Hi Werner-- Yes, I understand the effect of 19.2b. I'm asking about the effect of 19.2a.

Maybe 19.2a has no effect and is just context for 19.2b? But I don't know. That's why I'm asking.

Say 2 boats are sailing for the same tack, with overlap, with a collision course for both with an obstructions. The obstruction can be sailed on either side, so the boats can either bear away and pass the obstruction in leeward, or luff and pass the obstruction in windward. 

19.2a gives the right to make decision to the right of way boat, in this case the leeward boat as rule 11 applies between them. 19.2b also then says that if the leeward boats decide to bear away and sail leeward of the obstruction, the leeward boat must leave room for the windward boat room between itself and the obstruction.

If there is a "right of way boat" - there ist a "NOT right of way boat" too! 19.2(a) makes clear, that the right of way boat decides, what will happen!

Even without 19.2a, the other boat needs to keep clear of the right-of-way boat. So I would think that generally the right of way boat has their choice of side even without 19.2a. Hence my question about how 19.2a affects things.

For example: 16.2 ! 

In the example of two port tack boats approaching an obstruction, the windward boat could claim room, as entitled by 19.2(b), to leave the obstruction on her port side.  19.2(a) makes it clear that the windward boat would only be entitled to room if the leeward boat chose to pass the obstruction on her port side and it is not the windward boat's choice to force leeward to give her room.

> the windward boat could claim room, as entitled by 19.2(b), to leave the obstruction on her port side.

Murray: Thanks, that clarifies a bit -- you're saying 19.2b could be interpreted as constraining the right-of-way boat too much if we didn't have 19.2a.

Still I don't really see it how 19.2a makes a difference there -- 19.2b only constrains the outside boat. If the leeward port boat in your example heads up enough that both of them are leaving the obstruction to starboard, then the leeward right-of-way boat isn't outside.

I suppose if 19.2.a is just making it easier/simpler to arrive at that interpretation, that's a good thing.

Dave Perry would call this a "belt and suspenders" rule.  While it provides context for 19.2b, it probably isn't absolutely necessary.

Thanks Eric! That makes sense.

To further clarify my reason for asking: I think if it were necessary, that would expose something I don't understand yet about the other rules and so would help me learn. But it also makes sense if it isn't necessary.

Right, David, it is for context. There was probably a "however" at the end at one time.

Thanks Philip.