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Old 14-12-2018, 22:27   #1
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"A" versus "B" AIS - worth transmitting?

So my somewhat limited knowledge based on a shaky foundation is that there are two classes of AIS - A and B. Commercial and large vessels use A and the rest of us use B. My understanding is that the larger vessels can shut of the B transmissions, which actually makes sense in a very busy (mostly small craft) area so that they only receive A transmissions.

Now one of my purposes for AIS is logically so that larger vessels know where I am, but if they are shutting of B transmissions, is it worth even transmitting using class B AIS?
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Old 14-12-2018, 23:14   #2
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Re: "A" versus "B" AIS - worth transmitting?

This is the clearest breakdown I can find: https://www.oceantimemarine.com/clas...on-system-ais/
If offshore with limited traffic (where AIS is most useful), it seems all class B transmissions will be received, but perhaps with some non-vital information missing such as vessel type and name. In crowded areas class B transmissions can be sometimes deprioritized.

But if you can handle the added cost, complexity and power draw, class A has some benefits.

Either would be a great boon for any boat. Neither are a panacea.
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Old 14-12-2018, 23:18   #3
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Re: "A" versus "B" AIS - worth transmitting?

Absolutely it is. The solution is when every vessel has working ais at all times. If you're not transmitting then you're a part of the problem. It's a shame receive-only devices were ever made.

As to class A, no you certainly don't need that. It's a whole different ball game really. But you can be hit by anything, not just a class a ship.

I don't know about shutting off class b transmissions, but certainly the better units can hide class b transmissions that aren't close to an approach.
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Old 15-12-2018, 06:00   #4
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Re: "A" versus "B" AIS - worth transmitting?

We transmit class B and Iíve seen enough big ships alter course to avoid us that Iím happy we are visible. Itís definitely not a waste of money.
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Old 15-12-2018, 06:13   #5
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pirate Re: "A" versus "B" AIS - worth transmitting?

[QUOTE=Tillsbury;2781755]Absolutely it is. The solution is when every vessel has working ais at all times. If you're not transmitting then you're a part of the problem. It's a shame receive-only devices were ever made.
/QUOTE]

I do think it should be mandatory that AIS should be switched of when tied up in ports/marinas.. the stationary blobs smother to much mobile traffic.
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Old 15-12-2018, 07:15   #6
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Re: "A" versus "B" AIS - worth transmitting?

I've read a lot of very strong opinions against class B AIS.

After three years using it, I can't agree. Even in busy harbors like NYC, Boston and Halifax, it's been very useful. We've been contacted by VTS and other vessels, so if anyone is turning off display of class B, they are in the minority, or only do so in places where it doesn't matter, like close-in by the docks and marinas in good visibility.

As the price comes down, it may make sense to look at class A for future-proofing reasons, but I've seen no down side to using class B. Other vessels can see my location, name, length, heading, speed and MMSI. They don't really need to know my last port of call, turning rate, destination or navigation status. And even with class A, many of those values are wrong. I've seen many freighters with a status of "at anchor" doing 20 knots, or "underway" while tied to the dock.
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Old 15-12-2018, 09:57   #7
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Re: "A" versus "B" AIS - worth transmitting?

We've been told that the commercial fishing fleet turns theirs off when they hit a big catch - now I understand not wanting to advertise where the fish are, but it seems that at the point I need to see the most, most everyone goes stealth.....

I don't know if the above is true - I'll find out in the spring and get back to you.
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Old 15-12-2018, 10:01   #8
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Re: "A" versus "B" AIS - worth transmitting?

If you spend much time ashore with your boat anchored out of sight, it sure is nice to glance at an app and see your Class B transmitting boat is still right where you left it
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Old 15-12-2018, 10:18   #9
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Re: "A" versus "B" AIS - worth transmitting?

The Houston/Galveston VTS only picks up Class A AIS. Many large commercial vessels transiting the Houston Ship Channel will turn off the reception of Class B signals.
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Old 15-12-2018, 10:27   #10
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Re: "A" versus "B" AIS - worth transmitting?

Quote:
Originally Posted by redhead View Post
We've been told that the commercial fishing fleet turns theirs off when they hit a big catch - now I understand not wanting to advertise where the fish are, but it seems that at the point I need to see the most, most everyone goes stealth.....

I don't know if the above is true - I'll find out in the spring and get back to you.
I did pass commercial fishing boats off the FL coast last year, at night, with no AIS on. They were well lit, but I've got to believe they had AIS on board that had been turned off.

I am in process replacing my receive-only unit with a Class A transceiver. I am doing an overall upgrade of electronics, and I believe it's worth the extra money to be transmitting.
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Old 15-12-2018, 10:29   #11
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Re: "A" versus "B" AIS - worth transmitting?

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Originally Posted by redhead View Post
We've been told that the commercial fishing fleet turns theirs off when they hit a big catch - now I understand not wanting to advertise where the fish are, but it seems that at the point I need to see the most, most everyone goes stealth.....



I don't know if the above is true - I'll find out in the spring and get back to you.


I can guarantee you itís true, also youíll see in their data if you look most often they donít put anything in there except the MMSI number, no vessel details at all, cause they donít want others to know who they are.
Same for the ones that search for treasure, they turn theirs off too.
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Old 15-12-2018, 10:30   #12
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Re: "A" versus "B" AIS - worth transmitting?

FWIW: another vote definitely in favour of transmit. The only place we turned ours off in 5 years was in the river Hamble, off the Solent - alarm was on constantly because of the density of traffic. As I understand it, a big ship will only filter out class b transmissions for the same reason - alarms going off constantly, warning of traffic which is -say- actually outside the shipping channel.
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Old 15-12-2018, 10:32   #13
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Re: "A" versus "B" AIS - worth transmitting?

There are very good reasons that AIS was divided into two classes. It was to keep un-necessary traffic in busy areas to a manageable level. There is only so much bandwidth available. If every little boat had a Class A AIS the whole system would collapse if every boat was broadcasting on higher power and more frequent updates.

In a truly logical world, Class a transmitters would not be allowed on Recreational vessels below a certain size.

The idea that "I want a Class A so the big ships can see me in a busy harbor" is silly. In those kinds of places they have NO maneuvering room. All they can do is call you on the radio and yell at you to get out of the way. Rule 9 anyone????

In congested, busy waters it is YOUR job to stay clear of the big boys. They can't do it for you. You'll see them on your screen. What more do you want?

In open water it is a different story, and the Class B transmitter gives you everything you need.
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Old 15-12-2018, 10:36   #14
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Re: "A" versus "B" AIS - worth transmitting?

Quote:
Originally Posted by Tillsbury View Post
Absolutely it is. The solution is when every vessel has working ais at all times. If you're not transmitting then you're a part of the problem. It's a shame receive-only devices were ever made.

As to class A, no you certainly don't need that. It's a whole different ball game really. But you can be hit by anything, not just a class a ship.

I don't know about shutting off class b transmissions, but certainly the better units can hide class b transmissions that aren't close to an approach.
I fully agree with Tillsbury. What I do not understand is why this question keeps coming up. In our circumnavigation the norm was that we were being received. The norm by far, was that commercial ships used their AIS, as they must . AIS is the next best thing to your eyes and it watches, sends and listens, all the time. It is essential to passage making and we would pay multiples of what it costs.

Ships are also much more likely to respond by a factor of ten or more if you can call them by name and if you send they already know about you. We have proved that numerous times.

See and be seen. There is no reasonable argument against that imo.

Jim
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Old 15-12-2018, 10:49   #15
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Re: "A" versus "B" AIS - worth transmitting?

Quote:
Originally Posted by rsn48 View Post
So my somewhat limited knowledge based on a shaky foundation is that there are two classes of AIS - A and B. Commercial and large vessels use A and the rest of us use B. My understanding is that the larger vessels can shut of the B transmissions, which actually makes sense in a very busy (mostly small craft) area so that they only receive A transmissions.

Now one of my purposes for AIS is logically so that larger vessels know where I am, but if they are shutting of B transmissions, is it worth even transmitting using class B AIS?
AIS is a winner in all cases. Once you have it you'll love it.

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