A boat came up from behind and slotted in to leeward — do I really have to give way?
Yes. The moment she establishes an overlap to leeward, Rule 11 applies and the windward boat keeps clear — it makes no difference that she arrived from clear astern. She owes you only what Rule 15 requires: room to keep clear when she acquires right of way. If you had time to respond and didn't, the penalty is yours.
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<p><a href="https://sailing.kiwi.nz/v/N7AUNwiQacw/">Source: Sailing Kiwi</a></p>