Rule 18.5

Section C: At marks and obstructions: Section C rules do not apply at a starting mark surrounded by navigable water or at its anchor line from the time boats are approaching them to start until they have passed them. When rule 20 applies, rules 18 and 19 do not.

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.