As I see it the most difficult point about assesing whether a target is clearing or not is caused by the usually large difference in speed between say a yacht at 5-6 kts and, a large container ship at say 18-20 kts and perhaps a ferry at nearer 30 kts. You only have to try and asess this in daylight to see the difficulty.
The target seems to be on a constant visual bearing for maybe 20 minutes and then suddenly appears to zip forward and cross 1/2 mile in front of you.
This is why I like to plot their positions as waypoints, Tim you misunderstood my point, taking a spot distance & bearing on a radar target is simple, but remember one needs to take the ship's head at the same time, because what one wants is a compass bearing, not a bearing that just relates to the ships head at thast moment. if these are taken at 5 min internals and then either just plotted on a paper or on a plotter, one can build up a good picture of what is happening.
The big thing to be aware of though is that a yacht seen as a target by a fast moving ship (that's if we are lucky and the useless radar reflectors we are sold actual works) will appear to the ships bridge as virtually stationary (on a relative motion radar). This is why AIS is such a great idea, particularly if the info is actually overlaid on the radar display. it is also why Class B AIS for yachts is so brilliant, ships will then have information actually showing the course and speed of a yacht, whereas on a normal radar screen, they will be getting a small intermittent blip that could be interprited as being anytthing from a lobster pot to a stationary fishing boat. Lets be clear about this, ships coming in from deep sea and ships normally based outside of Europe are not expecting to see yachts, you just don't get them in most parts of the world.
I am more than ever convinced that there is a problem with the terminology in use here.
A relative motion display has your own vessel represented by a fixed point, usually at the centre of the screen. The display may be unstabilised (i.e. "Head up") or stabilised (usually "North up").
Radars in Head up mode usually have no compass input, so bearings can only be given in degrees relative. In that case, as you say, one needs to note the direction of the vessel's head at the moment the bearing is measured.
In order to obtain a north up display, some kind of compass input is required, so it is easy (and normal) for the the direction of the EBL to be displayed in degrees compass (= degrees true if the compass has been corrected.
A true motion display has your own vessel represented by a moving point, which moves across the screen in a drirection and speed corresponding to your movement through the water (sea stabilised) or over the ground (ground stabilised)
The presentation of the display and the quality of the data are two quite different things, but I suggest that if your heading data is is inaccurate when the radar is in relative motion mode, the situation is unlikely to be improved by switching to true motion mode, in which inaccurate speed data will be added to the innacurate compass data .
So far as plotting is concerned, I entirely go along with the idea of radar plotting. But there are several steps in radar plotting, and one can stop at any point. If it is obvious that the approaching vessel is going to pass clear, one can stop after the forst couple of plots. If it is looking a bit iffy, one might continue plotting, to make a better assessment of the CPA. It is probably relatively rare that a small craft skipper ever needs to calculate the other vessel's course and speed -- after all, if I can deduce that someting big and fast is likely to be sharing the centre of my radar screen with me in 12 minutes time, I don't really need to calculate that he is going to hit me at 27 knots!
But however far one decides to continue the plot, the conventional way of doing so is either on screen or on a relative motion plotting sheet. Plotting on the ground stabilised display of a chart plotter is misleading. And the conventional plotting interval is 6 minutes, rather than 5, because six minutes is one tenth of an hour. So if you work out that the other vessel has moved 1.9 miles between plots, you can immediatly tell that she is doing 19 knots
I entirely agree with you about AIS B. Brilliant idea. It will be even more brilliant when professional mariners start using AIS, rather than treating it as another irrelevant box of tricks foist upon them by the IMO!
Sure relative motion has you as the centre (or off set) and unless compass fed, shows your bow as up.
The problem with this is that bearings are relative to your heading. on small boats that heading is extremely unlikely to be consistent. This is why I like to use an autopilot in poor visibilty as it gives a much more comnsistent course.
But to avoid a collision one needs to plot and track the compass bearing!
The reason is that the collision avoidance bit has no relation to your heading at a specific moment but is to do with the relative compass bearings.
As an example ( and this is because of the usually large speed differences) one can prove mathematically that if one runs away from a bullit fired at one, the bullit never hits you, however we know thatsnot so. So if you have a target on you relative motion radar and alter course 90 dedgees, you will think it is not now not on a constant bearing, but if the compass bearing to the target remains the same, it will still collide with you. (or of course if goinfg away from you then get further away on constant bearing)
A stabilised relative motion set (eg north-up) *usually* displays compass bearings, not relative bearings.
And on a stabilised relative motion display, and you alter course by ninety degrees, the blob on the screen will stay in the same spot on the screen while you alter course. (of course it will -- neither the range nor bearing are likely to change significantly in the space of a few seconds) But your heading mark will move round the screen, in step with your change of heading.
An unstabilised relative motion display *may* display compass bearings, if it has an input from a heading sensor.
The crucial advantage of using a relative motion display is that a contact that is dangerous will be moving towards the centre of the screen. On a True motion display, it will appear to be heading for the as-yet-unknown spot in the water at which the collision may eventually happen.
Don't take my word for it: see www.mcaorals.co.uk/Radar%20Displays.htm
and see page 509 of the Admiralty Manual of Navigation Vol 1 -- available on the web at books.google.co.uk/books?id=GCgXCxG4VLcC&pg=PA509&lpg=PA509&dq=%22True+motion%22+radar&source=web&ots=Z6LkemD6xT&sig=idLvzo-FCmNHKZ3b7lsjpPI6fuo&hl=en&sa=X&oi=book_result&resnum=7&ct=result
But how many yachts have a stabilised screen not many I reckon - virtually all are head up, so this is why the bearing of a target must be taken in conjunction with the actual compass course being steered and plotted as the actual compass bearing of the target. With big speed differentials just relying on the bearing of a target relative to one's heading at a given moment won't help unles you are steering a very accurate course.
The point about plotting targets onto a piece of paper or your chart plotter is to give you an idea of the eventual destination and possible actions of your target. Without that hoped for understanding you are subject to uncertainty. This is particularly relevant when say crossing the North Sea when it it likely that there will be several targets, by plotting ther courses and speeds, you can determine that some may be altering course to avoid other targets, or altering course to stay in a particular lane in a separation zone (remenber not all these lanes are straight) this helps one to understand if a ship is starting to alter course onto you, to overtake or allow a right of way vessel to pass ahead of it.
It seems from virtually all of the major collisions that I have studied, that it is the unexepected course alteration of one ship for another than often leads to a third being hit!
And in busy shipping lanes our pathetic radar signatures are very easily overlooked when a ship is altering course for other ships.
I am now absolutely convinced that we have a conflict of terminology here, and nothing more.
I 100% agree that one cannot rely on relative bearings: they are only a rough guide, at best.
I 100% agree that compass bearings are vital for accurate collision avoidance. Whether one achieves this by using a stabilised display or by mental arithmetic and a paper plotting sheet is a matter of what resources are available.
I suggest we agree to differ about Relative Motion v True Motion, because I know for certain that this discussion is confusing other readers and is making them think that radar collision avoidance is far more complicated than it really is.
You two are taling same - but in different ways ... I think if both of you slowed down and read both scripts - you are singing same song !!
Recently I was on a friends boat who had only just fitted a typical radar set for small - medium sized sail-boats. He had no compass input but was able to input GPS data to stabilise the display. IMHO not the best - but must be better than nothing.
Even so I had extreme difficulty with it. The displayed info was blurring on the screen as the boats heading swung and GPS wasn't fast enough to react. The comment about autohelm is extremely vaild after that experience. Engaging it brought an improvement but still not the best.
Given that Fluxgate Compasses are not that expensive anymore, the Nasa in fact is less money than a good magnetic job !! There really should be a move to include compass source with the gear. Blimey a tillerpilot has a reasonable fluxgate built in - why can't a radar have it also !! Food for thought ??
