Assessing the risk of collision.

john_morris_uk

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Rather than drift the thread further in the OUZOU enquiry report thread, I thought I might pose a question and see what others think on the way we assess the risk of collision.

When crossing the channel with relatively novice crew, I am often called to give my opinion as to whether a ship is a 'threat' or not. The crew have taken bearings, but due to a mixture of their lack of skill with the handbearing compass and/or their impatience, they are not sure if the bearing is changing or not.

The reality is that the bearing often DOESN'T change very much at first, especially with a ship that is coming up fast and might be passing a mile or so ahead of you or behind you. As far as the ship is concerned he knows this from his Radar. He may even have tweaked his course a fraction to ensure that its a mile and a half and not just a mile. (He hasn't failed to comply with the IRPCS as he wasn't on a collision course anyway and is just enlarging the CPA) There is no risk of collision and nothing needs to be done other than monitor the situation carefully from the deck of the yacht.

Could this be one reason why we have ships who believe that yachts often don't know what they are doing? The bottom line is that people who 'give way to everything' are not obeying the IRPCS and are just as guilty of causing confusion as the ship that steams on regardless. And before anyone wades in, I too have been forced to avoid ships steaming up the channel with no-one to be seen on the bridge. I am not suggesting we stand on regardless - just make reasonable judgements.
 

Robin

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The voice of reason, John!

We watch initially, in good visiblity without radar, using simple visual aids if a ship has been spotted that might be a problem. Lining the other vessel up with a stanchion or an eyelet in a dodger whilst we are spot on course is one method and often easier than using a handbearing compass where small changes are not easy to spot with the card not being steady.

If radar is running (we don't have MARPA), the simple visual check as above probably equates to the 'tail' or 'plot' trail features on the radar which shows the approximate relative course of the other vessel. This simple radar aid often eliminates some vessels from being potential problems and allows concentration on the important one(s).

I fully agree that often the bearings do not change much until the range closes, the same is true using the stanchion/eyelet check as well. This is especially true if the other vessel's course is not at 90 degs to your own, like for example crossing the lanes east of the Ushant TSS when headed for Le Four from Dartmouth. The oblique angle in this case means that approaching vessels appear on a collision course for a very long time, then as they do get closer the angles change rapidly and the clear by significant margins. In these circumstances making immediate diversions in a 'might is right' move is unhelpful and probably just as likely to create a problem with yet another vessel as it is to remove a risk from one that really isn't a risk.

I remember once when crossing the Western Channel in thick fog, in company with another yacht. We were going south and approaching the west going feed into the Ushant TSS and the radar was full of targets, two of which were giving some immediate concern. Our friends in the other yacht called us on VHF and asked our opinion and I said I'm sure the first one will cross ahead easily but the second might be too close to pass ahead of safely. As soon as I stopped transmitting the call was interupted by another voice 'we will miss you by bl00dy miles, keep your course!' and indeed he was right as we could see for ourselves as we got closer, the CPA was over a mile and that was after we were out of his track line.

I've probably given the opening now for the 'always cross at right angles' debate but it really isn't that simple always. Bear in mind I'm not talking about crossing a TSS, only the 'lanes' between them that most ships follow as being the straightest/quickest route up and down Channel.

Robin
 

Bergman

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Absolutely right. Refreshing to see such clarity.

Of course

You are now going to be subject to torrents of abuse from those who consider standing on as somehow irresponsible.

Special prize for the first to post the "here lies the body" nonsense
 
G

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When crossing busy lanes you would be zigzagging back and forth like a yoyo if you changed course for every ship on the horizon. I stand on and just keep taking bearings until they are within about, very roughly speaking, 5 minutes of contact, before making an assessment of whether there is a collision risk. After all, if you make the decision too early then not only will you be spending an unnecessarily long time being erratic in the shipping lanes, they'll probably have changed course and speed anyway by the time they get near - not necessarily to avoid you, maybe just because they want to for whatever reason. So making too early a decision is completely pointless, and increases the risk.

It is just not possible to judge accurately at long distance which side the ship is likely to pass. Take a decision too early and you are as likely to be putting yourself into the ship's path as not.

If I do decide to make a change of course finally, then it is radical and takes me out of the ship's path as decisively as possible.
 

Poignard

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What I do is get a piece of paper, or use one of those waterproof notebooks if it's raining, and rule several columns. There is one column for each ship that I'm watching and at the top is some note to identify it, e.g. "gas tanker", "Maersk", "destroyer", "green hull", "orange hull" and so on.

Down the left hand side I enter the time the bearings are taken and enter the bearing of each ship under it's "name" at the time. How often the bearings are taken depends on the circumstances and the visibility. You can soon see if a ship is likely to pass ahead or astern or if she alters course. Any ship whose bearing remains constant is, as we all know, going to hit us [unless we are following it!]

I find this a great help, especially when there are numerous ships around.

I only take bearings with a compass. Bearings taken with respect to a stanchion etc are only reliable if your own course is steady, which implies calm conditions and/or a higher standard of helsmanship than is usually found on my boat.
 

Silverado

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For a long time - and it happened again recently - we are sailing/motoring along and a large vessel is encountered. When I am skippering I will decide to stand on or give way as required because it is nearly always obvious what you are expected to do. Granted when crossing a TSS like the one at Ushant it is better done in good vis and preferably under power in daylight. However what really annoys me is when we are sailing along at night and I am off watch and the crew feels that we need to alter course for an overtaking ferry (we're Med based). When I quiz them (having seen the plotter track) later I am told that we had to or we could have been run down. I ask what the ColRegs say about overtaking vessels, I ask about confusing the helmsman of the overtaking vessel. They argue that they felt unsafe. Do I change my crew or just give them a hundred lashes?
 

Poignard

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[ QUOTE ]
... the crew feels that we need to alter course for an overtaking ferry (we're Med based)...

[/ QUOTE ]

Maybe they have Googled "Greek ferry accidents"!
 

Richard10002

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[ QUOTE ]
When crossing the channel with relatively novice crew, I am often called to give my opinion as to whether a ship is a 'threat' or not. The crew have taken bearings, but due to a mixture of their lack of skill with the handbearing compass and/or their impatience, they are not sure if the bearing is changing or not.



[/ QUOTE ]

Very good - takes the heat out /forums/images/graemlins/smile.gif

With novice crew, if there is time, I quickly establish the concern, discuss what the rules say, then one of the crew will come up with a solution which works, or I will make a decision which complies with the rules and explain what I am doing and why.

http://www.sailtrain.co.uk/Irpcs/rule7.htm

particularly:

"(a) Every vessel shall use all available means appropriate to the prevailing circumstances and conditions to determine if risk of collision exists. If there as any doubt such risk shall be deemed to exist."

http://www.sailtrain.co.uk/Irpcs/rule7d.htm

After the event, I try to encourage lots of practice in taking bearings of other vessels, and judging whether bearings are steady or changing. I would discuss the importance of aspect, (helps identify vessels you can ignore in the short term), the use of radar, (distance off, use of EBM and ERM - we have 2 of each), and possibly do a few plots on paper.

In addition, I would discuss the various actions available to the give way vessel... substantial alteration, (30 degrees or so), show a different aspect, slow down or stop, turn round and go the other way......

That's probably more than enough for novice crew, but as they get more experienced, I introduce the concept of what I call "sphere of influence", i.e. the density of traffic helps determine how far I look in assessing a situation. If it's very busy, I might set the radar at a range of 2 or 3 miles, and concentrate on what's happening within this range, (flicking to larger ranges every now and then to keep an eye on the global picture). If it's not busy, 6 or 12 miles may be correct.

On the slowly changing bearing thing - it's a judgement call which is not dissimilar to the reefing decision, and it is reinforced by Rule 7. If you think a collision situation may exist, or have any doubt, then assume it does and act accordingly. If you are the stand on vessel, you continue to monitor the bearing, and the radar, and you will either see the other vessel alter course for you, or its' bearing will change as it gets closer, or you will decide that you have to take some action. If you are the give way vessel and you end up zig zagging all over the place, then so be it.

As people get more experienced, they become more able to assimilate a situation, and less unecessary zig zagging will occur.
 

snowleopard

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The wobbles in a hand-bearing compass make any collision risk assessment worthless until the vessel is within a mile or two.

I've just done a spreadsheet working out the rate of bearing change for a yacht at 5 knots crossing the path of a ship at 15 knots...


Yacht will pass 1 mile ahead of ship

time to CPA Distance Bearing
5 min 2.3 80
10 min 3.6 77
15 min 4.9 75
20 min 6.2 74
25 min 7.5 74
30 min 8.9 74


Yacht will pass 1 mile astern of ship

time to CPA Distance Bearing
5 min 0.5 31
10 min 1.7 61
15 min 3.0 66
20 min 4.3 67
25 min 5.6 68
30 min 7.0 69

If you take it that the minimum bearing change you can reliably work on is 5°, you will only see that when the ship is less than 2 miles away and 5-10 mins from the point where the tracks meet.

So, wait till you see the whites of their eyes!
 

Sea Devil

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Just to add 2p worth...

I find lining up the ships two masts or failing that the angle of the bows very useful - works great at night when they have the two white lights on....

Basically if you can see a gap between the masts or the light you are OK if you can only see one side of the bow... If the gap is tiny or you can see a hint of the other bow you have a problem...

Inshore it is also useful to see if the land is 'moving' behind the ship ... if its not you have a problem...

I would suggest that it is better to use several methods - or be able to use several methods than rely just on bearings which with truly big ships in lumpy seas are not slow to reveal the problem..
 

peterb

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Just one tip, which only works on clear nights. Compare the ship's lights with the star nearest to them. If they stay in the same relative positions then there is risk of collision. It's far more sensitive than a hand bearing compass.
 

FAITIRA

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Thats a good one,
I find fishing vessels a greater prob than large commercial vessels, due to unpredictable course changes and deck lights obscuring their nav lights.
 
G

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It's a good tip, tho be aware that the star Earth spins surprisingly quicky, so the star will have shifted 4 degrees during the, say, 15 mins from first sighting a ship on the horizon to it getting close.
 
G

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[ QUOTE ]
I only take bearings with a compass. Bearings taken with respect to a stanchion etc are only reliable if your own course is steady, which implies calm conditions and/or a higher standard of helsmanship than is usually found on my boat.

[/ QUOTE ]
I think that the biggest problem with the taking a sight along a stanchion approach is more if you've shifted your head an inch one way or another it can throw you off several degrees. It is extremely difficult to keep your head in the same position for several minutes with that degree of accuracy, especially on a moving boat.
 

rwoofer

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So good to hear that I'm not the only heretic, when it comes to standing on. Given the mismatch in speeds, ships actually have far more control over the CPA than a yacht does. It's like trying to cross a fast road on foot, it's very hard for a pedestrian to judge a gap, but actually much easier for a driver of the fast car to judge and alter. My personal experience fits in with the theory above, you can't really judge collision course until the ship is within 2 miles and to be really sure I would say closer to a mile, which is when I might make the decision to adjust speed or course.

I reckon that in the last 5 years I have not had to change course once whilst crossing the shipping lanes. Virtually all ships that I come across do steer to good CPAs whether they are right of way or not.
 

Richard10002

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[ QUOTE ]
Basically if you can see a gap between the masts or the light you are OK if you can only see one side of the bow... If the gap is tiny or you can see a hint of the other bow you have a problem...


[/ QUOTE ]

Cant quite agree with that.

If you are sailing say due south, and a crossing ship, forward of your beam, is heading due east, you would see a gap between the masts and have a clear view of his port side, but you could still be on a collision course.
 

Robin

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[ QUOTE ]
I think that the biggest problem with the taking a sight along a stanchion approach is more if you've shifted your head an inch one way or another it can throw you off several degrees. It is extremely difficult to keep your head in the same position for several minutes with that degree of accuracy, especially on a moving boat.

[/ QUOTE ]

It works well enough in the initial stages when contact is first established. You have to take 'note' at a time when you are spot on course (as you do with radar) and it goes without saying you need to stay in the same spot. As has been ably demonstrated by Snowleopard's spreadsheet, at long ranges not much changes! Compasses swing around too and we generally have a pair of binos with built in compass as well available if needed.

I agree too that things are much easier at night (fishing boats excluded!) because in daylight I sometimes find it difficult to tell the front from the back of some modern ships, like those with the bridge at the front. At night the mast light angles are very helpful in judging if you are crossing clear and finally get the red/green (phew) then clear across to the other side!
 

shmoo

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Nothing in this post is meant to suggest AIS should be used alone without lookout, or that it is the answer to everything. By AIS I mean passive rx only on the yacht.

We pick up ships on AIS a little while before we see their masthead lights (at night, obviously). Often as far away as this, if you really glue yourself to the screen of the nav computer you can see the Rate Of Turn suddenly jump up and then return to zero as they change course to miss us. Sometimes they leave this late enough for our collision alarm to sound for a while, then go off again as they change course.

If they haven't seen us or look like they are not going to give way, that starts to become clear round about about a TCPA of 10 minutes and the other big benefit of AIS comes into play - we can call them BY NAME and CALLSIGN. I am not going to stand on regardless but it is reassuring to negotiate our giving way with KNOWING they are going to hold their course.

All the while we take barings in the usual way. I have got much better at estimating distance and assessing the situation generally by having both methods available.
 
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