On a race course
Sailormon6 mentions something that is very important, and that is the effect of "dirty air" on another boat. A boat in your wind shadow can be slowed down very much by the disturbed or decreased wind caused by your sails. I have had one instance where some one planted himself just to windward of me, and I simply asked if he was racing. He said no, and immediately got what I was saying and headed off, getting a big "thanks" from me. However another time I had a boat with a mast about twice as tall as mine, motoring with the main up, get about two boat lengths in front of me in a light air race, effectively stopping me dead in the water, and not even noticing. He was oblivious to anyone else, figuring if he didn't actually hit any boats it must be okay, I guess. I went from a possible first to a fourth in that one. I always try to notice where I am in regards to any racer and stay as far away as possible. It's just courtesy.