Case 9
Definitions, Proper Course
Rule 10, On Opposite Tacks
Rule 18.1(b), Mark-Room: When Rule 18 Applies
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.

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?

No; rule 10 applies. Rule 18.1(b) states that the boats are not subject to rule 18 because they are on opposite tacks and the proper course for one of them (S), but not both, 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.