Sailing vessel and RORO ship on collision courses - what do you think of the outcome?

[chrisr]

Originally Posted by Dockhead

Good that you're asking this question, so that there is no doubt in anyone's mind -- NO, AIS systems (at least recreational ones) calculate CPA between two dimensionless points -- the antenna locations of the two systems. Discussed here: https://forums.ybw.com/index.php?thr...ntenna.472710/

AIS DOES also broadcast as part of static data information about the location of the antenna. Calculating CPA between the closest parts of each vessel would be theoretically possible, but would be a very complicated calculation. OpenCPN, which has the best AIS implementation I've ever seen, will DISPLAY the dimensions of the vessel to scale based on the static data, which is very helpful. But CPA calculation itself is based on antenna location (incidentially, the GNSS antennae, not the rx/tx anntennae) -- discussed here: https://opencpn.org/flyspray/index.p...desc&pagenum=2

Beware false precision. For a host of other reasons, you cannot safely navigate to pass so close to a ship that this will matter. The dimensions of the ship are not really important. A safe CPA is a much greater dimension than +/- the length of the ship will matter.

agree completely. i never allow a CPA of less than 1nm in perfect conditions, and much more if conditions are less than perfect or if i'm crossing ahead

probably this is why i've never seriously considered this question of antenna placement v CPA

cheers,

[boatman61]

Originally Posted by Dockhead

Yes, maybe where Ping is few yachts have radar, but virtually all ocean-going yachts in this part of the world have radar.

Now whether the owners know how to operate the radars or not may be a different question. . .

To my mind radar is the most useful instrument on board after only depth sounder and compass, far more useful than chart plotter, AIS, etc.

Target trails are great, but to run stabilized mode usefully you need a better compass than most yachts have. I'm giving my boat a Furuno satellite compass this year for her birthday, specifically for this (and radar overlay).

EBL and index lines work a treat for the task we are discussing now. Immediate visul representation of the evolution of the relative bearing -- supremely useful.

As to fog, I'm sure any one of us could tell multiple stories on this subject, but my favorite one was from year before last, as we were on passage from Helsinki to Kalmar, on the SE coast of Sweden, couple of days and nights. Anyone who has been in these waters knows the treacherous winding channel between the rocks in Kalmarsund leading down to the bridge. Well, luck would have it -- night fell as we sailed down Kalmarsund towards the entrance to the channel. Then torrential rain. Then fog so thick I couldn't see my own forestay. What would we have done without radar? In the event we picked our way through, identifying each channel marker on the radar, threading our way through. We had a scare when a strong return appeared right in front of us like 100 meters ahead -- crash stop, then the return dissolved into a bunch of small dots, and then we heard the birds.

I don't really like my 4G radar all that much (particularly the useless MARPA), but it does have a couple of strong points, and one of those is the lack of main bang which allows you to see objects practically up under your bows. That is really really useful in pea soup fog like that.

And so we tied up safely in Kalmar harbour with an extra ration of grog.

Dockhead.. not a one of my boats has had radar and out of all my deliveries maybe 10‰ were fitted with radar..
Its not as common as you seem to think..

[jtsailjt]

Originally Posted by boatman61

Dockhead.. not a one of my boats has had radar and out of all my deliveries maybe 10‰ were fitted with radar..

Its not as common as you seem to think..

That may be more a reflection of the sort of boats you get hired to deliver than what is typical on seagoing boats in general. Outside the Caribbean charter fleets where fog is almost unheard of and night sailing Isnt allowed, I’d say that amongst cruising sailboats over about 35’ long, in my part of the world it’s pretty rare to see one without radar. I think DH is right on this, outside a depth sounder, radar is about the most useful electronic tool onboard and can be used for collision avoidance as well as navigation when you’d otherwise be nearly blind. I can’t imagine intentionally going to sea in poor visibility or on a moonless night without it.

[boatman61]

Originally Posted by jtsailjt

That may be more a reflection of the sort of boats you get hired to deliver than what is typical on seagoing boats in general. Outside the Caribbean charter fleets where fog is almost unheard of and night sailing Isnt allowed, I’d say that amongst cruising sailboats over about 35’ long, in my part of the world it’s pretty rare to see one without radar. I think DH is right on this, outside a depth sounder, radar is about the most useful electronic tool onboard and can be used for collision avoidance as well as navigation when you’d otherwise be nearly blind. I can’t imagine intentionally going to sea in poor visibility or on a moonless night without it.

Actually it's a fairly broad spectrum, from 65ft down.. ages between brand new and 45yrs old.. Lagoons, Bali's, Moody 45DS and other assorted mono's..
Click on the picture link below where you'll find a selection of boats I have delivered.
As for foggy, moonless nights.. done quite a few of those on boats from 21ft to 31ft across the English Channel on 'booze cruises' from Poole to Cherbourg over the years..
Like DH I've survived.. does not make either of us better than the other.

[Dockhead]

Originally Posted by boatman61

. . . Like DH I've survived.. does not make either of us better than the other.

Heh, heh. Lot of wisdom in that phrase. I agree

[requiem]

Originally Posted by Dockhead

Calculating CPA between the closest parts of each vessel would be theoretically possible, but would be a very complicated calculation. OpenCPN, which has the best AIS implementation I've ever seen, will DISPLAY the dimensions of the vessel to scale based on the static data, which is very helpful.

There are a few ways it could be "uncomplicated". A safety buffer method could simply take the greatest of the vessel's dimensions and subtract that from the CPA. For a more precise value, once the aspect is known it's a simple right-triangle calculation in plane geometry.

Of course, either method assumes the data have been correctly entered in the first place.

Originally Posted by Dockhead

Beware false precision. For a host of other reasons, you cannot safely navigate to pass so close to a ship that this will matter. The dimensions of the ship are not really important. A safe CPA is a much greater dimension than +/- the length of the ship will matter.

Completely agree!

[Lodesman]

Does anyone have an AIS that shows Bearing at CPA (BCPA)? I know my Vesper unit doesn't and I don't believe the Norwegian Cl A unit I've had some exposure to, provided that info either.

[Dockhead]

Originally Posted by Lodesman

Does anyone have an AIS that shows Bearing at CPA (BCPA)? I know my Vesper unit doesn't and I don't believe the Norwegian Cl A unit I've had some exposure to, provided that info either.

Never heard of one that gives that data.


But OpenCPN gives a graphic representation of positions of vessels at CPA -- extremely useful. Calculating BCPA from that would be trivial. If you need that data, post a Feature Request in the OpenCPN forum and I bet they'll whip it up for you. But maybe you don't need it -- I certainly don't, when I can see the crossing graphically.


OpenCPN has a so much superior AIS implementation that this is my primary tool for AIS in complicated situations. In really difficult situations like the Elbe approaches I like to have a good sailor at the helm using eyeballs whilst I sit at the nav table with OpenCPN running on ship's computer, radar on the B&G MFD, and a plotting sheet, notebook, and radio close at hand. I find that visualizing crossings on O to be tremendously helpful in thinking out solutions to complicated problems involving multiple vessels. You instantly see BCPA, aspect of the ship, how the crossing would change with different course changes -- everything.

[requiem]

Originally Posted by Lodesman

Does anyone have an AIS that shows Bearing at CPA (BCPA)? I know my Vesper unit doesn't and I don't believe the Norwegian Cl A unit I've had some exposure to, provided that info either.

Sorry for the terrible screenshot, but this is what I currently can show from land...



The lower magenta circle is my projected position at CPA, and the magenta dashed line leads to the other vessel's position at CPA (upper magenta circle). So, it has the proper numbers for any calculation even if not displayed.

[HeinSdL]

Originally Posted by Lodesman

Does anyone have an AIS that shows Bearing at CPA (BCPA)? I know my Vesper unit doesn't and I don't believe the Norwegian Cl A unit I've had some exposure to, provided that info either.

I was under the impression this would be quite standard, didn't realise this may not be the case. On my iNavX combined with an AIS from Weatherdock it does also indicate this parameter. Find it quite useful to further confirm my planning ahead. (In addition of course to watching what is physically happening, before I get reprimanded here for steering by AIS )

[requiem]

Originally Posted by HeinSdL

I was under the impression this would be quite standard, didn't realise this may not be the case. On my iNavX combined with an AIS from Weatherdock it does also indicate this parameter.

Are you sure it's actually BCPA and not just the course vector?

My first (and quite incorrect!) thought was that one could simply use the current aspect to determine this, which is somewhat workable for a CPA of 0, but Lodesman's question made me remember that both bearing and aspect will (quite obviously) continuously change for any ship on a passing course.

This would be particularly a problem for dangerously close CPAs, and it is at the CPA that the bearings are changing the fastest and the shorter the CPA the greater the impact of any measurement error on them. So, I think that makes the calculation not only complicated as Dockhead mentioned, but also more dangerous.

I do still like the idea of a "safety bubble" based on vessel size; it should have negligible impact for small boats but might deter risky passes.

[Lodesman]

Originally Posted by requiem

So, it has the proper numbers for any calculation even if not displayed.

On the Vesper I can get a rough idea of where the crossing will be based on the angle of the little triangles, but it's not terribly clear when CPA is relatively close. Yours has the benefit of the dotted magenta line, but I imagine that also is less easy to determine for close CPA without zooming into a smaller range scale. I'm not a fan of cluttering up my screen with long leaders and such, and would rather read that useful bit of info from the tote. Personal preference.

[Dockhead]

Originally Posted by Lodesman

On the Vesper I can get a rough idea of where the crossing will be based on the angle of the little triangles, but it's not terribly clear when CPA is relatively close. Yours has the benefit of the dotted magenta line, but I imagine that also is less easy to determine for close CPA without zooming into a smaller range scale. I'm not a fan of cluttering up my screen with long leaders and such, and would rather read that useful bit of info from the tote. Personal preference.

Try OpenCPN -- will be revelation.

[Lodesman]

Originally Posted by Dockhead

Try OpenCPN -- will be revelation.

I might try that just to see how it works. I don't actually intend to use AIS for anti-collision planning/tracking, and my thinking is the makers left out BCPA as they intended for it to be used as a warning device solely, and not to determine the where's and how's of anti-collision.

[fxykty]

Originally Posted by Dockhead

Try OpenCPN -- will be revelation.

Is a separate connection required in OpenCPN to pull the AIS data from the multiplexer? I’m using a Yacht Devices wifi gateway https://www.yachtd.com/products/wifi_gateway.html and it pushes out all instrument and MFD info from my SeatalkNG network, but seemingly not AIS.

I have an ICOM AIS B unit that uses 0183 to provide AIS data to the Raymarine MFD via the SeatalkNG network, but I can’t see that AIS data in OpenCPN nor iSailor, both of which get theor NMEA data from the Yacht Devices wifi gateway.