to make a trial trip

A, moving faster, passed and was clear ahead of B at position 3.

If a boat is clear ahead when she reaches the zone, the boat clear astern at that moment shall thereafter give her mark-room.

This rule does not require a boat clear ahead to take any action to keep clear as a windward boat before the boat clear astern becomes overlapped to leeward.

If a boat is clear ahead when she reaches the zone, the boat clear astern at that moment shall thereafter give her mark-room.

AS was clear ahead of BP when she reached the zone.

A boat clear astern that is required to keep clear but collides with the boat clear ahead breaks the right-of-way rule that was applicable before the collision occurred.

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A boat clear astern that is required to keep clear but collides with the boat clear ahead breaks the right-of-way rule that was applicable before the collision occurred.

As that sentence states, it applies only if a boat was clear ahead when she reached the zone.