AIS Marine traffic v. what I am receiving aboard - beginner expectations

We have just come back from Ireland 11 hours in zero viz fog. We have a rail mounted transponder aerial. All large shipping avoided us by giving us at least 1 nm CPA. The only thing that came close to colliding was a big hallbberg raspberry. CPA was less than .25 of a mile and he decided to cross directly in front of me as stand on vessel. Never saw him until 10 minutes later when fog lifted and we had 6 mile visibility. Rail mount option also provides an emergency vhf should the big stick or mh unit fail. Splitters are the work of the devil! IMHO.
 
TSB240 said:
We have just come back from Ireland 11 hours in zero viz fog. We have a rail mounted transponder aerial. All large shipping avoided us by giving us at least 1 nm CPA. The only thing that came close to colliding was a big hallbberg raspberry. CPA was less than .25 of a mile and he decided to cross directly in front of me as stand on vessel. Never saw him until 10 minutes later when fog lifted and we had 6 mile visibility. Rail mount option also provides an emergency vhf should the big stick or mh unit fail. Splitters are the work of the devil! IMHO.
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:encouragement:
I am always amused by the reports of great distance AIS target acquisition ... for what are they good for? Those ships 20, 30 or 100 nm away are not the ones likely to hit you, which is why we carry our AIS receivers and transponders - to warn us and others of potential closing threats.

Even the cowboy fast ferries I encounter along the Croatian coast, with closing speeds of 30-40kts, I only need to know about within a 5nm radius; even with my less-than-efficient antenna position on the pushpit and spasmodically shielded by the wind-generator mast, I know I am received within that range and I certainly can see them, and with a closing CPA am alarmed by OpenCPN accordingly.

As the OCPN screen-shot shows in message #13 I am receiving target reports from ships in Monfalcone and Trieste, over 30nm away. Such reports are just unnecessary clutter for the software to process.
 
TSB240 said:
We have just come back from Ireland 11 hours in zero viz fog. We have a rail mounted transponder aerial. All large shipping avoided us by giving us at least 1 nm CPA. The only thing that came close to colliding was a big hallbberg raspberry. CPA was less than .25 of a mile and he decided to cross directly in front of me as stand on vessel. Never saw him until 10 minutes later when fog lifted and we had 6 mile visibility. Rail mount option also provides an emergency vhf should the big stick or mh unit fail. Splitters are the work of the devil! IMHO.
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If the HR crossed ahead as stand-on vessel, my interpretation would be that you as give-way vessel chose to close to this distance, unless I have misunderstood the situation. (On re-reading, I see that you might have referred to yourself as stand-on vessel but it is unclear) We can probably assume that he had AIS and probaby radar, so perhaps he thought the situation under control. 1/4 mile wouldn't concern me, I think. Splitters are brilliant.
 
Barnac1e said:
:encouragement:
I am always amused by the reports of great distance AIS target acquisition ... for what are they good for? Those ships 20, 30 or 100 nm away are not the ones likely to hit you, which is why we carry our AIS receivers and transponders - to warn us and others of potential closing threats.
Click to expand...

I'm with Barnacle on this. I would suggest that moving the AIS antenna to the masthead is, in many cases, a retrograde step. Using a properly mounted dedicated pushpit antenna (I'd never use a splitter on my VHF) routinely offers me effective ranges of 8 - 12 miles. I want to ensure that within that range, by AIS picks up all relevant transmissions but without the additional clutter of excessive, irrelevant hits.
 
Sea_Spray said:
I'm with Barnacle on this. I would suggest that moving the AIS antenna to the masthead is, in many cases, a retrograde step. Using a properly mounted dedicated pushpit antenna (I'd never use a splitter on my VHF) routinely offers me effective ranges of 8 - 12 miles. I want to ensure that within that range, by AIS picks up all relevant transmissions but without the additional clutter of excessive, irrelevant hits.
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I have no issue with that. My reason for mast mounting is primarily for VHF range. I know splitters split opinion but for my tentative steps outside the comfort zone I'm not going to blow the budget yet. My AIS is receive only so range is slightly more significant for maintaining my calm demeanor crossing the channel:)
 
FairweatherDave said:
I have no issue with that. My reason for mast mounting is primarily for VHF range. I know splitters split opinion but for my tentative steps outside the comfort zone I'm not going to blow the budget yet. My AIS is receive only so range is slightly more significant for maintaining my calm demeanor crossing the channel:)
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So for a receiver-only AIS, what target range qualifies for a calm demeanor?

Is it necessary to see ships 20nm instead of 10nm away? If so, why?

Should all motorboat skippers who have no mast, be denied calmness? After all, their AIS target range must be almost as limited as mine is - truly enough to induce severe lack of calm, I suspect ... :D

The only vessels that interest me are the ones that could prove lethal, the big ones. Because they are big their AIS antennas are invariably up high. VHF propagation range depends, not just on your antenna height, but on the other vessel's antenna height too. Your potential antenna height is by far the minor component of the combination. Let them have all the cost and complexity of achieving antenna height.

My pushpit AIS antenna will consistently receive target reports from commercial shipping at 20nm and beyond. Since I replaced my AIS receiver with a transponder some years ago, I regularly see such vessels approaching in the Adriatic and deliberately alter course some 2-3 nm away with the only reason being, as far as I can tell, because of our closing CPAs.

Friends with whom I regularly cruise and who have themselves AIS systems, report my position from at least 10nm away, on the occasions when we all do not reach the same destination.

I see no logical reason to contemplate changing to a masthead AIS antenna, with all the cost and lack of redundancy that implies.
 
TSB240 said:
We have just come back from Ireland 11 hours in zero viz fog. We have a rail mounted transponder aerial. All large shipping avoided us by giving us at least 1 nm CPA. The only thing that came close to colliding was a big hallbberg raspberry. CPA was less than .25 of a mile and he decided to cross directly in front of me as stand on vessel. Never saw him until 10 minutes later when fog lifted and we had 6 mile visibility.
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I do not believe that the stand-on/give-way Colregs rules apply in fog or other restricted visibility at all. :confused:

This would mean that you were just as much in the wrong as the HR as you were definitely not the stand-on vessel. :ambivalence:

Perhaps a crash-course in Colregs is required. Or a non-crash course. ;)

Richard
 
Richard we missed end of col regs drift........?
Just to confirm it was the only close encounter less than .25 mile in a 100 mile leg in dense fog and a pushpit mounted antenna was adequate to avoid this target.

We happened to observe within a very limited range of less than 50 metres some other non AIS carrying porpoises and dolphins, the odd lobby pot and clumps of seaweed, a gannet sat in a plastic fish tray too lazy to get up and fly... ?
At one point the sea was so flat the boat so motionless it was difficult to establish any real horizon the only reference was the mast.....All a bit like being in an isolation tank blind folded and with ear plugs in.
Radar was picking up lobby pots and cardinals.
 
Barnac1e said:
I see no logical reason to contemplate changing to a masthead AIS antenna, with all the cost and lack of redundancy that implies.
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+1
I decided to have my antennas where they can be easily maintained/tested/replaced.
Doing the arithmetic for the range of the main VHF radio, there is a small theoretical possibility that, mounting at the pushpit as opposed to the top of the mast, I will not have line-of-sight to a CG mast only when I am mid Channel. When next the mast comes down, I'll make sure the MH antenna is refurbished - I love having redundancy.
Cheers
Bob
 
RIBW said:
When next the mast comes down, I'll make sure the MH antenna is refurbished - I love having redundancy. Cheers Bob
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Having seen several boats masts that came down in a less controlled manner is why I have aerials on the raised stern frame over the boarding "gate". Indeed as its easier to maintain I get better signal from stern frame than masthead - which is probably more useful than line of sight range
 
I totally get the arguments about ease of maintenance and redundancy. And I am with anybody whose AIS range is 10 miles. My reason for raising the antenna as I said is because it is my only antenna and for VHF I believe consensus is it should be top of the mast. However mine is currently on the rail and going to stay there while I see how it all goes. Appreciate MOBO's don't have the mast option but when crossing shipping lanes they do have the option to blat out of (or into) trouble :)
My distinction between receive only and a transponder is obvious, you can relax a bit more with a transponder. Bearing in mind I have no experience of using AIS for real (yet) I believe the value of AIS is in the range of say 1 or 2 to 7 seven miles where you are planning to cross a shipping lane. That is obviously an oversimplified description but is why I bought AIS. At less than a mile I doubt I am going to be looking at a screen. To start with given I am learning how to use the tool the longer I get to study a target and feasibly enter their MMSI into my radio the better.
 
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