World Sailing Official Case Book
Case 9
When a starboard-tack boat chooses to sail past a windward mark, a port-tack boat must keep clear. There is no rule that requires a boat to sail a proper course.
Question
Two close-hauled boats on opposite tacks meet at a windward mark to be left to starboard. S has adequate room to tack and round the mark with due allowance for wind and current but instead of tacking, S holds her course with the intention of forcing P to tack to keep clear. Can P disregard rule 10 if she considers S to be sailing beyond her proper course and to have sufficient room to round the mark?
Answer
No.
Rule 10 applies. Rule 18 does not apply if the boats are in any of the four situations described in rules 18.1(a)(1), (2), (3) or (4). In this case, both rule 18.1(a)(1) and rule 18.1(a)(2) apply to S and P. They are on opposite tacks on a beat to windward (see Case 132), and the proper course for S, but not P, is to tack.
When S chooses to hold her course, P must keep clear. While in certain circumstances boats are prohibited from sailing above a proper course, there is no rule that requires a boat to sail her proper course.