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Inside Overlap and Room

Inside Overlap and Mark-Room are the two central concepts of Rule 18 in the Racing Rules of Sailing (RRS). They determine which boat receives room at a mark – and which boat must give that room. At windward marks, lee gates, and offset marks, most protests arise precisely here: Was there an inside overlap in the zone? Did the outside boat give sufficient mark-room?

This guide explains the terms precisely, shows typical race situations, and helps helms and tacticians apply Rule 18 safely in competition. It builds on the parent article Rule 18 and Mark Roundings and assumes knowledge of the right-of-way system.

What Inside Overlap Means

An overlap exists when the keel lines of two boats are parallel and the boats overlap laterally (Rule 16). Inside Overlap means: the other boat lies between your boat and the mark that both boats must round or pass.

Specifically for Rule 18:

  1. The inside boat is the boat closer to the mark
  2. The outside boat is the boat farther from the mark
  3. This assignment applies only while an overlap exists and Rule 18 is active

Important: Inside and outside always refer to the mark, not to the starboard or port side of the course. At a windward mark, the inside boat may lie to starboard – what matters is position relative to the mark.

When the Overlap Counts

Rule 18.2(a) states: A boat has mark-room only if it had inside overlap when either boat first entered the zone and then maintained the overlap.

The decisive moment is therefore:

  1. Entry into the zone – three boat lengths around the mark (Rule 18 defines the zone)
  2. Overlap status at that moment – was the inside boat between the outside boat and the mark?
  3. Continuity – did the inside boat maintain the overlap until passing the mark?

Inside Overlap Check at the Mark

1
Approach to mark
2
Zone reached (3 BL)
3
Overlap yes/no?
4
Determine inside/outside
5
Give mark-room
6
Mark passed

The Zone: Three Boat Lengths

The zone is a circle with a radius of three boat lengths around the mark. Measurement is from the closest point of the hull of each boat to the mark – not from the bowsprit tip or the stern.

Practical consequences:

  • A boat that establishes an overlap only inside the zone receives no mark-room under Rule 18.2(a)
  • The outside boat must know the overlap status at zone entry – tacticians and helms should actively track the distance
  • Boat length is defined in the sailing instructions or class rules

Tip: Train to visualise the zone: three boat lengths correspond to roughly 12 metres for an ILCA. Many experienced crews internally count “three lengths” as soon as the mark becomes clearly visible.

What Mark-Room Includes

Mark-room (Rule 18 defines the term in the definitions) is the space a boat needs to:

  1. Pass the mark on the required side
  2. Sail the course to the next mark immediately afterwards
  3. Manoeuvre if necessary – tack, gybe, or sail change

The outside boat must give the inside boat this room, even if another right-of-way rule would otherwise apply – for example Rule 11 (windward boat keeps clear) at the windward mark.

Obligations of the Outside Boat

Under Rule 18.2(b), the outside boat must:

  1. Give the inside boat mark-room
  2. Not sail into the inside boat’s path before it has passed the mark
  3. Allow the inside boat to continue its course immediately after the mark

If the outside boat breaches these obligations although a valid inside overlap existed, a rule infringement has occurred – typically with a protest and possible DSQ under Mark Roundings and Penalties.

Rights and Limits of the Inside Boat

The inside boat may claim mark-room, but must sail the course correctly (Rule 18.1(b)), use only room needed (Rule 18.2(c)), and give the outside boat room to pass (Rule 18.2(d)).

Inside Overlap at the Windward Mark

The windward mark is the most common protest zone on a windward-leeward course. Both boats approach from leeward, tack, and sail to the mark.

Typical scenario:

  1. Boat A (outside) and Boat B (inside) approach the windward mark
  2. At zone entry, B has inside overlap – B lies between A and the mark
  3. A must give B mark-room: B may pass the mark on the required side and continue its course after the tack
  4. A must not bear away so tightly that B cannot reach the mark or sail its course

Windward Mark – Inside vs. Outside

Role
Rights
Obligations
Typical Error
Inside boat
Claim mark-room, pass mark, continue course
Sail course correctly, use only room needed
Sail too wide around the mark, restrict outside boat
Outside boat
Own course after mark once inside has passed
Give mark-room, do not squeeze
Bear away too late or too tightly
No inside overlap in zone
Normal right-of-way rules (e.g. Rule 11)
Keep clear under Rule 11/12
Wrongly expecting mark-room
Overlap only in zone
No mark-room under Rule 18.2(a)
Observe right-of-way
Late bear-away without protection

Inside Overlap at Lee Marks and Gates

At lee marks and gates, the same principles apply. Rule 18 applies only when both boats round the same mark. After a lee mark, room for a gybe is part of mark-room.

Gate approach with inside overlap: Top-down view of a lee gate with two marks forming a gate – Boat A (outside, farther from the gate) and Boat B (inside, closer to the gate) approach from windward. The zone is a dashed circle around the chosen gate. The green area marks mark-room for B; the red zone the prohibited bear-away angle for A. The arrow shows the course of both boats after passing the gate.

Losing and Regaining Overlap

Rule 18.2(a) requires the inside boat to maintain the overlap. If it loses it before passing the mark, the mark-room claim lapses – unless the second sentence of Rule 18.2(a) applies (clearly ahead/astern before the zone, then overlap in the zone). If the outside boat overtakes the inside boat within the zone, the inside boat loses protection.

Timing
Overlap Status
Mark-Room?
Note
Before the zone
Inside overlap
Not yet Rule 18.2(b)
Zone entry decisive
At zone entry
Inside overlap
Yes
Outside must give room
In the zone
Overlap lost
No (general rule)
Right-of-way under Rule 11/12
In the zone
New overlap (without clear ahead)
No
Rule 18.2(a) first sentence
After mark passed
Any
Rule 18 ends
Normal encounter rules

Typical Protest Situations

Most Rule 18 protests revolve around the same questions. A protest jury checks in this order:

  1. Did Rule 18 apply at all? (Start line, wrong course, different legs?)
  2. Was inside overlap present at zone entry?
  3. Did the outside boat give mark-room?
  4. Did the inside boat sail the course correctly and claim only room needed?

Rule 18 Protest Decision Tree

1
Rule 18 applicable?
2
Zone entry
3
Inside overlap?
4
Mark-room breached?
5
Decision outside/inside according to fault

Practical examples: If the outside boat (X) bears away too tightly and prevents the inside boat (Y) from passing the mark, although Y had inside overlap at zone entry, X breaches Rule 18.2(b) – Y should protest (Protest Procedure). If a boat establishes overlap only inside the zone, it has no mark-room and must keep clear under Rule 11.

Checklist for Helms and Tacticians

Before every mark rounding, the crew should internally review these points:

  • Distance to mark: Are we in or near the zone (3 boat lengths)?
  • Overlap status at zone entry: Are we inside or outside?
  • Did our boat have inside overlap when entering the zone?
  • If outside: reserve mark-room for the inside boat – do not squeeze
  • If inside: sail course correctly, claim only room needed
  • After passing: Rule 18 ends – observe normal right-of-way rules
  • If in doubt: have protest flag ready and document incident for hearing

Tactical Recommendations

  1. Position early – inside overlap before zone entry is often more important than speed in the final boat lengths
  2. Outside defensively – better to give one boat length more room than risk a protest
  3. Inside realistically – mark-room does not entitle you to unrealistic bear-away manoeuvres
  4. Decide gate early – without inside overlap, choose the other gate early

Summary

Inside Overlap means: boat between another boat and the mark – decisive at zone entry. Only a boat that had inside overlap then and maintained it may claim mark-room; the outside boat must give that room. Details in the Racing Rules of Sailing.

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