Class B AIS

[roly_voya]

I am fitting out a yacht for offshore and after all the publicity about inefective radar reflector have been looking at class B AIS as a means of increasing visability to shipping. Question is, do most ships efectively monitor for class B transmissions? I gather this is optional and can be turned off to prevent swamping in port areas but does it get turned back on at sea and is it regularly checked?

 

[landaftaf]

a class b ais will transmit on the ais frequencies - just like a class a ais, so any commercial vessel over 300 t will see a class a or class b transmitter if it is on.
the difference is, as far as I know, purely specification driven. the class a ais has to comply with a higher specification.

this web site will explain the different systems available

 

[rickp]

Class B transponders use some different messages that not all Class A sets can decode yet - that includes the 'static' information such as vessel name & MMSI (technically this change was to allow the static info to be sent in one timeslot, I believe). I guess that will be resolved over time though. Class B transponders are also on less power (2 watts, rather than 12, I think) which trials have shown is sufficent for about 6 nm range.

The interesting thing is what a captain on a commercial vessel will do when entering the solent with hundreds of Class B sets squawking away. The position and bearing updates are only be sent every 30seconds so I'm not sure that it will help on collision avoidance - so would we see them filter all non-Class A targets?

I seriously considered a Class B set but on balance decided to just get a AIS receiver for now and wait and see how class B pans out...

Rick

 

[Dyflin]

[ QUOTE ]


The interesting thing is what a captain on a commercial vessel will do when entering the solent with hundreds of Class B sets squawking away.



[/ QUOTE ]

I would suggest that he will stand in front of a radar screen with a good pair of bins in hand. He's only concerned about what's going to give him trouble. /forums/images/graemlins/smile.gif

 

[roly_voya]

I agree class B AIS would be no help in the solent as Rickp says too much traffic and anyway the sort of shipping I am interested in is going to have priority (constrained by draft) so I will be well out of their way. They are also folowing set channels and accompanied by pilot boats so there is no exuse for getting in the way.
My usual stratagy if I look like crossing in front of a large vessel is to stand off before it becomes close quaters and wait untill I can pass behind, I would not normally pass less than 3 miles ahead of a large vessel. Although this has always worked I realised after reading the Uzo report that she migh have been doing just that, the Pride then turned onto a colision course and there was not time to get clear. (may not be exactly what happend but could have been) You cant blame the ship, if they cant see you they cant avoid you. If they had looked at an AIS display and seen a yacht the turn would have been delayed. Given that I sail out of Milford and much of the shipping is the size of a small Island and about as manoverable such things are neither remote nor theoretical and the evidence about radar reflectors is worrying to say the least.

 

[AndrewF]

I'am the master on a rather large tanker. And personally do not rely on AIS targets for my collision avoidance. Party due to the fact that the course and track information comes directly from the transmitting vessels GPS. This means that it shows speed over the ground and ground track. When you really need speed through the water and vessels heading.
The other problem is that most comercial vessels display AIS information on the ECDIS sets and use Radar sets for collision avoidence. You can normally overlay the information from the ECDIS to the radar sets but this increase of information is confusing and in my view dangerous.
So rather than rely on a piece of electronic equipment, I would invest in a decent radar reflector and a good set of nav lights. Oh and possibly paint the hull any colour other than white and have sails that aren't white.

 

[landaftaf]

interesting perspective

without entering into a pi$$ing contest, I also have been sailing as Master for many years ..... though not around the solent since ais came into force.

the one over riding aspect of ais is it will get you seen. the info recd is incidental. most modern vessels have an ais display on either the ecdis or radar or both so this alerts the bridge to the presence of another contact, the rest follows its normal events.
I agree the info displayed might be inaccurate but at least you can see it, and the arpa plots the targets relative position to work out risk of collision, not the erroneous data transmitted, so both on ecdis and radar you get a reasonable cpa in minutes.

because all the data from a class b might not available, or because it 'only' has a range of 6 miles should not denigrate the usefulness of an ais, a or b

 

[homa]

I am another serving Master (on a large N.Sea Ferry).
I find AIS annoying and clutters the radar picture - particularly in congested waters.
I would agree with AndrewF that one should prioritise things.
Buy a decent radar reflector
Fit the best nav light you can afford - and keep them SWITCHED ON at night !
Keep sails hoisted when crossing shipping lanes - makes you much more visible against white horses - even at night.
Look over your shoulder from time to time - amazing how many yachts we come up on with poor or no nav lights on and poor radar detection.
I have purchased my own secondhand radar now and am now considering seame as my next bit of safety kit. This will be long before I consider AIS.

 

[roly_voya]

Re: Class B AIS - Just to clarify

I already have a SOLAS aproved 10sq/m reflector, nav lights at aquasignals 40 with 2/3mile range, sails are a combination of Tan and white so the boat is about a visable as possible although a fl orange staysail could be worth considering. I will be fitting radar over this winter which will be viewable both from deck and nav station so watch will be by both sight and radar if conditions indicate it. My current cruising ground is S/W wales & Ireland based ar milford haven but the boat is being refitted for a proposed trip to N spain. My concern started after reading the accident inverstigation results from incident with Pride of Bilbao and the yacht Uzo which suggested that even the best passive reflectors may not be visable to some shipping in some circumstances which is clearly worrying. I have looked at the SeaMe system but SeaMe only covers Xband so if a ships is only use S band for long range away from the coast (don't know if this is common) that could be a problem offshore and although better than passive reflectors it is still only a reletively small target particularly when the boat is heeled. I am considering whether AIS is a better option as it should have longer range and more reliable reception as it transmits on VHF. The BUT is that it is only as good as the recieving system/gear, hences the request here.

 

[landaftaf]

Re: Class B AIS - Just to clarify

you will no doubt make your own decision

however, had the POB had ecdis with ais superimposed(meaning the OOW didnt need to spend alarmingly large amounts of time in the chartroom), and had the ouzo had ais she would have picked up the ouzo a long time before she was seen ...... cos if you have an ais target you look for it visually - in which case it wouldnt have hit it QED - allegedly

take it from me, the ais is normally superimposed on the ecdis (and can be on the radar) ........ the merch vessel will probably see you well before it detects you on radar, which will often be before he sees you visually.

incidentally, all the class a ais kits I have sailed with have an independent CPA alarm one can program, so even if the merch isnt using an ecdis he will still be alerted to a class a or b ais if his alarm has been set, or if he gets off his arse and looks at it now and again.

you have thought it through, relax and use the ais for what it was designed to do ........ the other (negative IMO) contributors have taken their parochial view in crowded/busy waters, even then the merch vessel can reduce the range on the ecdis to check up on the individual yachts in or around his intended path - and to get an almost instaneous CPA.

if you are seen by the merch vessel the rest will fall into place /forums/images/graemlins/cool.gif

 

[roly_voya]

Re: Class B AIS - Just to clarify

Thanks all, interesting comments. On balance I think it is worth fitting primarily as it is a third and independent means of alert to visual and radar dectection which might quite often mean we both know each other is there much earlier.

 

[Amphitrite]

Re: Class B AIS - Just to clarify

BTW, Class B AIS uses a new subset of AIS messages to transmit static AIS data (name, callsign etc) not all older Class A units can decoede, but the MMSI is always transmitted together with the dynamic data (position, speed, course etc) as it is used for identification purpose by the system.