Wahkuna .. again

[jimi]

Very interesting analysis in the Cruising Association mag, whereby they draw the conclusion was by stopping and drifting about with the radar on head up diplay led the crew to beleive all was well, whereas if they had North up or maintained a constant heading it would have been apparent all was not well. A lesson to us all, and one I feel that should get wider attention.

 

[Robin]

Jimi

The lesson is that there is no point in buying toys if you don't know how to play with them. Same will go with knobs on for NASA AIS IMHO BTW FWIW. Too many folks think what is on the heading line from one quick look is what you will hit or be hit by, whereas in truth you need to watch the video not the stills and the line that matters most is probably the EBL.

Robin

 

[doris]

I have always felt that not enough emphasis has been put on the fact that by being dead in the water and 'drifting about' the Wakuna was a target looking to be hit. I hadn't thought about how that would have made 'head up' irrelevant and I thank you pointng that out.

 

[ParaHandy]

there's a crucial bit in his analysis which doesn't quite ring true which is that halfway through the turn to starboard - whilst all but stationary - the CPA increased to 1.5nm. There wasn't any condition that I foresaw where that could be the case (I could be and probably am, wrong) ....

 

[jimi]

My simplistic interpretation of the article was that an EBL had been put on the target but because the Wahkuna's head was drifting about it moved the target off the EBL and led to a misleading impression of the CPA. Had the head been constant or the display been north up then the EBL would have showed a clear picture.

 

[tome]

I've not read the article yet but will take a gander this evening. CPA's are notoriously inaccurate, especially very low speeds where the vectors jump all over.

 

[Robin]

Stopping is however still a useful tactic on occasion but those (most) of us without North Up radar facilities can still do it by keeping a minimum amount of way on, only just enough to maintain the heading. Best with an autopilot steering because you need to be only just moving, and best also if done quite a bit farther away than right under the nose of the ship you are trying to miss as did Wahkuna!

 

[peterb]

The real CPA never changed from the direct hit that ensued. The problem was that Wahkuna's turn caused the display to look as if the ship was passing across the bow. At the moment that Wahkunah was heading directly at Vespucci, Vespucci was about 1.5 miles away, and that made it look as if the CPA was about 1.5 miles.

Interesting to see that so many of you are CA members.

 

[ParaHandy]

ah .. yes ... of course, he's heading straight for her. That implies that wahkuna had (by then) lost all spacial awareness and hadn't a clue .... nothing to do with cpa's etc ...

ahh ... snap .. the article was written by you? well done. it fits

(jimi's only joined cos he needs a new flag)

 

[ParaHandy]

there's an interesting bit abt AIS in the CA mag which offers a reasoned argument for it ... more than you might be able to cope with ...

 

[philmarks]

I haven't done the radar course, and I may be doing the wrong thing, but when there is a close quarters situation I zoom in on the range. The article in the CA mag shows the range rings at 2nm, presumably as they were set. On occasion I have gone down to 1/2nm range rings (those occasions were not in fog thanks goodness!). I also maintain a plot of any dubious targets.

Very valuable point though about maintaining steady heading. I am usually under A/P particualrly in poor vis, as 2 more eyes then available for lookout.

One other point, surely it is better to maintain at least some steerage way (and on steady heading), then at least you have some control over aspect in dire last minutes. I think Wakhuna lost some time getting into gear and starting to move.

 

[Robin]

Sorry, we are no longer members as of some years now.... /forums/images/graemlins/tongue.gif

I'm not anti AIS either, just the idea that it is the poor man/small boat's answer to fog! I see it as a useful addition to radar, not as a alternative and I see it as a second choice not as first choice purchase. How many packs of ciggies equals one radar set I wonder? /forums/images/graemlins/laugh.gif

 

[peterb]

[ QUOTE ]
I haven't done the radar course, and I may be doing the wrong thing, but when there is a close quarters situation I zoom in on the range. The article in the CA mag shows the range rings at 2nm, presumably as they were set. On occasion I have gone down to 1/2nm range rings (those occasions were not in fog thanks goodness!). I also maintain a plot of any dubious targets.

[/ QUOTE ]

The MAIB report says that Wahkuna's radar was set to 6 mile range. Range rings aren't mentioned, but 2 mile rings are normal on the Raymarine radar when it is set to 6 mile range.

I doubt whether dropping down in range would have been helpful. Once they had made the decision that the ship was missing them by 1.5 miles they stopped looking at the radar. If they had been looking at it, it would have been obvious that the ship was closing rapidly, even on the 6 mile range.

 

[Robin]

Close quarters in fog with the big stuff is bad news big time!

If you are plotting either using the EBL or doing a paper plot or even using a 'trail' plot it is important only to take the bearing when your vessel is right spot on course otherwise you will get a false idea of whether the real bearing is changing. It helps if the range is set so that the target vessel is at the edge of the screen, especially on the small radar screens most of us have. Routinely, we watch on 6ml range and only drop to 3ml for the reason mentioned as far as ships are concerned, but in areas away from the big stuff we might look more often on 3ml range in case there is small stuff about that doesn't show up as well on 6ml. Ranges under 3ml are really only of use entering harbour or whatever and you may need to adjust the gain control even then as the screen can get swamped if there are strong targets, including land. For initial use 3ml is too short a range for big ship detection, they can cover that distance too quickly, hence we use 6ml but in the lanes we might take occasional looks at 12mls to see what is just off the screen.

Personally I don't like range rings especially on small screens as they clutter the view and could even hide a small target. You soon get to visualise approximate ranges (1/3rd of radius on 6ml range = 2mls for example) and if you need to know better than that then put the curser on the target.

Radar is a wonderful tool but you do need to practice with it regularly in clear weather, that way gives more confidence when fog comes down and a good time is one of those boring no wind days especially if crossing the lanes.

It also takes a little thought beyond the initial target, it is all very well taking avoiding action for number one but not to forget the next in line could then also be a problem!

 

[graham]

If the person on watch is tired confused or has no training or experience it wont matter if North up or Head up or Arse up.In fog I think safest option is for the most experienced person to have control of the radar with a helmsman to take his orders.

You cant concentrate on collision avoidance when you have other things to do.

Generally I allways leave range rings on as it quickly makes it obvious if the range to a target is reducing.You can turn them off momentarilly if you need to look under them.

 

[Robin]

[ QUOTE ]
If the person on watch is tired confused or has no training or experience it wont matter if North up or Head up or Arse up.In fog I think safest option is for the most experienced person to have control of the radar with a helmsman to take his orders.

You cant concentrate on collision avoidance when you have other things to do.

[/ QUOTE ]

Well said! I would just add that the helmsman should be standing by but with the autopilot steering, autopilots don't get disorientated in fog as us humans sometimes do.

 

[peterb]

EBLs

[ QUOTE ]
If you are plotting either using the EBL or doing a paper plot or even using a 'trail' plot it is important only to take the bearing when your vessel is right spot on course otherwise you will get a false idea of whether the real bearing is changing.

[/ QUOTE ]


You're quite right about the EBL moving. The obvious way to find out whether you're on a collision course is to mark the position of your target by aligning a VRM and an EBL with it, then leaving it there to see if the target moves. The problem, of course, is that your ship isn't stable, so that the EBL keeps on moving. But I wonder whether the radar manufacturers could deal with that.

If the radar is being used in head-up mode then one might expect the EBL to move with the ship. But if the radar is being used in one of the stabilised modes (north-up or course-up) then the radar has within it data on the heading relative to north. In that case, it shouldn't be beyond the wit of man to provide an EBL which is stabilised with respect to north, rather than with respect to the ship's head. Such an EBL would make it much easier to find out the relative motion of your target.

At LIBS I went round all the radar manufacturers asking how their EBLs were stabilised. Almost all of them said that if you were using a stabilised display then the EBL would be stabilised with respect to north, irrespective of ship's head. At the last stand I visited (can't remember whether it was Furuno or Simrad) they said that their EBLs were fixed with respect to the ship's head under all conditions, then added "just like everyone else's". I immediately queried it, but they were supported by one of their distributors who happened to be on the stand, and who also sold other makes. Then I made a second round of the stands, making sure this time that I got an 'expert' rather than a salesman. They all agreed, their EBLs were stabilised with respect to the ship's head!

Maybe the most interesting point was that when I described the need for a stabilised EBL, their 'experts' agreed that it would be a good thing, that it shouldn't need any hardware changes, and should be achievable by a relatively simple software change. I'm going to be asking the same question next year!

 

[Robin]

Re: EBLs

[ QUOTE ]
You're quite right about the EBL moving. The obvious way to find out whether you're on a collision course is to mark the position of your target by aligning a VRM and an EBL with it, then leaving it there to see if the target moves. The problem, of course, is that your ship isn't stable, so that the EBL keeps on moving. But I wonder whether the radar manufacturers could deal with that.


[/ QUOTE ]

I'm not sure I would trust a north up display either for the reason that unless you have a stable fast acting compass heading reference like a gyro it isn't going to be accurate enough. This is also the reason I don't feel happy about MARPA on small boat radars, it is open to the 'calculator/computer error' by which I mean we believe the answer simply because it says so. If the calculator say 2 + 2 = 6 then 6 it must be mustn't it? This is also a bit like working out tide heights the RYA way to 3 decimal places when in my book the 12ths system with a large safety margin might be better in practice!

In our case we have radar only at the chart table but we also have a 'multi' instrument head and an autopilot control adjacent to it, as well as a chart plotter if relevant. The 'multi' can be set to read ship's head taken from the fluxgate compass that runs the autopilot. So we set the EBL and range ring on the target as you, then compare the target's position relative to the EBL/range periodically but only make the comparison when the multi shows we are STEADY on course, not just 'passing through' the course mid-yaw! In fog I am happiest when the target is passing ahead of us, if it shows us passing ahead of the target I like to see this as being by a large margin and by more of a margin than I would willingly accept in clear eyeball visibility.

 

[jimi]

Re: EBLs

Peter, I'm not sure if I understand exactly what you are saying here. If I set my display to north up (using the interface to the flugate compass used in the autopilot) and put an EBL then surely that then is relative to North?

 

[peterb]

Re: EBLs

Surprisingly, no. The EBL will be locked to your boat head. The readout may show the EBL as relative to boat head (in which case it will give a steady reading even if the boat turns) or it may give it as relative to north (in which case the EBL reading will change as the boat turns). As far as I could tell from the answers at LIBS, all makes work in this way. Bearing in mind what Colregs say about ships being on a steady bearing, I was surprised that no-one seemed to have latched on to the idea of an EBL which did what it said on the tin; i.e. gave you an electronic line of constant bearing.

Come to think of it, if Wahkuna had put an EBL on to the ship's echo, then the turning of Wahkuna would have quickly taken the EBL off the echo and given the impression that there was no imminent collision.