I was clear ahead on the run and a boat hit me from behind — doesn't clear astern keep clear?
Only on the same tack. Rule 12 — clear astern keeps clear of clear ahead — applies only when the boats are on the same tack. Two boats running with booms on opposite sides are on opposite tacks, so Rule 10 applies: the port-tack boat keeps clear, even from in front. A boat is on the tack matching her windward side, so boom out to starboard means port tack. Downwind, ask which side the boom is on before you ask who is ahead.
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<p><a href="https://sailing.kiwi.nz/v/l34Bd_k_df0/">Source: Sailing Kiwi</a></p>