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Old 24-08-2017, 03:36   #31
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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Originally Posted by Dockhead View Post
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.
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Old 24-08-2017, 03:39   #32
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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Originally Posted by Pelagic View Post
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?
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Old 24-08-2017, 03:49   #33
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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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...
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Old 24-08-2017, 03:50   #34
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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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
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Old 24-08-2017, 03:57   #35
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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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.
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Old 24-08-2017, 04:13   #36
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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Originally Posted by rramsey View Post
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.
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Old 24-08-2017, 05:03   #37
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Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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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

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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
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Old 24-08-2017, 08:21   #38
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

One of the best threads I have ever read.


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Old 24-08-2017, 08:34   #39
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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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.
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.
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Old 24-08-2017, 08:39   #40
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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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.
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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Old 24-08-2017, 09:35   #41
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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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
All wise words WJ but that only works when you have good visibility......
knowing how to plot radar targets in restricted visibility is another matter as is the measure of a safe CPA.
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Old 24-08-2017, 09:38   #42
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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[...] 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. [...]
When we are at sea and the boat is corkscrewing around with seas on the quarter I really can't reliably use the stanchion trick. This is why I love my hand bearing compass. In these conditions, AIS calculated CPA can also be difficult since your COG and speed are skewing so much (which is part of that "cone of uncertainty"), but with the proper filtering / smoothing you can still get useful numbers.
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Old 24-08-2017, 09:43   #43
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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Old 24-08-2017, 09:44   #44
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

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All wise words WJ but that only works when you have good visibility......
knowing how to plot radar targets in restricted visibility is another matter as is the measure of a safe CPA.
My advice to anyone in a small vessel especially if they have wooden or fibre glass hulls is unless you are caught at sea in bad visibility is to stay in port it is much safer, no matter what anyone tells you about radar reflectors unless the weather is flat calm there is good chance your radar return will be lost in the clutter, AIS might be Ok but radar is the main collision avoidance tool used in poor visibility.
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Old 24-08-2017, 10:57   #45
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Re: Collision Avoidance, Cones of Uncertainty, and Appropriate CPA

Good thread. The thing about different opinions in the end is that everyone can sail their own boat their own way. But it is good to hear what others do. My offshore sailing is in the central English Channel, including crossing overnight to or from France. This has got a lot less stressful since I fitted an AIS Transponder & a plotter at the helm which can integrate AIS & Radar and gives me vectors, CPA & TCPA. The shipping lanes are always busy. My philosophy is to stay out of their way wherever possible, and maintain a minimum of 1 mile CPA, especially at night, except where I am passing astern closer (say half a mile) so as to have the maximum distance in front of the next ship. Given the way CPA can flick around in a seaway, I dont otherwise like anything less than a mile and I dont like crossing the bows anyway. During the day and good visibility I'm happy to go a bit closer, but basically I just like to stay out of their way and make any course changes absolutely clear. Musnt forget all those vessels out there without AIS transponders, though.
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