There are different types of boats racing in a Yardstick race. Two boats (different kinds and different boat lengths) are approaching a leeward mark. In the diagram, the three-length zones for each of the two boat’s hull lengths are drawn.

Position 1 Blue is clear ahead of Yellow.

Position 2 Yellow (sailing faster) establishes an inside overlap on Blue, but still Blue is the boat nearer to the mark.

Position 3 Before Yellow reached the smaller circle, she became the boat nearer to the mark.

How does rule 18 apply in respect of the mark rounding in these circumstances? Who, if any, is entitled to mark-room?

In position 3, Yellow is entitled to mark-room under rule 18.2.

The zone in rule 18 is the area around the mark within a distance of 3 hull lengths of the boat nearer to the mark. In position 1, Blue is nearer to the mark and therefore the zone based on three of Yellow's hull lengths is not relevant.

If Blue is still the boat nearer to the mark when she enters 'her' zone (before position 3), then Blue will be required to give mark-room to Yellow under the first sentence of rule 18.2(b).

However, if Yellow has become the boat nearer to the mark before Blue came within three of her hull lengths from the mark, the first sentence of rule 18.2(b) cannot apply. If they remain overlapped at the mark, rule 18.2(a) applies and requires Blue to give mark-room to Yellow while the boats are overlapped.