As Rob has said I have an active AIS system ("transponder") and also an Echomax X-band radio transponder. I have them because each time I leave the jetty in the River Itchen I pass through part of the Port of Southampton and then proceed to sail on Southampton Water and the Solent. Rob''s AIS plot looks quite quiet to me... At this time of the year at midday at the weekend there are typically 300 to 400 pleasure craft in clear view of the Coastwatch station at Calshot Tower!
Apart from the initial cost, the disadvantage of an active AIS system is the power consumption - it needs its own GPS receiver and it is transmitting - the Digital Yacht AIT2000 I have uses typically 0.3A at 12V. "Receiving only" has much less power cost. As Andy says, for that the best way is to have a radio with a built in receiver.
Advantages of an AIS receiver: I find having AIS displayed on the chart plotter is most useful for knowing where the fast "Redjet" ferries are when I want to cross the shipping channel. Also I can check where a large "clear channel" ship is if I've heard from VTS that one is in the area. Redjets travel at over 30kts, a clear channel ship might be doing near 20 kts. It also alerts me to other yachts which are behind the sails and on a collision course - yes, you have to keep looking yourself, lots of boats don't have AIS, but it's another source of information. Well worth it in my view.
Is it worth having an AIS transmitter? I don't know! I see it as a belt and braces approach. In bad visibility it will allow the Redjets and other ships to see where I am, but there are lots of fast pleasure craft out there who may not be looking at AIS. In the Solent it's probably a good idea, elsewhere perhaps less necessary.
A note on radar: be aware that the small tubular radar reflectors are more or less useless. Indeed, worse than that, they give a false sense of security (there is a Qinetiq report CR0704527 ). An efficient radar reflector is bigger than most of us would want up the mast of a Swallow Yacht. I have an Echomax X-band transponder because ships in near shore waters are more likely to be watching x-band - dual band (S+X) is better if you can afford it and if you are likely to be going offshore e.g. across the channel. The main disadvantage of the transponder is you can see LEDs flashing on the electronics box but you don't actually know that it is working! And, again, it is another sink for electrical power. But it is better than having a useless small radar reflector!
Peter
