In a mark rounding you have to provide room for the overlaped boat to execute a rounding. You do not have to provide them room to do a rounding where they go wide/sail where they wish.
Not quite. The tactical considerations depend on the positions of the boats. Rule 18 no longer supercedes the rules of Section A (rules 10-13), so port tack boats still have to keep clear of starboard tackers, windward has to keep clear of leeward, etc. If the inside boat has right-of-way, that gives her the ability to take more than just "mark-room". That means she can make a "tactical rounding". If the outside boat has right-of-way, then she need not give the inside boat anything more than "mark-room". That is pretty much (technically a little different but very similar to) room for a "seamanlike rounding".
Given this why if in Robi's case, assuming that there was plenty of time to avoid the obsticle to either side, would the leward boat be obligated to allow them to decide how they were to pass it. I see where they could not come up to force closure at the last minute.
That is not quite Robi's scenerio. That one involves "room" at an obstruction, not "mark-room" at a mark. Take a look at the definitions of "room" and "mark-room" and you'll see that they are different.
Let's look at this situation without involving rule 20 (room to tack). Suppose that two overlapped boats are sailing towards an anchored boat. The sizes and sailing characteristics of the boats involved are such that the leeward boat (L) can either pinch up and shoot above the obstruction, or bear away to pass below. Rule 19.2(a) gives L the choice of which way to go. The windward boat (W) has no say in the matter.
If L chooses to pass above the obstruction, then she is the inside boat. W must must give her room under rule 19.2(b) and keep clear under rule 11. Rule 16 (changing course) still applies so L must act early enough to allow W to keep clear.
If L chooses to pass below the obstruction, then W is the inside boat. L must give her room under rule 19.2(b). W must keep clear of L under rule 11.
I hope that helps,
Eric
