Source File: BasicRacingRules.pdf
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.
Boats are on opposite tack but the red line boat is gybing to go around the mark (not tacking). So can’t call ‘buoy room’. However, before gybing the red line boat is on starboard tack and has right of way over blue. The red line boat can use this to create room for itself to round the mark. After gybing overlapped windward boat (red) must stay clear of leeward (blue).
Note the gybing boat (red) has to gybe as soon as practically possible and not delay:
When an inside overlapped right-of-way boat must gybe at a mark to sail her proper course, until she gybes she shall sail not further from the mark than needed to sail that course. Rule 18.4 does not apply at a gate mark.