A boat slid in to leeward and hailed 'Leeward boat!' — do I have to move instantly?
No — although Rule 11 makes the windward boat keep clear, Rule 15 says a boat that has just gained right of way must first give you room to keep clear, and room means time to respond. React promptly — trim, build speed, head up — and you've kept clear; you never have to anticipate an overlap before it exists. (World Sailing Case 53.)
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<p><a href="https://sailing.kiwi.nz/v/DAocUIe-d_c/">Source: Sailing Kiwi</a></p>