[MARGEMDOAZUL]

Peço desculpa por estar em inglês esta explicação da regra 11, da federação Americana de vela
RULE 11 - ON THE SAME TACK, OVERLAPPED
When boats on the same tack are overlapped, rule 11 applies. When boats are on much different angles of sail, it is often difficult to know which is the leeward boat. The boat that will hit the other's leeward side or be hit on her own windward side is the leeward boat. As a good rule of thumb, the boat that is on the point of sail closer to the wind is typically the leeward boat; i.e., between a boat sailing downwind and a boat sailing close-hauled, the close-hauled boat is usually the leeward boat.


In each situation, both boats are on the same tack. The white boat is the leeward boat and the black boat must keep clear under rule 11.
"I realize that when I'm the windward boat I have to keep clear of the leeward boat, but how far away do I need to stay?"
Far enough away so that while the leeward boat (L) is sailing on a straight line, you do not hit L or force L to take any avoiding action to miss you; e.g., have to change her course, ease her spinnaker pole forward or require any of her crew to duck or move to avoid being hit.
Furthermore, you need to be far enough away so that the leeward boat can change course in both directions without immediately making contact with you. If you allow yourself to get so close to L that she is physically unable to change her course without immediately hitting you, you are not keeping clear under the definition Keep Clear and are breaking rule 11. Note that the word "if" in the second part of the definition suggests that L does not need to actually hit you to prove she couldn't change course without hitting you. If the protest committee decides that L couldn't change course without immediate contact, then you have broken rule 11 simply by your extreme close proximity to L.
More importantly, anytime L cannot sail her course without a need to take action to avoid you, you have not kept clear and have broken rule 11. Therefore, you will be smart not to allow yourself to get so close that you will possibly interfere with L in the least. Notice also that when L is head to wind, it is quite possible that you will be required to go beyond head to wind (i.e., change tacks) in order to keep clear. If this is the case, you must do so. Also, if you are converging with L and both of you are sailing on what you each think its you prper course.