Between boats leaving the same mark on the same side when at least one is in the zone. It does not apply between boats on opposite tacks on a beat, between a boat approaching and one leaving the mark, when one but not both must tack at the mark on opposite tacks, or when the mark is a continuing obstruction.
Rule 18 is a snapshot rule. Look at the two boats at the moment the first of them reaches the zone (three hull lengths of the boat nearer the mark). That moment fixes who owes mark-room for as long as rule 18 applies between them.
Mark-room is space to sail to the mark when your proper course is close to it, to round or pass it on the required side, and then to leave it astern. It is not a right to sail any course you like; it is the space you need to make the rounding a competent crew would make.
Section C rules do not apply at a starting mark surrounded by navigable water, or its anchor line, from the time boats are approaching it to start until they have left it astern. Committee-boat end barging is governed by Section A and rules 14–16, not rule 18.
18.1 When Rule 18 Applies
Two boats, same mark, same required side, one of them in the zone. The four exceptions are what most rule 18 protests are really about:
- opposite tacks on a beat to windward: rule 10 and rule 13 decide;
- opposite tacks where one boat's proper course at the mark is to tack and the other's is not;
- one boat approaching the mark and the other leaving it;
- the mark is a continuing obstruction (an island, a long breakwater), where rule rule 19 applies instead.
Rule 18 also stops applying once mark-room has been given.
18.2 Giving Mark-Room
(a) When the first boat reaches the zone: if they are overlapped, the outside boat owes the inside boat mark-room; if they are not overlapped, whichever boat has not yet reached the zone owes mark-room to the one that has. That obligation continues even if the overlap is later broken or a new one begins.
(b) The obligation under (a) ends if the boat entitled to mark-room passes head to wind or leaves the zone.
(c) If (a) does not apply (for example it has ended under (b)) and the boats are overlapped, the outside boat gives the inside boat mark-room.
(d) An inside overlap from clear astern, or from tacking to windward of the other boat, earns nothing if the outside boat has been unable to give mark-room from the moment it began.
(e) If there is reasonable doubt whether an overlap was obtained or broken in time, the rule presumes it was not. The boat claiming the change has to prove it.
18.3 Tacking in the Zone
Applies at a mark left to port when a boat tacks from port to starboard inside the zone and another boat on starboard is fetching the mark. Rule 18.2 does not apply between them: the boat that tacked gets no mark-room from the fetching boat. If the fetching boat has been on starboard since she entered the zone, the tacker must also not make her sail above close-hauled to avoid contact, and must give mark-room if the fetching boat becomes overlapped inside her.
The practical lesson: tacking onto the layline inside three lengths in front of starboard traffic is high risk. You have only the protection of rule 13 and rule 15.
18.4 Gybing in the Zone
An inside overlapped right-of-way boat that must gybe at the mark to sail her proper course may not sail farther from the mark than her proper course needs until she gybes. It stops the inside boat from carrying the outside boat wide past the mark before gybing. It does not apply at a gate.
Common situations
Clear ahead at the zone, wide rounding
Boat A is clear ahead and reaches the zone first. She is entitled to mark-room from B even if B later gets an inside overlap. If A swings very wide and B, still clear astern, reaches the zone before A does, B is the boat entitled to mark-room. The 2025 wording makes the first boat to reach the zone the one that matters when there is no overlap.
Leeward mark, outside boat with an overlap at the zone
Two boats arrive overlapped. Inside is owed room to sail to the mark, round it and leave it astern. If the inside boat is also the right-of-way boat she may sail her own course through the rounding, subject to rule 16 and, when she must gybe, rule 18.4. If she is the keep-clear boat (inside and to windward), she is protected only while she sails within the mark-room she is owed; a wide, sweeping rounding that takes more space than the rounding needs is outside it.
Port-tack boat at the windward mark
On a port-rounding windward mark, a boat approaching on port tack and a boat on starboard are on opposite tacks on a beat. Rule 18 does not apply: starboard has right of way under rule 10. If port tacks in the zone, rule 18.3 governs.
Common misreadings
"Three boat lengths means three of my lengths"
The zone is three hull lengths of the boat nearer the mark. In mixed fleets the zone depends on which boat is closer.
"Inside overlap at the mark means I get room"
Only an overlap that existed when the first boat reached the zone counts under 18.2(a). An overlap obtained later from astern gives nothing under 18.2(a), and 18.2(d) and (e) make late overlaps hard to claim under 18.2(c).
"Mark-room lets me luff the outside boat"
Mark-room protects you while you sail to and round the mark. If you sail outside that room, for instance by luffing above your proper course to push the outside boat away, you lose the protection of rule 43.1(b) and the ordinary rules apply.
Changed in 2025
Rule 18.2 was reorganised for 2025. The case that used to be 18.2(b) (who is overlapped when the first boat reaches the zone) is now 18.2(a); without an overlap, the first boat to reach the zone is entitled to mark-room even if she is behind. Rule 18.3 now applies to any port-to-starboard tack in the zone against a fetching starboard boat, and continuing-obstruction marks were moved to rule 19.