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Old 06-09-2016, 10:11   #271
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Re: AIS, Receiver or Transmitter

By the way, this is fascinating reading for anyone interested in collision avoidance:

http://www.titanicology.com/AndreaDo...n_Analysis.pdf

A lot of very important lessons here.

For those naive ones among us, who think that you can just dodge out of the way, and that there's nothing more to collision avoidance than that -- the story of this collision, minute by minute, shows graphically how far ahead some collision situations have to be resolved, before they become inevitable. You can't do collision avoidance by spontaneous inspiration.

This horrifying case happened because Andrea Dora and Stockholm were approaching each other on courses which would have had them pass close by green to green, with CPA of a couple of cables. This is one of those really dangerous traps where lack of accurate data about how your crossing can really kill you -- where a turn to starboard too late will make a collision inevitable. You MUST KNOW WHICH WAY TO TURN.

Andrea Doria was not keeping a radar plot, and was not even using the EBL. Mark 1 eyeballs on Andrea Doria's radar screen did apparently perceive the green to green pass, but did not perceive that the bearing was steady and CPA as not improving. Stockholm was keeping a radar plot, but screwed it up, and thought that she was passing red to red with Andrea Doria. So Stockholm ordered a turn to starboard, guessing wrong that this would increase the CPA. By a mile out already, nothing either ship could have done, could have prevented the collision -- see the interesting analysis of various what-ifs in the article.

The Andrea Doria tragedy is often described as a "radar-assisted collision", but I'm not sure this is an illuminating way to describe it. The collision was caused by lack of accurate data about how the vessels were crossing, and -- maybe in this sense it really was "radar assisted" -- failure of the crews to understand what they didn't know. Did radar make them overconfident? Andrea Doria crew didn't even make a radar plot; Mark 1 eyeballs on the screen was all they used, and assumed they understood what was happening. But they didn't.

It was also caused by general sloppiness -- failure to take enough action, soon enough. Something we sailors are constantly guilty of in collision avoidance situations. We crossed a hundred times and never collided; we'll cross this time too -- human nature. Why go to the trouble to do a proper radar plot -- I can see well enough.

Proper radar plotting on both vessels would have made it clear to both vessels that they were going into a green to green crossing, and that both vessels needed to either set up a red to red crossing well ahead -- at least 10 miles out -- or both turn to port to increase the CPA. Normally what ships do in this situation today is to call each other on the VHF (notwithstanding the MCA note against that), and specifically agree on a green to green crossing. The radio call is to make damned sure that there is no misunderstanding about it, so that someone suddenly turns to starboard and causes a collision.


What if this had been 1924 and not 1954, so no radar on either vessel? Would the crews have known, what they did not know? Would that have meant that they took action earlier, and more cautious action? The accident occurred in fog, and the vessels saw each other only in the last moments before the collision. I think it's very unlikely that without radar and navigating in the fog, that the crews could have done anything at all. Perhaps they would have seen each other just as they flashed by a cable apart, green to green, never having perceived each other and never having made the fatal course alterations. But that would have been just luck -- which is what we all rely on to a great extent, and some of us -- almost entirely -- whether we realize it, or not.

Properly used, radar, like AIS, reduces the necessity of relying on luck.
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Old 06-09-2016, 12:04   #272
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Re: AIS, Receiver or Transmitter

Well, it seems lack of radar assisted collision, since the Andrea Doria was not keeping their plot. But also, (perhaps it would not have made a difference considering how confused they were,) how about calling them up and talking to them as you approach just to make sure you are both on the same page? I may have missed it but I didn't see any record of radio contact.
And they were approaching each other at a combined speed of 40 knots... no plots, and error in the radar of 4 to 5 degrees? Things are happening pretty fast at 40 knots...
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Old 06-09-2016, 12:44   #273
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Re: AIS, Receiver or Transmitter

The 'heading marker' in those days was mechanically generated , a small striker plate up the mast in the scanner unit. Adjustable. Probably all ok when it left the factory but the bloke only had to be a few degrees out when drilling the holes in the mounting plate... I've come a cross two on new ships, scary if you don't know its there.

Bridge VHF was still a novelty in the early sixties, doubt they had such a thing. Even if they had it I don't think 'ship on my port bow off New York ' would work.
Once bridge VHF was the norm radar assisted collisions were replaced by VHF assisted collisions.... collision avoidence conversations invariably involved a contravention of 'the rools'.

DH, sticking the EBL on a 'target' ( why is it always called that I wonder?) achieves little unless you note relative bearing and distance , do it all again 6 minutes later, and then plot it... slow and tedious especially in a multi risk environment.
'Tails' is effectively plotting everything that appears on your screen from the moment it appears. After an appropriate period of time ( speed and distance related ) CPA can be simply calculated in a flash using mk 1 eyeballs, pencil as a straight edge, and the range rings.

Single target on your screen?, sure, put the EBL on it , but all that will tell you is that bearing is opening ahead astern or steady... nada mas
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Old 06-09-2016, 13:42   #274
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Re: AIS, Receiver or Transmitter

Quote:
Originally Posted by El Pinguino View Post
. . . DH, sticking the EBL on a 'target' ( why is it always called that I wonder?) achieves little unless you note relative bearing and distance , do it all again 6 minutes later, and then plot it... slow and tedious especially in a multi risk environment... .
What? Have you never used your EBL? Stick it on and see if the "target" walks down. That's all you have to do as long as everyone is on constant speed and course. EBL IS the relative bearing -- that's what EBL DOES -- what noting do you need to do? None.

What is bloody brilliant about the EBL is that it does all the averaging in nature -- your eye can perfectly see the bearing errors.

I think you've killed a few brain cells since your last radar class!

But of course you of all people, deserve to have You've lost fingers to crocodiles? If I could tell that story in the bars in Moscow, my sexual organs would have been worn away to nothing by now .. . . .
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Old 06-09-2016, 14:12   #275
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Re: AIS, Receiver or Transmitter

EBL = Electronic Bearing Line

FWIW, I know it's been discussed on CF before, but Furuno's ARPA works really well (NavNet3D, haven't tried the TZTouch). I've ARPA targeted a AIS (class B) vessel, and the ARPA display was a lot smoother than AIS. CPA/TCPA were identical.
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Old 06-09-2016, 14:16   #276
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Re: AIS, Receiver or Transmitter

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Originally Posted by Dockhead View Post
What? Have you never used your EBL? Stick it on and see if the "target" walks down. That's all you have to do as long as everyone is on constant speed and course. EBL IS the relative bearing -- that's what EBL DOES -- what noting do you need to do? None.

What is bloody brilliant about the EBL is that it does all the averaging in nature -- your eye can perfectly see the bearing errors.

I think you've killed a few brain cells since your last radar class!

But of course you of all people, deserve to have You've lost fingers to crocodiles? If I could tell that story in the bars in Moscow, my sexual organs would have been worn away to nothing by now .. . . .
Oh dear, I think my work here is done... I'm off to the Golfers' Forum to tell golfers how to play golf....
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Old 06-09-2016, 14:29   #277
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Re: AIS, Receiver or Transmitter

Quote:
Originally Posted by DotDun View Post
EBL = Electronic Bearing Line

FWIW, I know it's been discussed on CF before, but Furuno's ARPA works really well (NavNet3D, haven't tried the TZTouch). I've ARPA targeted a AIS (class B) vessel, and the ARPA display was a lot smoother than AIS. CPA/TCPA were identical.
I've heard that, but never been so blessed. Raymarine, B&G -- all the same carp, as far as ARPA/MARPA is concerned. Bloody useless waste of time. I think it's because the antenna is too small to give a reasonably accurate bearing, without which ARPA/MARPA doesn't work.
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Old 07-09-2016, 00:43   #278
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Re: AIS, Receiver or Transmitter

DH, I think you'll find that the new heading sensors (solid state, with rate, pitch, roll and yaw) really helps the area/marpa stuff. Seems to me to work pretty well on the Navico broadband and Halo radars, with these heading sensors ( Navico one is the precision 9).
This thread - I was an early adopter of AIS, and have now had 2 transponders. I think they are a fantastic tool, that many don't appreciate because they have not experienced them sufficiently.
However, the basic rule of safe navigation is "Don't ever trust one source of information" two is better, 3 is good. So if the AIS says collision threat, the radar does too, and the mk1 eyeball says maybe, you better do something!
For shorthanded sailing , I would not like to be without the AIS any more. Does not mean I can't, I'd just rather not.



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Old 07-09-2016, 01:17   #279
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Re: AIS, Receiver or Transmitter

Quote:
Originally Posted by Neptune's Gear View Post
DH, I think you'll find that the new heading sensors (solid state, with rate, pitch, roll and yaw) really helps the area/marpa stuff. Seems to me to work pretty well on the Navico broadband and Halo radars, with these heading sensors ( Navico one is the precision 9).
This thread - I was an early adopter of AIS, and have now had 2 transponders. I think they are a fantastic tool, that many don't appreciate because they have not experienced them sufficiently.
However, the basic rule of safe navigation is "Don't ever trust one source of information" two is better, 3 is good. So if the AIS says collision threat, the radar does too, and the mk1 eyeball says maybe, you better do something!
For shorthanded sailing , I would not like to be without the AIS any more. Does not mean I can't, I'd just rather not.



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I have a good heading sensor -- Airmar H2183. MARPA still useless. I don't think the 4G radar with its very small antenna has enough bearing discrimination.

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Old 07-09-2016, 03:04   #280
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Re: AIS, Receiver or Transmitter

Quote:
Originally Posted by Neptune's Gear View Post
DH, I think you'll find that the new heading sensors (solid state, with rate, pitch, roll and yaw) really helps the area/marpa stuff. Seems to me to work pretty well on the Navico broadband and Halo radars, with these heading sensors ( Navico one is the precision 9).
This thread - I was an early adopter of AIS, and have now had 2 transponders. I think they are a fantastic tool, that many don't appreciate because they have not experienced them sufficiently.
However, the basic rule of safe navigation is "Don't ever trust one source of information" two is better, 3 is good. So if the AIS says collision threat, the radar does too, and the mk1 eyeball says maybe, you better do something!
For shorthanded sailing , I would not like to be without the AIS any more. Does not mean I can't, I'd just rather not.



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This is sensible. However one needs to consider how and where you sail. If you are heading off and will likely find yourself in "crowded waters" with a fair amount of fast moving commercial traffic it sure makes sense to add the TX feature. As tempting as it is to add one to my boat, I've decided that the way we sail these days and where we sail does not merit the expense. We are weekend sailors in southern NE and do some longer cruises when we can find the time. YES there are ferries and tugs pushing or towing up and down the sound and tankers etc going in and out of several ports along the coast. But most are easily seen visually and avoided... Which is our approach... avoid getting close to other vessels by altering course completely if necessary... time lost to us doesn't matter. Radar works well for us and how we sail. No need to chat up commercial traffic.

Sailing in the waters that DH does would probably be too stressful for wife and not pleasant. Too many boats in close proximity and worrying about them is not her ideas of sailing.
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Old 07-09-2016, 05:13   #281
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Re: AIS, Receiver or Transmitter

Once I was sailing towards Pulau Redang in the South Chinese Sea I saw an AIS signature going fast towards KL in the middle of the Malaysian peninsular. Its name was Lamborghini and registered in Singapore! I was impressed my VHF antenna could catch the signal - it is only 4m ASL. But there were a base station nearby on the coast and the signal must have been relayed... ;o|

Does anyone have experience in cheap Chinese AIS buoy/beacon for fishing nets? It's programmable with MMSI, vessel name, size and type. I've already AIS reception and don't want the much more expensive transponder solution.

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Old 08-09-2016, 01:33   #282
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Re: AIS, Receiver or Transmitter

Well this thread has pushed me to order a transponder. I can recall 2 incidents where I could have done with it instead of my Rx only one. One was when I fell asleep on my way to Bermuda and forgot to turn my AIS alarm on. Another was when I was in the Southport channel far to the starboard side sailing along when I remembered to turn my AIS on and lo and behold not half a mile in front of me was a big fishing boat hidden in all the shore lights coming straight at me. He ended up missing me by 20 yards or so but it was close. I can also remember talking to a cruise ship mid gulf stream about it. I had given them a scare and they really tried to convince me to get one. One of the things that make it difficult is that the name of the ship doesn't immediately appear so it takes a while before you can radio the ship that is heading towards you.

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Old 08-09-2016, 06:40   #283
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Re: AIS, Receiver or Transmitter

The ship or boats name is included in the static data transmitted by the AIS, which is transmitted at every 6 minutes (Class A).
Dynamic data is transmitted between 2 and 10 seconds if the transmitter is moving, the rate of transmitting depends on the speed.

With a Class B AIS, it is possible to miss one or more transmissions, hence the delay in getting the name of the target displayed.

More info here
Ais Reporting Intervals
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Old 08-09-2016, 07:07   #284
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Re: AIS, Receiver or Transmitter

Name of the vessel is not required for anything though. Is it?

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Old 08-09-2016, 07:11   #285
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Re: AIS, Receiver or Transmitter

Re an earlier MARPA drift.

An interesting side dish (warning: drift) is that new radars (not AIS but radar) distinguish the moving objects vs. the static ones and plot them in different colour.

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