Mark-room is the room a boat needs to sail to a mark and round it. Under rule 18, when boats are overlapped as the first of them reaches the zone, three hull lengths from the mark, the outside boat must give the inside boat mark-room.
Rule 18 of the Racing Rules of Sailing deals with boats rounding a mark together. It decides who must give room, and it applies when boats are in the zone: the area within three hull lengths of the mark, measured using the length of the boat nearer to it.
The basic rule. If boats are overlapped when the first of them reaches the zone, the outside boat must give the inside boat mark-room, whatever tack they are on and whichever is to windward. If they are not overlapped when the first reaches the zone, the boat clear ahead is entitled to mark-room from the boat clear astern.
The situation is fixed at that moment. A boat that gets an inside overlap from clear astern after the boat ahead has entered the zone is not entitled to mark-room. Equally, if an outside boat breaks the overlap after the zone, she must still give room.
What mark-room includes. Room to sail to the mark, to round or pass it on the required side, and to leave it astern, while sailing promptly and in a seamanlike way. It does not give the inside boat the right to take a wide, sweeping course.
Important exceptions:
- rule 18 does not apply between boats on opposite tacks on a beat to windward, so at a windward mark port still keeps clear of starboard;
- it does not apply to a start mark surrounded by navigable water from the time boats approach it to start;
- rule 18 also limits what a boat can do if she tacks in the zone of a windward mark.
Practical advice: call your overlap clearly before the zone ("Overlap" or "Room"), and if you are outside, start giving room early. Most mark-rounding incidents happen because one boat did not realise the other was overlapped, so look behind as well as ahead.
More in the book: Chapter 15, Your first race