18  MARK-ROOM
18.1 When Rule 18 Applies
Rule 18 applies between boats when they are required to leave a mark on the same side and at least one of them is in the zone. However, it does not apply
(a) between boats on opposite tacks on a beat to windward,
(b) between boats on opposite tacks when the proper course at the mark for one but not both of them is to tack,
(c) between a boat approaching a mark and one leaving it, or
(d) if the mark is a continuing obstruction, in which case rule 19 applies.

18.2 Giving Mark-Room
(a) When boats are overlapped the outside boat shall give the inside boat mark-room, unless rule 18.2(b) applies.
(b) If boats are overlapped when the first of them reaches the zone, the outside boat at that moment shall thereafter give the inside boat mark-room. If a boat is clear ahead when she reaches the zone, the boat clear astern at that moment shall thereafter give her mark-room.
(c) When a boat is required to give mark-room by rule 18.2(b), she shall continue to do so even if later an overlap is broken or a new overlap begins. However, if either boat passes head to wind or if the boat entitled to mark-room passes head to wind or leaves the zone, rule 18.2(b) ceases to apply. (effective 1 Jan 2010)
(d) If there is reasonable doubt that a boat obtained or broke an overlap in time, it shall be presumed that she did not.
(e) If a boat obtained an inside overlap from clear astern and, from the time the overlap began, the outside boat has been unable to give mark-room, she is not required to give it.

18.3 Tacking when approaching a Mark
If two boats were approaching a mark on opposite tacks and one of them changes tack, and as a result is subject to rule 13 in the zone when the other is fetching the mark, rule 18.2 does not thereafter apply. The boat that changed tack
(a) shall not cause the other boat to sail above close-hauled to avoid her or prevent the other boat from passing the mark on the required side, and
(b) shall give mark-room if the other boat becomes overlapped inside her.

18.4 Gybing
When an inside overlapped right-of-way boat must gybe at a mark to sail her proper course, until she gybes she shall sail no farther from the mark than needed to sail that course. Rule 18.4 does not apply at a gate mark.

18.5 Exoneration
When a boat is taking mark-room to which she is entitled, she shall be exonerated
(a) if, as a result of the other boat failing to give her mark-room, she breaks a rule of Section A, or
(b) if, by rounding the mark on her proper course, she breaks a rule of Section A or rule 15 or 16.

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