US36 offers a similar (counter) example and explanation of the rules … except in this instance the leeward boat did not provide room prior to 19.2(b) turning off.

When M became overlapped to leeward of W, the applicable rules changed. Even though L’s bow was well behind W’s stern, the overlap between L and W began at that time because M was between and overlapped with both of them (see the definition Clear Astern and Clear Ahead; Overlap). Rule 12 no longer applied, and rule 11required W to keep clear of M and L. W was no longer a right-of-way boat or an obstruction to L and W; therefore, L was no longer required by rule 19.2(b) to give M room between herself and W. Rule 15 required L and M to initially give W room to keep clear, and since both M and W were required to keep clear of L, L became an obstruction to M and W.


Just before M and L became overlapped with W, L failed to give M room between her and W as required by rule 19.2(b), as shown by the rules breaches and contact that occurred shortly after the overlap was established. As a result of L’s breach of rule 19.2(b), there was not space for M to give W room to keep clear after she became overlapped with W, as she was required to do by rule 15. 

Obstruction:  An object that a boat could not pass without changing course substantially, if she were sailing directly towards it and one of her hull lengths from it.  An object that can be safely passed on only one side and an object, area or line so designated by the sailing instructions are also obstructions. However, a boat racing is not an obstruction to other boats unless they are required to keep clear of her or, if rule 22 applies, avoid her.  A vessel underway, including a boat racing, is never a continuing obstruction.

Obstruction:  An object that a boat could not pass without changing course substantially, if she were sailing directly towards it and one of her hull lengths from it.