When a right-of-way boat changes course she may break rule 16, even if she is sailing her proper course.
Assumed Facts
S on starboard tack and P on port tack are on a leg from the leeward mark to the finishing line. S has overstood and bears away to clear the stern of the committee boat at the starboard end of the line. P can cross S clear ahead if S maintains that course, and P hails S to hold her course. After S passes the stern of the committee boat, her proper course is to luff to a close-hauled course. S luffs to a close-hauled course at which point there is less than a hull length between S and P. Both then luff to head to wind, and they manage to avoid contact by the narrowest of margins.
Question
What rules govern the relationship between the two boats?
Answer
Rule 18 does not apply because the boats are on opposite tacks on a beat to windward (see rule 18.1(a)). Therefore, S is not entitled to mark-room from P. P is subject to rule 10, which requires her to keep clear of S, but rule 16.1 prohibits S from changing course without giving P room to keep clear, even when S is sailing her proper course. In this situation when S luffs to a close-hauled course just after passing the stern of the committee boat, she does not give P room to keep clear and, therefore, breaks rule 16.1. P breaks rule 10, but is compelled to do so because S breaks rule 16.1. Therefore, P is to be exonerated under rule 64.1(a).