AIS

[Genoa]

Why change your course? If you are stand on - stand on, and let them change. Its pretty easy for them, and far less confusing for all.

How the **** do you know if you or they are stand on at 4 miles???
Even with the best bins in the world, in the cockpit of my boat I couldn't see if that (big) ship is showing any shapes to indicate I should keep clear, and in the interests of safe navigation I have to assume I am give way unless I am confident of the opposite.

"Restricted in ability to manouvre" can apply to any vessel under the control of a VTS or similar.

AIS gives you valuable data to determine well in advance what the possible scenario might be and plan for it. You don't wait til you are sailing to get the weather forecast. You plan!
Why is everyone so keen to shoot down another information source?? Using it to the exclusion of other information is clearly wrong, but ignore it at your peril (RIP Ouzo). Yes the Collregs state power gives way to sail, but they also say, keep a good lookout, and in my book that means eyeball AND available information from other sources. If Notices to Mariners tells you of a bouy that's light has failed you consider it and keep clear of the area or increase your lookout, even though you can't Mk 1 eyeball it. AIS is no different.

 

[haydude]

Because at 2 miles off with a CPA of 0.0 and a TCPA of 7 minutes or so I don't want to wait any longer to see if they are going to give way.

Does your plotter give you these values? Also please could you spell the acronyms for me, I understand that CPA is something about minimum distance ...

 

[claymore]

Webbmeister

.
Two or three long threads recently along the lines of 'you see three AIS targets on bearings bla bla bla twelve miles away"

AIS is surely for use when you are within normal visual range and the target isn't visible? Same as radar?

Anyone who in normal vis (+3nm say) makes navigational decisions based solely on AIS or radar is surely missing the point and likely to be disappointed.

- W

I read your post with interest. Naturally you are taking a virtual stance on this rather than one where a hint of reality kicks in.
Let us begin with your sailing ground: The NW coast of the Land of God. Just remind me where we have a 12 mile straight stretch. And whilst you are at it - just remind me how many "Ships" we have that will be equipped with AIS. I assume they will include the Northern Light vessel, the Sanda Vessels and the good old Calmacs.
Why are you getting steamed up and involved in a topic that is patently beyond your scope, my ageing passion fruit?

 

[boatmike]

Closest Point of Approach

 

[Genoa]

Sorry but in my view this predictive avoidance at long ranges is plain daft when you have the ship visual anyway. If you work a similar scheme whilst crossing say the central English Channel or the lanes east of the Ushant TSS you might still be there now waiting to cross.

QUOTE]

There are two problems with your statement here.
First - "plain daft"! - Never would I say that using any source of fairly accurate data to determine the liklihood of a catastrophe as daft. Daft is ignoring information that can help you make better decisions where ever it comes from

Second - Au contraire! If you try and cross busy shipping lanes without looking for a big picture it is getting increasingly difficult. Mid channel you regularly get 2-3 lanes of ships going in one direction. It is often difficult to work out which side a more distant inbound is going to go of the one close by. Because such vessels are usually following a fairly strict course/speed agenda, AIS makes it simple to decide what your track should be between several vessels. Without it, you take the "one ship at a time" approach which is absolutely fine (we all did it) but can lead to you having to take dramatic avoiding action between ships, and I find that normally leads to tipping crew out of bunks or spilling my G&T. I'd rather predict positions and find a likely gap to head for.

On a different point, I agree CPA (when on a variable speed yacht) can be misleading but the prediction of ships positions is usually pretty accurate. You have to time your approach. Incidentally as far as stand on / give way is concerned. If you are crossing, you are give way even if you are sailing.

 

[Chris_Robb]

Because at 2 miles off with a CPA of 0.0 and a TCPA of 7 minutes or so I don't want to wait any longer to see if they are going to give way.

Quite happy with 2 miles, I would wait longer. Because yachts are so unpredictable in their actions due to their failure to abide by the rules, ships are leaving it till much later to change course.

 

[shmoo]

Does your plotter give you these values? Also please could you spell the acronyms for me, I understand that CPA is something about minimum distance ...

CPA - predicted target range at closest point of approach
TCPA - predicted time to CPA

[Radar and ARPA manual By Alan G. Bole, Alan Wall, W. O. Dineley]

 

[Bat21]

I read your post with interest. Naturally you are taking a virtual stance on this rather than one where a hint of reality kicks in.
Let us begin with your sailing ground: The NW coast of the Land of God. Just remind me where we have a 12 mile straight stretch. And whilst you are at it - just remind me how many "Ships" we have that will be equipped with AIS. I assume they will include the Northern Light vessel, the Sanda Vessels and the good old Calmacs.
Why are you getting steamed up and involved in a topic that is patently beyond your scope, my ageing passion fruit?

I found that AIS was invaluable with the Rhona vessels in that neck of the woods if you have a transponder, they have on several occasions altered course to open up the CPA whilst not in sight, their change of course would not have been necessary but to avoid a close encounter with me. The Calmacs however appear to home in on AIS targets to see how close the encounter can be before the victim gives way!

 

[Bilgediver]

It was my understanding that the AIS system, using VHF channels 87B and 88B, has 'quasi-optical' range and I struggle to understand how use of these frequencies achieves 'over-the-horizon' and 'through-terrain-masking' capabilities.



Shall all be revealed?

In actual fact VHF does seem to get over headlands and around islands so it is definitely a good possibility that ships will get AIS returns when no radar returns are available. Also in many cases the antenna may be so high it is peeking through the trees on the headland.

You are more likely to get a VHF signal round a headland reflecting of something than a radar signal.

I can monitor VHF from home and listen to ships from out in the north sea on the east all the way to Grangemouth on the west yet I can see very little of the coast.

 

[huzar30]

Well, as it happens, I was motoring!

If you have him visual and he's crossing in front of you, then the only way you can tell if you're going to collide is with a hand bearing compass. The reliability of this method leaves, in my opinion, something to be desired. AIS will also give you the relative bearing far more accurately and easily than with a hand bearing compass.

I had 40 minutes warning that we were going to come close to each other. I was able to check every five minutes or so what the situation was.

I could also see that other ships in the vicinity had a CPA of 2 miles or more, at which point I basically ignore them.

I think the comment about Ushant doesn't match reality. I'd be far happier crossing the lanes at Ushant with AIS than without. Without AIS, I'd have to scan the horizon and take bearings on likely suspects, or watch their aspect. Now I can look up the ship bearing xxx degrees, see its course and speed [knowing the speed is quite useful], and where and when the CPA will be.

I wasn't using a plotter, but instead a laptop with SeaPro [a very nasty bit of software, but it does interface with an AIS]. As to passing in front/behind, the software also draws two little dots - one for where he will be at CPA, and one where I will be. Thus I can also see whether I passing in front or behind him.

It's not a substitute for radar. Radar does a different job. However, the AIS reduces the workload very considerably [and I sail singlehanded], and gives me much more peace of mind than standing on a swaying deck holding a compass to me eye and trying to estimate the bearing to within five degrees.

These arguments about AIS are like those about GPS 20 years ago, VHF 30 years ago, and the use of the lodestone 400 years ago.

 

[bilbobaggins]

These arguments about AIS are like those about GPS 20 years ago, VHF 30 years ago, and the use of the lodestone 400 years ago.

Gosh, you've been around a bit!

As for 'these arguments', I learn something every time I'm wrong.

 

[Robin]

Sorry but in my view this predictive avoidance at long ranges is plain daft when you have the ship visual anyway. If you work a similar scheme whilst crossing say the central English Channel or the lanes east of the Ushant TSS you might still be there now waiting to cross.

There are two problems with your statement here.
First - "plain daft"! - Never would I say that using any source of fairly accurate data to determine the liklihood of a catastrophe as daft. Daft is ignoring information that can help you make better decisions where ever it comes from

Second - Au contraire! If you try and cross busy shipping lanes without looking for a big picture it is getting increasingly difficult. Mid channel you regularly get 2-3 lanes of ships going in one direction. It is often difficult to work out which side a more distant inbound is going to go of the one close by. Because such vessels are usually following a fairly strict course/speed agenda, AIS makes it simple to decide what your track should be between several vessels. Without it, you take the "one ship at a time" approach which is absolutely fine (we all did it) but can lead to you having to take dramatic avoiding action between ships, and I find that normally leads to tipping crew out of bunks or spilling my G&T. I'd rather predict positions and find a likely gap to head for.

On a different point, I agree CPA (when on a variable speed yacht) can be misleading but the prediction of ships positions is usually pretty accurate. You have to time your approach. Incidentally as far as stand on / give way is concerned. If you are crossing, you are give way even if you are sailing.

1) Sorry I think it IS plain daft to be worrying about a ship that is 7 mls away that you already have visual. It would be different if the visibility was bad. I don't think there is any likelyhood of catastrophe at 7mls unless the ship is firing missiles. Sure take note from AIS that it is there and confirm you have it visual but to alter course for it at this range IS daft.

2) We could be classed as frequent flyers across the Channel and on occasions, in thick fog even, have had over 10 ships on radar within the 6ml range so well understand the concept of the big picture. I counted 14 ships in visual range at night in the lanes east of Ushant this August. Predictability is very much the name of the game but trying to dodge the multitude in one action isn't in my view feasible and makes you the unpredictable one. One step at a time, like one shot at a time in golf is best in my view.

3)

You have to time your approach. Incidentally as far as stand on / give way is concerned. If you are crossing, you are give way even if you are sailing.

NO NO and a thousand times NO! You are confusing crossing shipping in open sea that is lined up with crossing shipping in a TSS which is entiely different.

Sorry if that sounds pompous, I'm not really like that but tend to write like it sometimes. I am just getting increasingly concerned that for some people AIS is seemingly being used to make detailed advance plans (or overplans) rather than taken as useful extra information to be used with caution.

By the way when you get a CPA from AIS do you know if it was calculated from a GPS position at the front back or middle of that huge tanker? Was the ship and your GPS on the right datum? Did the ship's GPS have a momentary woopsee and not update for 5 minutes?

 

[Genoa]

These arguments about AIS are like those about GPS 20 years ago, VHF 30 years ago, and the use of the lodestone 400 years ago.

So true. It is another tool which we should embrace for what it does for us. Not reject out of hand.




Also, to an earlier post about the difference between TSS and shipping lanes :

A vessel proceeding along a narrow channel must keep to starboard.
Small vessels or sailing vessels must not impede (larger) vessels which can navigate only within a narrow channel.
Ships must not cross a channel if to do so would impede another vessel which can navigate only within that channel.

It is my understanding that parts of the English Channel fall into this category as well as the TSS. For some large ships the navigable parts of the English Channel are "narrow"

 

[bilbobaggins]

One step at a time, like one shot at a time in golf is best in my view.

I'm not going to disagree; this brings another, related, topic to mind, and I'll open a different thread for that.

 

[Robin]

If you have him visual and he's crossing in front of you, then the only way you can tell if you're going to collide is with a hand bearing compass. The reliability of this method leaves, in my opinion, something to be desired. AIS will also give you the relative bearing far more accurately and easily than with a hand bearing compass.

I can usually tell visually if I'm about to run into a wall of steel! The use of a handbearing compass is to determine if there is a potential problem arising to be aware of. In practice in good visibility I cannot remember when I last used my handbearing compass! The important thing is not the bearing itself but if the bearing is changing and which way. I have a favoured perch in the cockpit (complete with backrest and armrest padding on the gantry) from which I will monitor relative visual bearings, lining up the target on a particular dodger eyelet, stanchion or the like but only when we are spot on our course. It is plenty accurate enough in good vis because we never aim to cross too close in front of something, too close being a relative distance and varies to suit the occasion.

I think the comment about Ushant doesn't match reality. I'd be far happier crossing the lanes at Ushant with AIS than without. Without AIS, I'd have to scan the horizon and take bearings on likely suspects, or watch their aspect. Now I can look up the ship bearing xxx degrees, see its course and speed [knowing the speed is quite useful], and where and when the CPA will be.

Me too but I can tell you I'm still more interested in what I actually see than what the laptop says! But whilst I would be very pleased to have AIS data available I wouldn't be using it to plot a course in and out of ships showing at 7mls range.

As a matter of interest for all the ships out there and the number of times we've been in amongst them the reality is we rarely have to alter course at all whoever was stand on and irrespective of being under sail or power. We do have to alter sometimes of course but with care and without double guessing too far out it often proves unnecessary as the situation evolves.

 

[Robin]

Also, to an earlier post about the difference between TSS and shipping lanes :

A vessel proceeding along a narrow channel must keep to starboard.
Small vessels or sailing vessels must not impede (larger) vessels which can navigate only within a narrow channel.
Ships must not cross a channel if to do so would impede another vessel which can navigate only within that channel.

It is my understanding that parts of the English Channel fall into this category as well as the TSS. For some large ships the navigable parts of the English Channel are "narrow"

Which bits? The English Channel where the 'lanes' are certainly isn't narrow to anything.

I'm not talking about crossing shipping in Southampton Water, Needles Channel or the Solent but out in mid Channel. In the narrow bits around Dover Straits the ships are in TSSs anyway.

 

[Genoa]

1) Sorry I think it IS plain daft to be worrying about a ship that is 7 mls away that you already have visual. It would be different if the visibility was bad. I don't think there is any likelyhood of catastrophe at 7mls unless the ship is firing missiles. Sure take note from AIS that it is there and confirm you have it visual but to alter course for it at this range IS daft.
?

To me one of the problems here is that situations are answered that are not the ones used in original examples. As I said, the scenario I faced was a vessel hidden be another in good viz, which I find increasingly common. I never mentioned any distance, and certainly not 7 miles. However, now you've arbitarily thrown in 7 miles, how about the the two vessels appraoching at combined 21 knots, you with the kite up. Suddenly 7 miles (20minutes) starts to look like an important time to decide what to do, or at least PLAN.

2) We could be classed as frequent flyers across the Channel and on occasions, in thick fog even, have had over 10 ships on radar within the 6ml range so well understand the concept of the big picture. I counted 14 ships in visual range at night in the lanes east of Ushant this August.

Using radar is great. And visual contact is better, but what is wrong with another source of more detailed data?

Predictability is very much the name of the game but trying to dodge the multitude in one action isn't in my view feasible and makes you the unpredictable one. One step at a time, like one shot at a time in golf is best in my view.

Agree that predicability is the name of the game, so trying to make as few course alterations as possible must be helpful? If you take the one at a time approach (and the golf analogy - a straight walk from T to pin never happens if your shots are like mine
) then each time you pass one ship you change course again. If I was 1 of those 14 ships you counted and I could see a blip "bouncing from one avoiding strategy to another I'd be more confused than see a vessel maintaining constant course (admitedly potentially varying speed, under sail, but something you could average out)

NO NO and a thousand times NO! You are confusing crossing shipping in open sea that is lined up with crossing shipping in a TSS which is entiely different.

You are partially right here, but for some of the vessels in the lanes in the Englsih channel between the Casquets and Dover they would consider they are in a "narrow channel" and therefore rule 9 applies. See attachment

Sorry if that sounds pompous, I'm not really like that but tend to write like it sometimes. I am just getting increasingly concerned that for some people AIS is seemingly being used to make detailed advance plans (or overplans) rather than taken as useful extra information to be used with caution.

Not at all Robin - keep up the healthy debate. But the more people use these tools the more likely we all are to use them better. Like Radar, so often folk only turn it on in the fog and forget how.

By the way when you get a CPA from AIS do you know if it was calculated from a GPS position at the front back or middle of that huge tanker? Was the ship and your GPS on the right datum? Did the ship's GPS have a momentary woopsee and not update for 5 minutes?

As I've said, AIS is no substitute for standing in the cockpit and watching. But it does provide valuable information. Why else is it becoming a tool for all larger vessels? The quality will improve as usage increases. As for not updating for 5 mins, if a vessel is travelling at more than 5 knts you easily see the lat long of their transmissions change.

 

[ksutton]

To the non believers.

Make no mistake about it, all of these vessels are watching one another as a back up to enhance visual and as a support to radar in good and bad viz, thanks to AIS.

Typical AIS reception range will be 25 to 35 miles.

http://www.marinetraffic.com/ais/default.aspx?centerx=30&centery=25&zoom=2&level1=140

Ken

 

[Sandyman]

Different views on what constitutes a narrow channel I suppose.

 

[jonjo]

With respect at 7mls range and 40 minutes predicted to CPA the prediction is not very accurate simply because your COG/SOG will not be constant except possibly in a dead flat calm under engine.

If the figure is stable over a couple of minutes or 0.1nm is the average then it is a reasonable number to trigger early action. It will be more accurate than any any Marpa yacht radar trying to infer crossing vectors from minute relative changes in the position of the distant ship.

The CPA by the way isn't necessarily how close you would be when in the area where you could be run down... The CPA is reporting how close the ship is when at it's closest point to you

You are stretching the significance of this point way too far and probably confusing readers struggling with the basic technical concepts.

0.1nm is way to close for me, either under the bow and passing down the side of a super tanker. Anyhow you are overlooking the point that given AIS bearing at CPA a you can infer whether the CPA is infront of the bow or on the beam of the ship.