21.3
Backing A Sail
A boat moving astern, or sideways to windward, through the water by backing a sail shall keep clear of one that is not.
Part 2 — When Boats Meet
When rule 21 or 22 applies between two boats, Section A rules do not.
A boat moving astern, or sideways to windward, through the water by backing a sail shall keep clear of one that is not.