Question
Three boats A, X, and B are approaching an off-wind mark to be left to
starboard. The proper course after the mark is a broad reach on port tack. X has
no proper course restriction, and is luffing A to slow her. When X reaches the
two-length zone, B is clear astern. When B is alongside the mark, X bears away
and gybes to prevent B from passing between her and the mark. B touches X
and the mark and protests. What should the call be?

Answer
At position 2 B is subject to rule 18.2(c), and is keep clear boat with respect to X until both B and X have passed the mark. X as right-of-way boat under 18.2(c) is not subject to rule 16 when she changes course to round the mark (rule 18.2(d)). X may not sail below her proper course unless she gybes (rule 17.2).
X’s change of course at position 5 is not to round the mark. Therefore rule 16 applies. B is so close to X that she cannot alter course to pass astern of X. She tries to keep clear by bearing away, but the presence of the mark prevents this.
X breaks rule 16.1. Penalize X, and exonerate B under rule 64.1(b).