking constant hand bearings and transposing them to paper down on a chart table, work out a solution, and then run up and get a new bearing, etc?

To me, the potential for errors in either transcription, calculation or timing seem very high.

Mark
I take the bearings from the chart plotter. As calculated in AIS, they are much more accurate than with a HBC. Note that for all my call to caution in interpreting AIS data, AIS data is for sure an order of magnitude more exact than what you can get from MARPA or from a hand bearing compass.

Potential for error is practically nil, because you write them down every two minutes and you will have a whole series of them. An error here or there will not invalidate the series; it will merely create outliers which are immediately visible as such.

What you get from this is the direction of change of bearing, which tells you definitely whether you're going ahead or behind, and it tells you the speed of change. From a series of bearings you will know without any doubt whether you are passing ahead or behind.
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