I said I would not post again on this thread. I recant only to provide information on AIS.
Who has to carry: Automatic Identification Systems (AIS)
1. In general big ships. In Singapore everyone by law - something that is likely to become more prevalent in very crowded maritime areas. One of the exemptions - fishing boats - sometimes a problem when making coastwise passages. Fishing boats tend to be brightly lit so they are pretty easy to see unless the weather is bad. I think that you will see the rest of the major maritime world moving toward the Singapore rule - you must have a transmit type AIS to cruise in their waters. This will be led by the harbors that already require all ships to report in when entering and leaving port (VTS Systems - see here: Vessel traffic service - Wikipedia, the free encyclopedia
2. Sailboats are hard to see on most radars - including both commercial and recreational radars. If you call another vessel on the VHF they can usually figure out where you are but are not likely to see you on the radar before you call. The bridge watches are not staring at the radar and the alarms typically will not be set to go off for a return that small. Radar reflectors help a little, but not all that much. One thing that surprised me: back in the days of paper charts if you gave a ship your lat and long they could pretty much pinpoint your location in a couple of seconds. Now with the glass bridges it seems to take a lot longer. I am not sure exactly why this is true.
3. What kind of collision is most likely? The one where you are not in the cockpit looking around to see who can hit you. Even with an AIS transmitter you are not complexly safe from large ships. I have been run down twice, once by a cargo ship, once by a Carnival Cruise Liner. In both cases my AIS was blipping away. CCL Captains have a very bad reputation in the shipping world. They seem to think that the rules of the road are they go where they want and everyone else gets out of the way.
(actually I have been run down three times - in the pre-AIS days I was hit by a "laker" off Alpina in bad rain and fog. Did a mess to the bow of the boat but we stayed afloat and made it into a safe harbor.)
Fog banks can be deadly. I was approaching the Blue Water bridge at the foot of Lake Huron. There was a big fog bank under the bridge. The rest of Lake Huron was clear as a bell. My radar painted a huge target - I thought the bridge of course. All of a sudden a huge laker (they look even bigger from 100 feet looking up at the bow coming right at you!) came out of the fog bank heading directly at me. Five shorts on my pathetic little horn, right emergency rudder, all ahead flank, I got out of the way (barely.) They never saw me until I was abaft the wheelhouse tooting my little horn. Then they hit their fog horn. Darn near blew me out of the water! (I guess this counts for run down #4) They never slowed or turned. Would not have mattered anyway unless they needed to pick up the pieces. They just can't maneuver that quickly. (This was, btw, in my pre-AIS days.) Another minute and I was out of the fog in the St. Clair river.
Slightly off topic but your deck navigation lights on a typical sailboat are not worth much either. Offshore your lights tend to be obscured by wave action. The exception is a tricolor. Most commercial ships are illuminated at night. They are pretty easy to see. The exception is ships carrying explosive material such as oil. They usually have a minimum of lights visible - just the required navigation lights. I always wondered how the bridge watch on a brightly lit ship could see at night. We could track ships visually 10 NM away. We would chat with the bridge crew and they would know where we were (we have transmit AIS) but could not see us visually unless they got less than 1 NM away. That is why I now have a tricolor 60 feet up on top of the mast. In general on the open ocean 1 NM was the courtesy separation - large ships would alter course to give us a CPA of 1 NM or more. An aside: When I first started cruising I was confused by the side lights on big ships. In the recreational world the "bow lights" are the red and green lights located near the bow. But the COLREGS require side lights, not "bow lights." Most commercial ships have the side lights on the house in the stern. I was looking at a colored light thinking I was looking at the bow when in fact I was looking at the stern. Everyone seemed to be steaming in reverse...
Should you cruise in US waters (elsewhere in the world too, my experience is off Norfolk) in places like VACAPES you will discover that United States Navy ships don't transmit on AIS. And they don't show up on radar. And they don't have a lot of lights on. In fact you will swear they are not there until your radio crackles and they talk to you!! Even then you will swear they are not there. And they only talk to you if they want you to do something. Otherwise they just ghost by. A good thing from a combat standpoint, a little scary when sailing. The military operating areas are marked on the charts but they do have to get to them and get back home.