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R11 EXPLAINED

définitions :
- Clear astern and clear ahead; overlap : one boat is clear astern of another when her hull and equipment in normal position are behind a line abeam from her aftermost point of the other boat’s hull and equipment in normal position.  The other boat is clear ahead.They overlap when neither is clear astern or when a boat between them overlaps both.  These terms do not apply to boats on opposite tacks unless rule 18 applies.
 - Leeward and windward : a boat’s leeward side is the side that is or, when she is head to wind, was away from the wind.  However, when sailing by the lee or directly downwind, her leeward side is the side on which her mainsail lies.  The other side is her windward side. When two boats on the same tack overlap, the one on the leeward side of the other is the leeward boat.  The other is the windward boat

- Keeping clear : One boat keeps clear of another if the other can sail her course with no need to take avoiding action and, when the boats are overlapped on the same tack, if the leeward boat can change course in both directions without immediately making contact with the windward boat.

explanations :
When two boats are side by side, the windward boat must not interfere with the leeward boat who can –to some extent, see rule 17– choose its own course.
In some circumstances, the leeward boat has luffing rights: she can luff up to head-to-wind, the windward boat has to stay clear (by luffing).

In both illustrations below, assuming blue has luffing right (see rule 17), blue who is the leeward boat is luffing yellow.  Yellow, the windward boat, must stay clear as per rule 11

             

tactical aspects of the rule :
- In some circumstances (see rule 17) a leeward boat can use a luff as a offensive tool against a windward boat.  Even in a downwind leg, with the spinnakers up, the leeward boat can luff up to head to wind.  However, the luff cannot be brutal, even if right-of-way, a leeward boat must give an opportunity to a windward boat to react to her actions.
- Trying to overtake a boat to her windward site is very likely to result in that boat luffing in defense.  However, we will see later that overtaking a boat to her leeward site does not grant you luffing rights.
- In pre-start, especially when approaching the line to start, a leeward position can help force a windward boat to start early or even miss the line by “pushing” her with a luff

bye rafat - translation bye amic.