SECTION C - AT MARKS AND OBSTRUCTIONS
To the extent that a Section C rule conflicts with a rule in Section A or B, the Section C rule takes precedence.
18 ROUNDING AND PASSING MARKS AND OBSTRUCTIONS
In rule 18, room is room for an inside boat to round or pass between an outside boat and a mark or obstruction, including room to tack or gybe when either is a normal part of the manoeuvre.
18.1 When This Rule Applies
Rule 18 applies when boats are about to round or pass a mark they are required to leave on the same side, or an obstruction on the same side, until they have passed it. However, it does not apply
(a) at a starting mark surrounded by navigable water or at its anchor line from the time the boats are approaching them to start until they have passed them, or
(b) while the boats are on opposite tacks, either on a beat to windward or when the proper course for one of them, but not both, to round or pass the mark or obstruction is to tack.