I'd just interject amongst the many fine posts here that the average merchant ship will acquire a decent sized sailboat on radar at about six to seven miles on radar, in decent sea conditions, unless the boat has some type of radar transponder. Even so equipped, you don't pick up smaller craft and some ships at twenty miles.
In the past it was quite common to come across sailboats offshore with no running lights or the lights so dim as to make them ineffectual. Hopefully advances in battery technology and charging options is lessening those occurences. But I never really expected to see lights on a sailboat and, given the size radar return they present, was never certain if I was looking for a sailboat or a submarine periscope from the information on my radar.
I'll confess to having woken up a few offshore sailors who obviously were not keeping watch either visually or on the VHF. I'd pass under their stern a couple of miles off and give them a blast on the whistle. If they don't see a ship in broad daylight, a ship in liner service (which means it's on a regular run usually shared by many other companies, ie...in shipping lanes), then there's a good chance they'll not see the ship fifteen miles astern of me overtaking me rapidly either. And, of course, there's no assurance that ship is as alert as I was. The other motivation was that, when seeing a sailing vessel well offshore apparently watchless, one cannot help but wonder if all is in order on board. One would hate to think that one steamed right on by a boat unable to keep watch due to something of an urgent nature. The blast on the whistle wasn't malicious in intent; just an, "I'm out here". If they needed help it would be more than welcome and, if they were sleeping, it might cause them to question the advisability of doing so when I was able to get so close unbeknownst to them.
Frankly, no matter how beautiful the boat, I hated seeing them. Too often a sailboat would fail to show up on radar, either due to weather conditions or no radar reflector, and would just appear somewhere off the bow in two or three mile visibility. Nothing will jolt you quite like your lookout saying, "Hey mate, look at that sailboat" with a tone of wonderment in his voice. Most able-bodied seamen don't even consider the fact that another vessel might not be keeping a lookout, a luxury the mate on watch does not have. I've caused more than one spilled coffee or soup on board by putting her over on her beam ends to get some distance off a sailboat appearing close aboard. You feel really angry when you have to take drastic action like that and it then becomes apparent that no watch is being kept on board the sailboat. Of course you're angry because you just had that cold, clammy fear run through you that you might have come very close to killing someone had things been different.
Nolatom is entirely correct in that it is the lack of collisions between sailboats and ships that make for so few court cases where this otherwise might be a more widely discussed issue. I ascribe that to two things; good watchkeeping and the fact that the shipping lanes are generally very different from the sailing lanes.
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.