MARPA - CPA values?

stiknstring

stiknstring

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What am I missing here - and does it matter?

Crossing the shipping lanes last weekend en route from Guernsey to Dartmouth, I had the radar on with the MARPA activated. I far prefer this to just AIS (which we also have), but don't want to get into that issue at the moment. The CPA on the MARPA targets gave good results but I can't get my head around one issue: On the closer CPA's (say, under 0.5M) for reasonably close targets of 2M or more, I am never sure if this means that the ship will pass in front or behind me. The MARPA just gives a CPA figure but no clarity on where the boat will pass. You may say that this does not matter as long as you are not going to hit, but plainly the lower the CPA value the more unnerving this becomes and certainly if I think it is going to be close I will want to make a judgement in a separation zone (where I do not have any stand-on "rights") whether to turn early to go behind the ship or go past the ship and then turn away. I am not so fussed when the CPA is 1-2 miles as I can then pass in front with little concern, but at lower values it would be reassuring to know what the picture is going to look like.

Am I missing something obvious in the MARPA set-up?

Thanks to any respondents...
 
Repeat posts

Sorry everyone - broadband was so slow this morning, it caused me to press the post button several times in the belief that it had not gone out. You may well draw the inference on my technical knowledge from this!
 
Use the wakes function (it might be called tracks or trails, depending which make of radar you have) to determine the direction the blob is moving across the screen (don't bother which way the ship is moving across the water -- it's the blob on the screen that matters at the moment). Using a short wake/track/trail will give you a quick answer, and using a long wake/track/trail will give you an accurate one ... sadly, you can't have both!

Either use a whiteboard marker or your own imagination to draw a line that extends the wake/track/trail past the centre of the screen.
... If it crosses the heading mark, you are relatively "safe", even with a small CPA, because it means the vessel is going to pass ahead of you, and you behind it.
... If it does not crosses the heading mark, you will probably want quite a big CPA, because it means the vessel is going to pass astern of you, and you in front of it.

It gets better ... if you measure the minimum distance between your whiteboard markered line (or you imaginary line) and the centre of the screen, then you have just measured the CPA -- and you will find it is considerably more accurate and consistent than is given by your MARPA
 
Collision Risk

"Am I missing something obvious..."

IMO, the obvious answer to the dilemna of to whether a ship at 1/2 or even two mile range is going to pass ahead or astern, would be that rather than looking at your variety of electronic displays, you perhaps venture into the cockpit and look at the ship itself.
 
Thank you for the one unhelpful and sarky reply so far. For your information, the radar is in the cockpit and is used as a backup to the visual reference. The radar is used to diffuse the uncertainty from looking at a target from a distance off. Isn't that the whole point of having it?

My thanks to the other useful posts. The trace on the screen and the wake trace is a really useful and I guess obvious (although not to me initially!) solution - mthanks for that, I appreciate it
 
The last time I used mARPA (proper) off Milford Haven at night I was tracking 4 or 5 big blobs. iirc the CPA is based on a circle around you, it is not directional. But, again if I recall correctly, the mARPA data box will attempt to tell you the bearing at the same time as the CPA; due to our platform being 26ft and bouncy I take this with a pinch of salt... though it is a nice to have when the CPA is a nice big number.

As well as marking the screen you can also use the EBL (electronic bearing line) I think my RM has 2 I can call on. Place the EBL over the contact and watch it. If it stays on the EBL as you approach the CPA then it is probable for blood pressure that you make an appropriate course change to ensure the contact changes so you are passing astern.

Well worth practising this on nice bright sunny days.
 
As I said in an earlier reply, I've only sailed a couple of times on a boat with radar and MARPA.

One such time was coming down the channel towards IOW, and it was a fairly bouncy trip. I was on watch and was having a "fiddle" with this MARPA thingy, as it was all a bit new to me. Just for fun, I clicked on a "neverbudge" target, a lightship I think. The MARPA gadget said it was doing anything from 3kts in a SW direction, to 7kts to the North!

I put it down to the radar scanner halfway up the mast swinging around in some violent gyrations that was confusing the poor old MARPA. I'm sure that there is some "dampening" control on the radar to overcome this, but as it wasn't my boat, I didn't want to fiddle too much.

Might be worth trying it the next time you're out in similar conditions to see what happens.

It certainly made me rather wary of trusting all these electrickery gismos.
 
DogWatch said:
The last time I used mARPA (proper) off Milford Haven at night I was tracking 4 or 5 big blobs. iirc the CPA is based on a circle around you, it is not directional. But, again if I recall correctly, the mARPA data box will attempt to tell you the bearing at the same time as the CPA; due to our platform being 26ft and bouncy I take this with a pinch of salt... though it is a nice to have when the CPA is a nice big number.

As well as marking the screen you can also use the EBL (electronic bearing line) I think my RM has 2 I can call on. Place the EBL over the contact and watch it. If it stays on the EBL as you approach the CPA then it is probable for blood pressure that you make an appropriate course change to ensure the contact changes so you are passing astern.

Well worth practising this on nice bright sunny days.
Click to expand...

We also had a plastic wipe clean radar plotting board bought from West Marine in the USA made by Davies I think. This is a square board marked up with a large compass rose and scale markings for distance. Easy enough to make or produce a paper version on computer even, but it's value was that it had clear user instructions on the back for when the mind was wandering at 4am.. However other than for practice I never used it, because it was only of real use (ie better than EBLs and track/trail plots) if something was expected to get really close and I much preferred to make a move to avoid that in fog and in clear vis I could see the problem anyway.
 
BobnLesley said:
"Am I missing something obvious..."

IMO, the obvious answer to the dilemna of to whether a ship at 1/2 or even two mile range is going to pass ahead or astern, would be that rather than looking at your variety of electronic displays, you perhaps venture into the cockpit and look at the ship itself.
Click to expand...

I agree with the comment on your sarky remark - its not that obvious at night or in the fog, but perhaps you wouldnt know about that.
(why would anyone want to use radar during the day during good vis??)
 
davidfox said:
......
(why would anyone want to use radar during the day during good vis??)
Click to expand...

To get familiar with using the bluddy thing and build experience in interpreting what it tells you. It's best to do this in good vis, then relating to the visible world will make 'reading' the radar screen second nature. Including it telling you which ships you go astern of.

I would not want to rely on me using a radar unless I had a bit of practice in fine weather the same year at least.

And all that microwave power keeps the helmsman warm.
 
Radar is v useful during the day as another check to see if the chart on your plotter is where it should be, to take bearings from headlands etc, to check for fronts, squalls and thunderstorms and to navigate using parallel indexing. It is even handy for spotting big boats and cooking pasties.
Having said that, I often find it much easier to tell where a boat is going at night. It is usually more obvious than in daytime, where its hard to tell the a%$e of a ro-ro, from its elbow!

Patrick
 
Rowana said:
It certainly made me rather wary of trusting all these electrickery gismos.
Click to expand...
It's a classic case (albeit an unwitting one) of "garbage in, garbage out".
The trouble is that accurate calculations demand accurate data -- and in the case of a small boat, the only way to get meaningful speed and heading data is to average it over a long time -- several minutes. But marpa doesn't: it takes "snapshot" figures of speed and heading. So if you are doing 3 knots up the side of a wave, it will work out a CPA based on 3kts. If you then slither down the other side, it will reassess the CPA on the assumption that you are doing 8 knots. Both answers will be precise -- but wrong.

Plotting by hand over a period of 6 or twelve minutes (one tenth or two tenths of an hour) may not give such a precise answer -- but it is likely to be very much more accurate.

I don't think you need to be wary of trusting the technology ... for me, it's just a matter of being aware of its limitations.
 
stiknstring said:
Thank you for the one unhelpful and sarky reply so far. For your information, the radar is in the cockpit and is used as a backup to the visual reference. The radar is used to diffuse the uncertainty from looking at a target from a distance off. Isn't that the whole point of having it?

My thanks to the other useful posts. The trace on the screen and the wake trace is a really useful and I guess obvious (although not to me initially!) solution - mthanks for that, I appreciate it
Click to expand...

IMHO - the reply you don't like may have been sarky ... but points to an aspect that I will take issue with you.

0.5nm .... in open sea and wondering if ahead or astern ? As an Instructor at my Marine College said to me ... why so bloody close when you have the entire sea ?
Why not g'tee which side the pass occurs by altering to MAKE sure ? You don't need 'iffy' radar info to do that.

My suggestion is that you go back to basic collision avoidance and use the radar as a 'back-up' .... to me your post indicates that you are a Radar baby as we called them in the MN ... sorry to be blunt ...
 
Refueler said:
0.5nm .... in open sea and wondering if ahead or astern ? As an Instructor at my Marine College said to me ... why so bloody close when you have the entire sea ?
Click to expand...
Is that not a question that should really be directed to the watchkeeper in the give way vessel, rather than the stand-on one?
 
timbartlett said:
Is that not a question that should really be directed to the watchkeeper in the give way vessel, rather than the stand-on one?
Click to expand...

Oh Dear - he we go ...

No in fact as he maintained : Collision of two vessels is due to two vessels wanting to be in same spot at same time .....

We have OP who asks how to determine whether 0.5nm is ahead or astern .... 0.5nm is OK in Solent or close to shore etc. but absurd in open waters ...
In my opinion if such a CPA is given with the suspect yactie apparatus ... I would most certainly take the 'cowards' way and stay clear.
Even 0.5nm on ships CAR is too close ... given the inputs to calculate, heading, ships head etc. - taking that a yachts gear will FAR less accurate into the MARPA calculator ....

Whats on the Gravestone ?

Marpa said it was OK ?

Seriously .... 0.5nm ??????????
 
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