I have an ICOM MA-500TR class B AIS (meaning it transmits and receives).
Transmission can be turned off when not wanted.

Power consumption when transmitting is 1.5A otherwise it is 0.7A. It transmits at 2Watts which is the norm for these devices.

It has a built in GPS unit that will share it's output via NMEA and has it's own VHF antenna. While I do not yet have first hand experience with the unit on the water yet (splash day is Feb 2013), I did my homework and my experience with ICOM products over the years has been very satisfactory.

Tony & Sally Summers wrote:Graham,Annie, you mentioned that AIS uses a lot of power ----  do they? Ijust looked up the consumption figures on my transponder and it is supposed to average 4 watts. Its also possible to switch the transmit function off until a "target" presents an issue. I tend to pick up targets at about 10 miles range so have over 30 minutes to decide whether to activate transmit, if our courses appear to be converging.Mine is also a stand alone unit with its own GPS built in, and a dedicated aerial ---  friends have all reported problems with splitters on the ships VHF. Not having radar[where do you put the radome on a junk?] I still think its a great bit of kit, especially in thick weather. Tony RON GLAS

Thanks for your feedback Tony, it is very interesting to hear from people who have experience with it.  My advice came from an electronics engineer who chose to put a receiver on his yacht because he claimed that an AIS transmitter would use 2.5 amps, which would equate to 25 watts in a 12 volt system.  Perhaps it only uses 25 watts - if it does - when it is actually transmitting and the average consumption is much lower.  If it is in standby mode most of the time that would certainly reduce power consumption.  I have not yet done my own research but will do so before choosing a system in 2013.  A transmitter certainly is the best option if you have the power to run it. I spoke to an Australian Reef Pilot recently - he pilots ships through the Great Barrier Reef - and he said he found it extremely helpful when he crossed paths with a yacht using an AIS transmitter, since it gave him the yacht's speed and course and allowed him to take effective action much sooner.

Sensible use of electronics can greatly enhance our ability to keep an effective lookout, especially if singlehanded or short-handed, though I must admit to a hankering for the old days when I sailed without engine, electrics or electronics, relying, like John Guzzwell and Annie Hill, on a combination of luck and good judgement as to when it was safe to snooze, and when you should spend the night on deck, which I have done on many occasions, especially when in dense coastal shipping lanes.  Life was a lot simpler back then - and cheaper - but I accept that I am unlikely to return to such Arcadian days.

Tony, can you advise what AIS system you have installed?     

Graham,Annie, you mentioned that AIS uses a lot of power ----  do they? Ijust looked up the consumption figures on my transponder and it is supposed to average 4 watts. Its also possible to switch the transmit function off until a "target" presents an issue. I tend to pick up targets at about 10 miles range so have over 30 minutes to decide whether to activate transmit, if our courses appear to be converging.Mine is also a stand alone unit with its own GPS built in, and a dedicated aerial ---  friends have all reported problems with splitters on the ships VHF. Not having radar[where do you put the radome on a junk?] I still think its a great bit of kit, especially in thick weather. Tony RON GLAS

You might also consider a Sea Me type Radar Target Enhancer, nowadays available to cover X and S bands.  They are relatively cheap and straight forward to install ie all the works are in the reasonably sized aerial and the aerial only requires a thin 12 vote line "supplied with the aerial) run to it.  They use next to no electricity on stand by and only operate and use a little electricity when pinged by an incoming radar.  The switch has a red indicator light which flashes when it is pinged but you can also have a bleeper turned on.  I found that the bleeper is enough to wake me up.  

The system used on Badger is ideal for a couple who can both handle the boat.  Three hours seems about right.  I have been in that situation myself for a few, delightful passages, and I am deeply envious of my friends who have such cruising partnerships.  I once sailed as part of a competent crew of three and we did four hours on and eight off.  There was plenty of time to rest off watch but the watches themselves were sometimes a bit of a struggle, particularly when it was cold, wet and rough.  Perhaps if we'd been junk rigged with a Jester pramhood things would have been different.

Singlehanders, of course, face a different set of challenges.  I have always adhered to Blondie Hasler's advice that you should take to your bunk if you are not tending to body or ship. I have found that if I get my head down as soon as possible, BEFORE I get tired, I can manage very well with short naps for extended periods.  I feel amazingly refreshed after a 10 minute nap, have a quick look around, do what has to be done and get straight back to my bunk.  I am a reluctant riser ashore (monsters under the bed) but at sea it is not a problem, perhaps because - with luck - I know I will be back in my bunk before it gets cold.  When I am well offshore and have not seen a ship in ages, I relax a bit and sometimes spend 30 minutes in my bunk.  Jumping up to have a quick look at things though remains an easy thing to do and I go back to sleep almost immediately - I suspect that I am not even fully awake most of the time I am up unless I have to do something.

Not everybody has this ability to catnap though.  I have known others who say it makes them slide into exhausted stupidity.  Phillippe Jeantot, who won the BOC round the world race twice, and who started the Vendee Globe race, found he need to sleep for an hour at a time to get any benefit.  At the opposite extreme, I have known some singlehanders who spend all night in their bunks and adopt a fatalistic approach.  These sailors include the great John Guzzwell, of Trekka fame, among others.

Before I go off on another long singlehanded haul I plan to fit an AIS system, (a receiver at least since the transmitters use a lot of power) and I feel that this will greatly assist singlehanded watchkeeping.  It won't protect me from other vessels without a functioning AIS of course but it will greatly reduce the odds.  I will still jump up for a look around at every available opportunity though.  I also keep a very bright set of LED navigation lights burning, so other small craft should be able to see me unless they are fellow singlehanders, and therefore tarred with the same brush.