Section C rules do not apply between boats when the mark or obstruction referred to in those rules is a starting mark surrounded by navigable water or its anchor line, from the time the boats are approaching it to start until they have left it astern.
18.1 When Rule 18 Applies
(a) 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
(1) between boats on opposite tack on a beat to windward,
(2) between boats on opposite tack, when the proper course at the mark for one but not both of them is to tack,
(3) between a boat approaching mark, and one leaving it, or
(4) if the mark is a continuing obstruction, in which case rule 19 applies.
(b) Rule 18 no longer applies between boats when mark-room been given.