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Old 24-10-2012, 04:47   #1
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Type "A" AIS for Yachts?

I will be replacing all my electronics (at least the radar and plotters) this winter in the course of my refit this year. My previous network did not support AIS, so I have been living without. The new system, based on B&G Zeus plotters running an NMEA2000 network, will obviously by free of this restriction.

During a recent discussion with professional mariners about collision avoidance procedures, I was advised over and over again to invest in a Type "A" AIS rather than the usual (for yachts) Type "B" system. The Type "A" AIS carried on commercial ships has a more powerful signal, transmits position updates more frequently, transmits more types of data such as rate of turn, and allows you to send and receive text messages. I had not considered it, but suddenly Digital Yacht is offering one at an affordable price -- a couple of grand.

What do you guys think -- overkill on a sailboat? I do sail in the busiest shipping lanes in the world, so collision avoidance, especially avoiding commerical shipping, is a pretty big deal for me. The Type "A" AIS looks like a lot more trouble to use -- you have to program it with your passage information every time you fire it up.

I mentioned the idea on YBW a month or two ago, and got a harsh drubbing by people who thought using a Type "A" AIS on a yacht is "selfish", and "eating up the 'commons'", since users of other Type "A" sets can't filter out other Type "A" signals as easily as they can Type "B" signals.

What do you guys think?

Here is the Digital Yacht Type "A" set: Digital Yacht - Light Years Ahead -- it looks like a very nice piece of kit.
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Old 24-10-2012, 05:47   #2
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Re: Type "A" AIS for Yachts?

Ours is Type B, and easily sufficient for our purposes (we have more pleasure and fishing traffic than commercial shipping, here).

I suspect there's more difference in the transmit side, than in the receive side... meaning our unit still takes most advantage (increased signal strength and periodicity) of all the Type A signals we receive.

IOW, I'm well informed about area shipping, which in turn means I can navigate/pilot accordingly easily enough.

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Old 24-10-2012, 07:26   #3
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Re: Type "A" AIS for Yachts?

It's money vs. benefits. You know the benefits--you listed them yourself--so you just have to decide if they are worth the money to you.

Personally, I see no advantage to Type A that would justify the additional cost. So for me the decision is easy.

As to using up the 'commons'... Utter nonsense. Heck! If you're worried about using up the 'commons' then you should probably just get out of the water and leave it to the professionals. Truth is that we all use common resources every day of our lives, in a myriad of ways that we never even think about. It's one thing to use them in a reasonable and responsible way, and another to hog and hoard them to the point that you deprive others.

If there was some risk that someday every little recreational boat out there would be using Type A AIS then, maybe, an argument could be made that the standards need further refinement and frequencies should be reserved for the commercial users. We're not there, though. We're not ever going to get there. It's a straw-man argument, as far as I'm concerned.

If you want Type A then get it.
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Old 24-10-2012, 07:41   #4
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Re: Type "A" AIS for Yachts?

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Originally Posted by Dockhead View Post
I was advised over and over again to invest in a Type "A" AIS rather than the usual (for yachts) Type "B" system.
Type A's do transmit at greater power (12.5watt vs 2 watt for class B).

Most cruising sailors would prefer to have only the 2 watt electrical drain. Are you willing to deal with the 12.5 watt continuous drain?

As to range . . . AIS range is mostly determined by line of sight, that is antenna height. Power output has relatively little to do with it. Type B's can be read 400km up in the sky (by sats). It is a fact that you see most type A's further than type B's, but that's mostly because the A's are mounted higher on the ships than the B's are on pleasure craft. So, you will get some range increase with the extra power, but not anywhere as much as people might expect.

I don't think the extra data transmitted by the A is of much value. At least I have not found it so when looking at class A targets.

As to the public commons question . . . it's a difficult question. Many of the folks on YBW have some experience with the Solent, where there are a lot of AIS targets. I am not aware of anywhere else in the world today with as crowded a public commons as there, but as AIS growth continues it will happen. I had 192 AIS targets in buzzards bay recently. Many AIS units have smarter filtering, where you can set CPA and distance and speed limits for the alarms. This smart filtering is the direction its going to have to go, as there will be too many even class A target to show them all.

Net net, I would get a B because of the lower power consumption and because you will not get as big a range boost with A as people are telling you.
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Old 24-10-2012, 07:41   #5
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Re: Type "A" AIS for Yachts?

Dock head,

Type A's can still be filtered by range, size, and collision potential, at least with more sophisticated units. I don't see a problem using one.
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Old 24-10-2012, 07:53   #6
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Re: Type "A" AIS for Yachts?

I have carried a Class B receiver with me now on deliveries all around Europe, transatlantic and the Med - probably 20K+ miles and I would say it's the most invaluable aid I have ever had.

A transponder would be nice - it's the idea of 'being seen by others' as well as 'seeing others'. I have had instances of calling ships which have a potential collision course and them telling me that they can't see me in a small boat against the sea. This is partly because from the bridge they are looking down and so we are not above the horizen.

Anyway, I digress. Is Class A overkill? Possibly, but for many it's not an option as the budget would be overstretched. If you can afford it, you will be more visible at a greater range and so safety margins would be increased.

Is it worth it? Your call, but I would probably stick with Class B, make sure I had a well mounted dedicated antenna and good watch keeping. The best safety aids are your eyes and ears.
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Old 24-10-2012, 07:58   #7
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Re: Type "A" AIS for Yachts?

I have been debating with the same decision myself. AIS is on the to do list.

I suspect class A will get the attention of the bigger ships much better than class B. there has also been some talk of vessels filtering out class B targets in busy waterways.
I do wonder if they will assume a class A target is a large power vessel rather than smaller, sailing vessel. The information from the transmission is readily available to clarify this, but I human nature sugests they will sometimes assume rather than checking. They could easily and wrongly assume they were the the stand on vessel on some occasions.
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Old 24-10-2012, 08:22   #8
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Re: Type "A" AIS for Yachts?

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Originally Posted by estarzinger View Post
Type A's do transmit at greater power (12.5watt vs 2 watt for class B).

Most cruising sailors would prefer to have only the 2 watt electrical drain. Are you willing to deal with the 12.5 watt continuous drain?

As to range . . . AIS range is mostly determined by line of sight, that is antenna height. Power output has relatively little to do with it. Type B's can be read 400km up in the sky (by sats). It is a fact that you see most type A's further than type B's, but that's mostly because the A's are mounted higher on the ships than the B's are on pleasure craft. So, you will get some range increase with the extra power, but not anywhere as much as people might expect.

I don't think the extra data transmitted by the A is of much value. At least I have not found it so when looking at class A targets.

As to the public commons question . . . it's a difficult question. Many of the folks on YBW have some experience with the Solent, where there are a lot of AIS targets. I am not aware of anywhere else in the world today with as crowded a public commons as there, but as AIS growth continues it will happen. I had 192 AIS targets in buzzards bay recently. Many AIS units have smarter filtering, where you can set CPA and distance and speed limits for the alarms. This smart filtering is the direction its going to have to go, as there will be too many even class A target to show them all.

Net net, I would get a B because of the lower power consumption and because you will not get as big a range boost with A as people are telling you.
Thanks for all that, Evan.

Concerning power consumption: the figures are transmit power, not power consumption, two different things. AIS doesn't transmit continuously -- it transmits in bursts, so I don't think there's that much difference in inherent power consumption. The Digital Yacht Class "A" set uses about 12 watts -- about like an anchor light. That's ok on my boat.

Concerning the "selfish" and "commons" issue -- Class "A" sets have priority for the TDMA slots, so if the whole system gets overloaded, Class "B" sets will start to be kicked off the system. I'm not sure we really care about that either way -- I don't need AIS at all in crowded, close-quarters situations like in the Solent, where you don't have real crossing situations at all since the only thing you can do is just stay out of the shipping channels. I would just switch my AIS off in the Solent. Where I need it is out at sea, in the shipping lanes, where there will not be that large a quantity of targets in a 10 miles radius.
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Old 24-10-2012, 08:25   #9
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Re: Type "A" AIS for Yachts?

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Originally Posted by noelex 77 View Post
I have been debating with the same decision myself. AIS is on the to do list.

I suspect class A will get the attention of the bigger ships much better than class B. there has also been some talk of vessels filtering out class B targets in busy waterways.
I do wonder if they will assume a class A target is a large power vessel rather than smaller, sailing vessel. The information from the transmission is readily available to clarify this, but I human nature sugests they will sometimes assume rather than checking. They could easily and wrongly assume they were the the stand on vessel on some occasions.
I don't think they will assume anything about the type of vessel from class of AIS once they have glanced at the static data for the target, which tells them you are a SV under sail (by the way, with a Class "A" set you have to tell it every time you switch from sail to motor, as it will be broadcasting your means of propulsion -- maybe a PITA).

But a Class "A" target will certainly get their attention quicker, maybe an advantage.

Talking to commercial mariners I was advised over and over and over again to get a Class "A" set if I could afford it.
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Old 24-10-2012, 08:41   #10
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Re: Type "A" AIS for Yachts?

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Originally Posted by Dockhead View Post

What do you guys think -- overkill on a sailboat? I do sail in the busiest shipping lanes in the world, so collision avoidance, especially avoiding commerical shipping, is a pretty big deal for me. The Type "A" AIS looks like a lot more trouble to use -- you have to program it with your passage information every time you fire it up.

What do you guys think?

Here is the Digital Yacht Type "A" set: Digital Yacht - Light Years Ahead -- it looks like a very nice piece of kit.
IMO absolute overkill to go for a class A transponder. I'm currently on a fall cruise and just came through New York Harbor with my class B AIS. Very helpful. The commercial guys can see me and I can see them. Had no surprises of them creeping up from behind. Can use the long range when away from busy areas but, I had to bring down the filter to 1/4 mile in the harbor. Got plenty of notice when they were heading your way before you even see them too. Great safety addition to the boat. I can see all the data of their boats that I need and including their name, course, speed, CPA and visa versa in case you need to make radio contact. Save your money just go class B.
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Old 24-10-2012, 08:54   #11
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Re: Type "A" AIS for Yachts?

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I would just switch my AIS off in the Solent. Where I need it is out at sea, in the shipping lanes, where there will not be that large a quantity of targets in a 10 miles radius.
I doubt that this will be the case. AIS can be addictive in terms of the information it provides. The first thing most people discover when switching to AIS is that there are a lot more boats out there than they thought--just beyond visual range. And it's really nice to know the vessel's course, speed, name, et cetera.

I'm also skeptical of claims that some ships are filtering out AIS B signals. Commercial mariners want all the collision avoidance information they can get, especially when approaching their ports of call.
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Old 24-10-2012, 09:06   #12
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Re: Type "A" AIS for Yachts?

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I doubt that this will be the case. AIS can be addictive in terms of the information it provides. The first thing most people discover when switching to AIS is that there are a lot more boats out there than they thought--just beyond visual range. And it's really nice to know the vessel's course, speed, name, et cetera.

I'm also skeptical of claims that some ships are filtering out AIS B signals. Commercial mariners want all the collision avoidance information they can get, especially when approaching their ports of call.
I talked to some commercial mariners about this. They say that most of their AIS sets don't have a specific filter for Class "B" signals, but they can set filters for size and speed, and that they do filter small and slow targets in very crowded conditions, besides filtering out targets with more than x CPA. I can well imagine that in the Solent, for example, where there can be thousands of boats out, and a large commercial vessel simply cannot track all of the possible targets. Besides that, Class "B" signals will get kicked off the system if the

But again, that's the kind of place where it seems to me AIS information is not that useful anyway, because potential collisions appear and disappear in a much shorter time period than you can process by looking at a screen. If I'm three cables away from someone in a potential collision situation, no way I'm going to have my head in a screen -- at least it seems to me, from my armchair. We'll see when I've got it fitted and start using it, of course.
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Old 24-10-2012, 09:23   #13
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Re: Type "A" AIS for Yachts?

I wonder what the Class "A" requirements are for correct dynamic data. Such fields as "destination/status" and "Vessel Type" should be updated; I've occasionally seen AIS targets on passage with a speed of over 15 knots but "at anchor" according to the AIS data. The vessel type would probably be a legal COLREGS matter if the AIS broadcasts a "sailing vessel" while the vessel is under way using the engine.

I have to concur with the previous posters about AIS being a wonderful supplement to the tools available aboard. As a singlehander having a transponder to augment the radar and eyeballs is a great method of keeping blood pressure down. I don't think I'm quite ready to cross the TSS Schemes in the channel and/or deal with lots of big traffic --- that raises too many visions of the old video game called "Frogger".

It is so nice to be able to call "Cargo Vessel 'Behemoth'" rather than "Hailing big cargo ship about to run me down..." on the VHF using the AIS information, or even better yet getting the MMSI from AIS and calling the station directly and unmistakably via a DSC VHF call.
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Old 24-10-2012, 09:34   #14
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Re: Type "A" AIS for Yachts?

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The Digital Yacht Class "A" set uses about 12 watts -- about like an anchor light. That's ok on my boat.
It sounds like you understand and are fine with the major potential drawbacks, and value the potential benefits of A.

Personally I am the opposite. I think B works very well and I don't see much extra value in the 'benefits' of A, while I don't want the drawbacks.

Drawbacks: the extra cost, power consumption, and need to fill out extra data fields.

Benefits: slightly longer range than B - but not as much as you think and honestly the range of B is more than satisfactory, a possibility in very crowded waters that your signal will find a slot when a B does not, a possibility that ships will take your signal more seriously - but probably not all that an big effect.
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Old 24-10-2012, 09:36   #15
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Re: Type "A" AIS for Yachts?

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I talked to some commercial mariners about this. They say that most of their AIS sets don't have a specific filter for Class "B" signals, but they can set filters for size and speed, and that they do filter small and slow targets in very crowded conditions, besides filtering out targets with more than x CPA. I can well imagine that in the Solent, for example, where there can be thousands of boats out, and a large commercial vessel simply cannot track all of the possible targets. Besides that, Class "B" signals will get kicked off the system if the

But again, that's the kind of place where it seems to me AIS information is not that useful anyway, because potential collisions appear and disappear in a much shorter time period than you can process by looking at a screen. If I'm three cables away from someone in a potential collision situation, no way I'm going to have my head in a screen -- at least it seems to me, from my armchair. We'll see when I've got it fitted and start using it, of course.
Dockhead:

My Vesper Watchmate display sounds an alarm that I can then mute and also has various settings for the type of area I'm sailing such as harbor, coastal and a setting for when I'm anchored. There are also other settings I can set that I have not yet explored. I also adjust the range on it which cuts down on the amount of ships in view in places like New York. But, as a new one comes in range the alarm will sound again. I find I'm not staring at the AIS screen but, when the alarm goes of I take a quick look and see why it went off and make adjustments as needed. I've noticed the commercial ships I encountered seem to be doing the same. It seems to me any commercial Captain is going to have a lot of explaining to do if he has a collision with a boat equipped with an AIS transponder either A or B. But, the important thing is even if they do filter your boat. You can also take the corrective action because you have their course, speed, CPA information too! And there name so you can contact them on radio.
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