When is it considered a "tack" and not just a luff to windward? When you cross the direction of the true wind? When you change the clew of the sail(s)?
Take a look at the definition of
Leeward and Windward, which states "A boat's leeward side is the side that is, or, when she is head to wind, was away from the wind.... The other side is her windward side", and the definition of
Tack, Starboard or Port which states "A boat is on the tack, starboard or port, corresponding to her windward side".
When a boat turns head to wind, she remains on the same tack. The moment she passes head to wind, she is on the other tack.
RRS 13 states "After a boat passes head to wind, she shall keep clear of other boats until she is on a close-hauled course. During that time rules 10, 11 and 12 do not apply". The boat is on the new tack the instant she passes head to wind, but she must keep clear until she turns down to a close-hauled course.
I hope that helps,
Eric
