How would you decide this?

Dave Perry’s 100 Best Racing Rules Quizzes highlights specific aspects of the racing rules in a fun format designed to help you become more familiar with The Racing Rules of Sailing. Here is one of the quizzes:

Quiz 45
Boats SO (on starboard tack) and PI (on port tack) are approaching the finishing line on a collision course. SO reaches the zone of the finishing mark first and hails, “Starboard!” PI hails, “Room!” When right at the mark, PI begins to gybe inside of SO and SO is forced to bear away to avoid hitting PI. SO protests PI.

You are on the protest committee…how would you decide this? Answer below.

Quiz 45 Answer:
Boat SO’s protest is disallowed; neither boat is penalized. Rule 18.2(a), Giving Mark-Room, begins to apply when SO reaches the zone (is within three lengths of the mark); see rule 18.1(a), When Rule 18 Applies, and the definition Zone. Though on opposite tacks, SO and PI are considered “overlapped” both because they are each sailing below ninety degrees to the true wind and because rule 18, Mark-Room, applies to them (see the definition Clear Astern and Clear Ahead; Overlap).

SO, as the outside boat, is required to give PI (the inside boat) mark-room, which includes space to gybe when gybing is necessary to pass between the mark and the outside boat in a seamanlike way (see the definitions Mark-Room and Room). SO complies with rule 18.2(a)(1) and PI does not take more room than she is entitled to. Though PI breaks rule 10, On Opposite Tacks, she is exonerated (freed from penalty) by rule 43.1(b), Exoneration, for her breach, because she is sailing within the mark-room to which she is entitled.

Source: US Sailing