Can the leeward boat luff me before the start?

Yes — before the starting signal there is no proper course, so Rule 17's proper-course limit does not apply. The leeward boat may luff as high as head to wind, and under Rule 11 the windward boat must still keep clear. (RRS Case 13.)

<iframe width="560" height="315" src="https://www.youtube.com/embed/Im-JehjclPY" title="Sailing Kiwi" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
<p><a href="https://sailing.kiwi.nz/start/case-13-luff-before-the-start/">Source: Sailing Kiwi</a></p>

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