The drawings are not to scale and the vectors are relative to each boat's motion.
I'm in boat A, I am standing-on for boat B who has adjusted his speed and course well in advance to give-way and pass behind me. Along comes Boat C, he clearly has to give-way to boat B, and should endeavor to pass me port to port. All he needs to do is steer to starboard a little, as shown by the arrow, and he'll pass well behind boat B. Any guesses at what the captain of Boat C does?
The yellow arrows indicate what actually transpired. Boat C decides to go between boats A and B. This makes me, in boat A, change course radically to starboard to maintain 150' separation from boat C (I actually changed course enough to allow boat C to turn to port even harder in order to maintain 150' separation from boat B as well, even though he did not). Boat B had to slow to a near stop and turn 90 degrees to starboard to avoid a collision and Boat C simply cruises on by with 30 feet to spare from boat B, without changing course or speed.