Eric Forsyth, one of the world's most experienced sailors, always says to make a course change of at least 20 degrees when there is any question about a potential collision. His reasoning is that if a course change is needed - make a course change that is significant and clearly communicates your intentions to the other captain. Forsyth has probably put as many ocean miles to his stern as any sailor in the world on his or her personal boat. His sailboat is Fiona, a Westsail 42.
When sailing I give my new crew members a very brief lecture about collisions at sea, tell them about Forsyth and his advice, and emphasize the 20 degree rule (so long as there is enough room for that course change without creating another potential collision). Before my first blue-water passage the captain told me to read the book, "Ten Degrees of Reckoning". Anyone undertaking an ocean passage with inexperienced or moderately experienced crew should make the book required reading. It also helps one understand the wisdom in Forsyth's advice.
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