Minor editorial correction, boat name Amy changed to Zoe in two locations, the fourth paragraph above the first illustration, and twice in the second to last paragraph.

Thank you for spotting my error Dan! 

Corrected now. 

Born "It is not obvious what right the boat with mark room has to alter course above proper corse in the mark rounding."

Perhaps the better question that you might care to address is:  "What rule, for which she is not exonerated, is it alleged that Amy breaks?"

Case 63 does not say that the outside boat is not exonerated for breaking RRS 16.1 If she is sailing other than her proper course.

One point - Europe class does not have spinnaker
The diagram is somewhat confusing. If you follow the diagram Amy enters the zone but does not sail to the mark. As she is sailing outside  the room to which she is entitled then RRS 43.1(b) does not apply.
If however Amy enters the zone and  sails to a point 1m from the mark, then she IS sailing within her mark room. When Amy luffs to round the mark she is also sailing within her mark room and is exonerated fro a breach of RRS 16 but not 14 under RRS 43.1(b). However, if the PC finds that she broke RRS14 then she can be exonerated under RRS 43.1(c) if there is no damage or injury.

Thank you Gordon for your thoughts!

The question is if Amy is being exonerated under 43.1b when she luffs Above proper course in the markroundning in an attempt to shut the door on Ben.

If Amy sails her proper course the answer is clear that she will be exonerated. See 18.2 c 2.

Interesting point regarding the mark room and taking more mark room than Amy is entitled to. Amy choose to sail her proper course to the mark. This is the way I think most sailors would sail to the mark in this situation. There is no overlap to begin with, so she has both right of way and mark room. Then when there is an overlap and she has still both right of way and overlap. In my interpretation she can sail her proper course to the mark when having both right of way and mark room during the whole mark rounding, not only room to sail to the mark.
 
No, Europe have no spinnaker, but I was hoping with the illustration to make the situation more easy to understand. It is the illustration that was used in the link to Sailing Worlds article. 

The definition of mark room part (b) does not mention proper course. Amy is entitled to room to round or pass the mark as necessary to sail the course without touching the mark. This can include sailing above close-hauled. 
However exoneration does not include RRS 14 if there is damage or injury.

I agree with the definition regarding Mark room, no proper course mentioned.

So Amy can sail sail up top head to wind to come close to the mark and "shut the door" for Ben? That is then the answer I am looking for, is that allowed?

What about 18.2 c 2?

That seems to set a limit the room Amy can argue for when steering above proper course, or why is proper course mentioned there? This is the base of my question. Is the word proper course needed, is it a limitation, or what does it add? To me it adds a restriction to how Amy can steer her boat, or at least outside proper course she is not entitled to room?

Proper Course A course a boat would choose in order to sail the course and finish as soon as possible in the absence of the other boats referred to in the rule using the term.

Maybe my understanding of proper course is wrong. I have always thought of it as the course a boat  would steer without any other boats around. 

Both you and the author of the article refer to that above close haul can be proper course. Can it be proper course to sail above close haul, ie to give the boat Ben more dirty air. Then proper course seems to take into account that there are other boats around to take into consideration. 

I am getting confused on a higher level, but thank you for pointing out all the details. 

 

A boat always has a proper course.  There are only a few occasions when she is obliged to sail her proper course, or not sail above her proper course.
When 18.2c2 applies the inside boat must give the outside, entitled to mark room, boat  not only room to round or pass the mark, but also room to sail her proper course, if this is different to room to round or pass the mark.

Re-reading your description of the incident I have some questions:
- did you luff before reaching the mark? If so did you luff on to a course heading straight at or above the mark?
- did you luff at the mark? Was sailing above close-haued part of the mark rounding?

Yes, I agree. 

Can you than apply this to the Amy/Ben situation? It seems to be an example of the 18.2c2? 

What happens if Amy is not sailing her proper course while overlapped. In this case above proper course  when Amy luffs to the mark above close haul? 

I think the way that John A has reframed the question is the best way to think of this.  The question should be: if a boat sails a particular course does she break any rules and, if she does, is she exonerated for breaking the rule?

I think it is much easier to think about rule 18 as one of obligation (to give another boat mark room per the definition of mark-room or by 18.2(c)) and exoneration (while sailing within the mark room she is entitled to she is exonerated for breaking a specific list of rules).  A boat entitled to mark-room is not obligated to sail in that mark-room at all times.  However, she is still entitled to that room from wherever she is at that moment until rule 18 turns off.  This means that she can sail from wherever she is at that moment to the mark and around it and be in the mark-room she is entitled to.  She is not required to sail in the mark-room the entire time.  The notion of seamanlike rounding or proper-course doesn't come into this beyond rule 18.2(c) expanding the definition of mark-room to sailing her proper course.

Consider Rule 14 separately.  Before you can consider rule 14 you have to know which boat was right-of-way or entitled to mark-room.  That information comes from the other rules analysis.

Looking at the specific case, in position 2, suppose Amy alters course rapidly from where she is to point at the mark, "closing the door".  Amy is a leeward right of way boat (rule 11).  As a right-of-way boat, when she alters course she has an obligation to give a keep clear boat (Ben) room to continue keeping clear (rule 16).  If Ben can't keep clear or hits the mark trying to keep clear, Ben breaks rule 11 but is exonerated for that because, by turning rapidly, Amy has broken rule 16 (rule 43.1(a)).  But Amy, per rule 18.2(b), is entitled to mark-room from Ben, i.e. the room to sail from where she is 'to the mark and around it'.  Since Amy is now pointing at the mark and has not gone around it yet, she is sailing within the mark-room to which she is entitled to and rule 16 is one of the rules she gets exoneration for if she breaks it, rule 43.1(b).  Because Ben has prevented Amy from sailing in the mark-room to which she is entitled, Ben has failed to give Amy mark-room and breaks 18.2(b) and is not exonerated.

So:

Hey - we covered all the exoneration parts! :-D

If there is contact between the boats, you then look at each boat.  Was it reasonable for the boat to have avoided contact?  Certainly, as a keep-clear boat and one obligated to give mark-room, Ben could have avoided contact by not putting himself there.  Amy, as the right-of-way boat and one entitled to mark-room, could have avoided contact by not turning so rapidly.  In my opinion, both boats should be found to have broken rule 14 and Amy would not be exonerated for her breach if there is damage or injury

Brilliant and very elaborate answer John! 

I follow you line of reasoning, step by step. Thank you for the time you put into writing it! 

I very content with your conclusion since I was Amy in the situation ;) 

I am sorry if I get hung up on the wording in 18.2c2. Does it matter if Amy sails her proper course when turning rapidly, or turning quickly close to irons pointing at the mark (not sailing proper course in my mind). 

I am so thankful for all of you contributing with your thoughts!  

Thanks Bjorn.

When you look at 18.2(c), what it is doing is adding an additional obligation to the boat that is already obligated to give mark-room (Ben).  When Ben becomes overlapped inside the boat entitled to mark-room (Amy) not only does Ben have to give Amy room to sail to the mark and around it (mark-room), Ben also has to give Amy room to sail her proper course.  It basically makes the area where Amy gets exoneration bigger (and never smaller).

Unlike the obligation on the inside, overlapped, right-of-way boat to sail no farther from the mark than her proper course that is created by rule 18.4, 18.2(c)(2) does not create an obligation on the boat entitled to mark-room to sail a proper course.  It gets them any extra room they need to sail a proper course if they want to. 

You can also think of this difference as that between a tactical (proper course) rounding and a seamanlike rounding.  Mark-room is typically room to make a seamanlike rounding.  This allows a boat any additional room needed to make a tactical rounding.

In most leeward mark situations it is hard to see how 18.2(c) comes into play as the inside boat is typically also a windward boat and thus a keep-clear boat under rule 11.  It is easier to illustrate the idea at a windward mark.

Yellow enters the zone clear ahead of Blue.  In position 4 Blue has established an inside overlap.  Blue is required to give Yellow mark-room under 18.2(b) based on position 1 and 18.2(a) will not apply because 18.2(b) does.  Because Blue is required to give mark-room under 18.2(b), we have to look at the two parts of 18.2(c) to see if they also apply.  18.2(c)(1) says that even though Blue is now an inside overlapped boat, she is still obligated to give Yellow mark-room.  Blue does not get mark-room just because she is now an inside boat, i.e. 18.2(a) does not apply.  18.2(c)(2) says that in addition to getting mark-room, Yellow gets the additional room to sail her proper course around the mark.

Suppose that Yellow thinks her best course is to turn 180 degrees at the mark and do a gybe set.  If all she was entitled to is mark-room (room to sail to the mark and around it) then Blue can reasonably argue that she gave mark-room because Yellow has been able to sail to and around the mark and that Yellow has broken rule 11 in position 4.  However, because rule 18.2(c)(2) applies, not only is Blue obligated to give Yellow mark-room, but Blue is also obligated to give Yellow the room to sail her proper course around the mark while they are still overlapped.

At Position 2, in the OPs post, two rules apply between Ben and Amy.

(1) Rule 11.  Ben is required to keep clear of Amy as windward boat. Amy DID NOT establish her overlap from astern so Amy can luff above close hauled. Ben is not exonerated under 43.1 because (a) Amy has not broken a rule and (b) Ben is not entitled to mark room.    As long as Ben can escape by luffing the wrong side of the mark such that he is not exonerated by Amy's breach of 16.
(2) Rule 18.2 (b).  Amy can sail directly at the mark because Mark room entitles her to sail "to the mark" even if that is above close hauled.


It gets tricky after position 2.  Once Ben has no options ,he is entitled to room under rule 16.  Amy can luff above close hauled provided she leaves Ben room not to break rule 31. She can luff to proper course (which is almost never above close hauled). She can luff to the the "course as necessary"  (which is also almost never above close hauled) . But if Amy has rounded the mark and the "course as necessary"   is close hauled,  then she takes great risk IMHO, luffing Ben above CHC into the right hand side of the mark.

Wow John, your commitment and explanations are so good! 

I now understand that 182.c2 gives more "rights" than mark room gives. I makes perfect sense the way you describe it. 

Justin, thanks for your thoughts as well. It is the risk Amy takes luffing almost into irons sailing towards the mark (if I exaggerate the situation) that is the core question. 

Thank you all for putting so much time inte answering! 

John C re: "A boat entitled to mark-room is not obligated to sail in that mark-room at all times.  However, she is still entitled to that room from wherever she is at that moment until rule 18 turns off.  This means that she can sail from wherever she is at that moment to the mark and around it and be in the mark-room she is entitled to.  She is not required to sail in the mark-room the entire time."

A while back I had an offline discussion with a rules-expert about this very description above. It is a bit of a catch-22 problem because if a boat's MR follows her around while she sails in the zone, how can a boat ever sail outside of the MR she is entitled to while in the zone?   What does this do to Case 75's description of the "corridor"?  

How do we parse and describe the distinction clearly?

Ang - that's the problem with analogies, they aren't perfect.  The concept of a corridor is useful when talking about where the boat can sail starting at that moment and be entitled to the exoneration that comes from mark room.  But that is predicated that that is where the boat is headed at that moment, i.e. a boat has to be sailing 'to the mark and around it' to be in the corridor.  You could think of it as the corridor not existing until you are in it.  Once you are sailing 'to the mark and around it', the corridor magically appears.

Consider these two scenarios (sailed in boats that can easily sail DDW)

In this case Blue is sailing 'to the mark' and would be considered to be in the mark-room 'corridor'.  She would be exonerated for breaking rule 10.

Now consider this case in the same boats.

While Blue could easily be sailing to the mark, she isn't.  So, at that moment, she is not sailing in the mark-room 'corridor' and does not get exonerated for breaking rule 10 if Yellow has to take avoiding action.

Putting it all together

In position 1, because Blue is not sailing 'to the mark' she is not in the mark-room 'corridor' and would not be exonerated for breaking rule 10 at that moment.  If she continues on this course and interferes with Yellow would break rule 10.

Between positions 1 & 2, Blue bears away and Yellow maintains her course.

In position 2, Blue has borne away to a course that will take her as close as she can go to the mark and because Blue is now sailing a course 'to the mark', the 'corridor' has magically appeared and she is in it and her exoneration 'turns on'.  Yellow has not had to take avoiding action yet so Blue has not broken rule 10 prior to this point in time.

In position 3, Yellow has had to take avoiding action to keep from hitting Blue.  Blue has broken rule 10, but because she has been sailing in the mark-room 'corridor' since position 2, she is exonerated for breaking rule 10.  And Yellow has met her obligation to give Blue mark-room.

I think this is all consistent with Case 75.  The case only talks about the corridor that exists at position S1 and not how that corridor changes as the boats progress to position S2.  Also, this case is as much about sailing outside the corridor.  I.e. a right-of-way boat is not required to stay in the corridor and can sail a proper course provided she complies with the other right-of-way rules.  The case actually gives a false sense that a keep-clear boat is somehow required to stay in the corridor.  This isn't true, it's just that when outside the corridor exoneration is no longer available.

Reference to proper course in Case 63

Before 2009, the last paragraph of Case 63 consisted of only text similar to the first sentence of the current version of the Case and there was no mention of proper course.

In 2009, RRS 18 was changed to refer to mark-room, and the Definition of mark-room, referring to room for a boat to sail her proper course at the mark, was introduced.

The 2009 version of Case 63 then included an additional sentence at the end of the last paragraph

The risk she takes is that the boat entitled to mark-room may be able to close the gap between her and the mark while sailing her proper course.

The reference to 'sailing her proper course', together with the then current definition of mark-room meant 'sailing within the mark-room to which she was entitled'.

As we know, in the 2013 rewrite of the RRS the Definition of mark-room was promptly amended to remove reliance on proper course.

Case 63 could have been amended in 2013 to remover reference to proper course or substitute it with 'sailing within the mark-room to which she is entitled', but this did not and has not happened.

It is certainly true that if a boat is sailing her proper course around a mark is sailing within the room or mark-room to which she is entitled, but proper course is not a necessary condition for that.

John C and Ang, Direct Corridor

John C, I don't think you are correctly applying Case 75 with reference to a direct corridor.

Case 75 says

The mark-room that P was required to give S was the space S needed in the existing conditions to sail promptly to the mark in a seamanlike way. That space was a direct corridor from S1 [the position where S reached the zone] to a position close to and alongside the mark on the required side.

I don't think there is any room for doubt that the direct corridor starts from the edge of the zone at the point where the boat reaches the zone.

The mark-room to which a boat is entitled is thus a frying pan shape, with the handle starting at the point where the boat reaches the zone, and the pan centered on the mark, something like a little less than a boat length in size.

Applying this to Bjorn's scenario

@2, A is not sailing within the mark-room to which she is entitled, but RRS 18.2(c)(2) entitles her to room to sail her proper course, which is clearly to harden up and sail towards the mark, and in doing so she is sailing within that room and, if she breaks RRS 16.1 in doing so, she is exonerated by RRS 43.1(b).  At this stage, A is well below her close hauled course and I don't think there is much room to suggest that she is not sailing her proper course.

@2.5, A will have reached a position close to the mark, and will now be within the mark-room to which she is entitled.  She now has no need to rely on RRS 18.2(c)(2) and room, and proper course:  it's straight up RRS 18.2(b) and mark-room. From here on, if she breaks RRS 16.1, she is exonerated, whether she is sailing her proper course or not.

John A - I disagree that the 'corridor' does not move with the boat as I describe.  That is how that concept has always been applied in my experience.  I know (and I'll have to look them up) there are team and match race calls that cover this.  It doesn't matter where the boat is, from that position she gets exoneration the under 43.1(b) if, starting at that moment, she is sailing 'to the mark and around it'.  This includes any turn she needs to make to sail to the mark.  In position 2, Amy can turn as hard as she wants to towards the mark and get exoneration for breaking 16 as the turn is a part of sailing 'to the mark'.

Because Amy has rule 14 obligations, she is prevented from sailing in the mark-room she is entitled to at every moment while she is in the zone because of the presence of Ben.  You cannot say she is 'not sailing in the mark-room she is entitled to' and therefore not be entitled to exoneration because she is prevented from doing precisely that.  The entitlement to mark-room does not go away.

In position 2, Ben has obligations from multiple rules; 11, 18.2(b), and 18.2(c)(2) and is required to meet all of them.  Multiple rules can apply simultaneously.  It isn't a question of which one Amy "needs", it's a question of what rules Ben breaks.  In this case, he breaks all three and that is what a PC should find.

John C, I don't think the calls support your argument.

 Team Race Call J2 is directly in point.

Rule 11 On the Same Tack, Overlapped
Rule 18.2 Giving Mark-Room
Rule 21 Exoneration
Definition Mark-Room 

Question
B and Y are approaching a leeward mark to be left to starboard. When B reaches the zone she is clear ahead of Y. B bears away and at position 2 Y gains a leeward overlap. B maintains her course and a short time later Y needs to alter course to avoid her. Y protests. What should the call be?

Answer
Penalize B.
When B r eaches the zone, rule 18 starts to apply and Y is required to give B mark-room, which she does. From position 2, B is windward boat and must keep clear of Y under rule 11. B fails to do so and breaks rule 11. The answer is the same if the boats are overlapped at position 1.
Mark-room includes room for B to sail to the mark, but because she is sailing below her course to the mark she is not sailing within the mark-room to which she is entitled and cannot be exonerated under rule 21 for breaking rule 11. 

John A - I disagree that it is completely on point.  The question isn't whether, in position 3, B is sailing within the mark-room to which she is entitled, she is not as she is not sailing 'to the mark and around it'.  The question is whether if, in position 3, B turns to go to the mark is she then sailing within the mark-room she is entitled to?  According to your interpretation of a fixed 'corridor', she is not sailing in the mark-room to which she is entitled until she is rounding the mark.  That is where we disagree.

Consider the following situation

According to your interpretation of a fixed 'corridor', in position 10, Yellow would not be exonerated for breaking rule 10 even though she is sailing 'to the mark' and not yet sailing 'around it'.

I believe that once Yellow turns to go towards the mark after position 9 she is now sailing within the mark-room to which she is entitled as she is now sailing 'to the mark'.  In fact, if anywhere between positions 2 and 9 Yellow turns to go to the mark, she would be sailing within the mark-room to which she is entitled.  It's just that prior to turning she is not sailing in the mark-room she is entitled to.

As far as the calls go, take a look at Team Race call J3. In positions 1 & 2, B is sailing above the mark and not sailing with the mark-room she is entitled to and would not be exonerated for breaking rule 11.  However, once she turns to go around the mark she is now sailing within the mark-room to which she is entitled and is exonerated for breaking rule 11.

The match racing calls are a little harder to relate as rule 18 is different.  However, match race call E6 covers the situation where a boat is first sailing within the mark-room she is entitled to, then not, then returns to sailing within mark-room.

E6
Blue and Yellow on starboard tack approach the windward mark, to be left to starboard, with Yellow overlapped on the inside of Blue at the zone. After passing the port-tack layline both boats luff head to wind. At position 3 Blue has given Yellow enough room to tack. Both boats continue forward with the gauge between the boats reducing until, before leaving the zone, Yellow  passes head to wind. There is contact as Yellow’s stern swings. Both boats display flag Y. What should the call be?

Answer
Penalize Blue. When Yellow enters the zone overlapped on the inside of Blue, rule C2.9 (18.2(a)(1)) applies and requires Blue to thereafter give Yellow mark-room, which is room for Yellow to sail her proper course to round or pass the mark. When Yellow continues straight after position 3, she is not sailing within the mark-room to which she is entitled. Rule 11 requires her to keep clear of Blue and she does so.  At position 5, while still in the zone, Yellow tacks which is her proper course, and Blue fails to give her room to do so. Blue breaks rule 18.2(a)(1). Because Yellow is sailing within the mark-room to which she is entitled, she is exonerated by rule 43.1(b) for breaking C2.6 (13.1).

I agree with John C. Consider the following scenario:

Yellow and Blue are approaching a reach mark:

Position 1 - Yellow, inside overlapped, gets mark-room, RRS 18.2(b). Yellow is also keep clear, RRS 11. Blue is ROW, RRS 11.  The red lines depict the corridor or the mark-room to which Yellow is entitled.
Position 2 - Blue, ROW, RRS 11, bears away in preparation of a tactical rounding. Yellow, keep clear, RRS 11, follows Blue down resulting in her leaving the protection of the corridor. Yellow must be very careful to keep clear. Note: when any part of a boat moves outside of the corridor or mark-room, she loses the protection of mark-room and the exoneration under 43.1(b). Her boom in normal position is outside of the corridor.
Position 3 - Blue, ROW, RRS 11, begins her turn upwind. Yellow, keep clear, RRS 11, must be careful to keep clear of Blue until she makes her turn to round which then establishes a new corridor depicted by the green lines as she is again sailing within the mark-room to which she is entitled.

The width of the corridor depends on the width of the boat including her boom in normal position which may move, wind speed, and sea state. Notice that the green corridor is not quite as wide as the red corridor.

A word about “closing the door”. In my experience, this term is used by sailors to describe luffing a windward boat before the start so that she cannot make the starting mark thereby closing the door. The windward boat must go head to wind and stop or tack, RRS 11. All perfectly fine so long as the luffing boat gives the windward boat room to keep clear.

The OP scenario is very common with dinghies. The scenario is a race to the zone. Amy wants to enter the zone clear ahead of Ben so that she gets mark-room. If she changes course up before entering the zone, she risks becoming overlapped with Ben, and if that happens, Ben gets mark-room. This is why Amy holds her course until she enters the zone.  If Amy wins the race to the zone clear ahead, and Ben sticks his nose in, Amy may simply change course to round the mark.  Amy may break 16.1, but will be exonerated by 43.1(b). If Amy must change course to avoid contact with Blue, she should protest Blue and change course to avoid contact because as Gordon pointed out, if there is contact and there is damage or injury, Amy will not be exonerated under 43.1(c).
 
 

Jerry and John's ... the way I finally came to think about it after my offline discussion (and subsequently try to describe it) is that, yes, the corridor follows the boat until the boat entitled to MR is close enough to another boat that owes her MR .. such that she may break a rule to which she can be exonerated under 43.1(b). This jives with JC's circle-route scenario. 

This also jives with what John A has focused us on in past threads, which is room and MR is about access for a boat to 43.1(b) exoneration as she maneuvers. 

To JC's circle scenario ... at #1-#9 Yellow and Blue are not close enough or converging in a way for Yellow to break any boat-boat rule covered by 43.1(b).  So, at #10, Y gets her corridor from where she was. 

I also note Case 75 is written in such a way that the corridor description is specifically describing the Case scenario. It's written as "... that space was a corridor .." given the facts of Case 75 which puts the 2 boats in close enough proximity that rule 43.1(b) could be applicable. 

Guys, how do you get around the plain words in Case 75

The mark-room that P was required to give S was the space S needed in the existing conditions to sail promptly to the mark in a seamanlike way. That space was a direct corridor from S1 [the position where S reached the zone] to a position close to and alongside the mark on the required side.

John A ... it's not "getting around" .. it's putting the words in Case 75 into context.

Case 75's decision is discussing a specific set of facts.  Looking at the distance between the boats at #1 (the moment the first of them reach the zone) they are separated by just over 2 BW's and 1 width of S when including her boom overhang.   Therefore the application of the corridor-concept is ripe the moment they reach the zone.

Given the boats proximity to each other .. in Case 75's scenario with its set of facts ... " ...That space was a direct corridor from S1 to a position close to and alongside the mark on the required side.". 

First, while the concept of a corridor can be useful in describing when a boat may get exoneration under 43.1(b), there is a danger in extending the concept too much.

Case 75 only describes the location of the corridor at one point in time, when S is in position 1.  It says nothing about the location of the corridor after that.  The key in Case 75 is actually in the sentence John A didn't highlight: The mark-room that P was required to give S was the space S needed in the existing conditions to sail promptly to the mark in a seamanlike way.  Case 75 then uses the analogy of a corridor to explain this idea.  However, the real point of Case 75 is the application of 18.4.  S was an inside, overlapped, right-of-way boat and had an 18.4 limitation to not sail farther from the mark than her proper course.   Without this limitation, S could have used her starboard right-of-way to force P to gybe and then luff P off the page as long as she didn't break 16.1, which she would not have been exonerated for because she wasn't sailing to the mark.   

In Jerry's diagram, I would argue that when keep clear Yellow first turns down between positions 1 and 3 forcing Blue to avoid her she is breaking rule 11 and because she is not sailing 'to the mark' she is not entitled to exoneration.  Once Yellow turns up at position 3 she is now sailing 'to the mark' and entitled to exoneration but has already broken rule 11 and should be penalized.  This is where the analogy of a corridor falls apart.  It has nothing to do with where her boom or any other part of the boat is.

John C re: "In Jerry's diagram, I would argue that when keep clear Yellow first turns down between positions 1 and 3 forcing Blue to avoid her she is breaking rule 11 and because she is not sailing 'to the mark' she is not entitled to exoneration. "

Jerry setup a diff fact set in his description where he describes Blue altering course to leeward and Yellow sailing into that space ... not Yellow forcing Blue to leeward. 

Ang - thanks for pointing that out.
Jerry, I apologize, I missed that.

John C re" It has nothing to do with where her boom or any other part of the boat is.

Consider a team racing scenario. Two boats approach mark 3 on starboard and enter the zone overlapped. They both gybe at the same moment. The inside boat's boom makes contact with the outside boat's port shroud during the maneuver.  The inside boat's boom was in it's normal position for a reach to mark 4. The boat's hulls did not make contact. Did the outside boat give the inside boat mark-room?

The corridor is simply a way to visualize mark-room.

No sweat John C.

Jerry -

I agree that the corridor is about visualizing how the rule applies.

In your example, I would say that the outside boat did not give the inside boat the mark-room needed to sail to the mark and around it in a seamanlike way and should be penalized under 18.2(b).  The inside boat would have broken rule 11 but be exonerated under 43.1(b).  You don't have to worry about 18.4, but the inside boat could be limited by 17 and so you would have to consider that when looking at the inside boat's course.  The inside boat would also have the obligation to not let her main & boom fly to the new side in a totally uncontrolled way but most likely it would be ok, especially since it only went to a reaching position.

Another twist here is that Amy’s right to exoneration under rule 18 may be determined by her position relative to the mark, or by the direction to the next mark. 

To shut the door Amy will want to steer a course to the mark or above.  Under the definition of mark room the room she is entitled to is room to sail to the mark, but only if her proper course is to sail to the mark. If the next mark is directly upwind then from where Amy is in the diagram her proper course will be close to the mark. But if the next mark is directly to the right of the current mark, then her proper course will be to the next mark. If she sails to the current mark she will not be sailing within her mark room and won’t be exonerated for say a breach of rule 16.1. 

 Regarding the corridor.

There is John Allan's perspective, which refers to a corridor from the moment you enter the zone, and there are others who refer to mark-room from the point you start sailing towards the mark.

Rule 18 comes into effect as soon as one of the boats reaches the zone, and from that moment, mark-room must be given.

When I review the definitions of mark-room and room, I see that the definition of room includes "promptly."

When you are entitled to mark-room from the moment you reach the zone, i.e., room to sail to the mark, ..., and that room implies "promptly," it seems to me that the mark-room is established at the moment the rule comes into effect and is limited to the zone to sail to the mark from the point of entry, round it, ...

John Allan already mentioned Case 75, which according to others is limited to that specific situation, but there is also Case 118 that seems to support John Allan's interpretation. It states: "The phrase 'room to sail to the mark' means space to sail promptly in a seamanlike way to a position close to, and on the required side of, the mark."

It seems to me that one cannot sail freely in the zone and then claim mark-room from 90° further in the circle to sail to the mark and from there claim mark-room. 

A boat entitled to mark-room can sail wherever they want to.  If they are not a right-of-way boat and they break a right-of-way rule then they are exonerated if, and only if, they are sailing in the mark-room they are entitled to.  There is no requirement that they were sailing in that mark-room before that.  Just that at the moment that they need exoneration they must be sailing in the mark-room they are entitled to.

A boat required to give mark-room must give a boat entitled to mark-room the room to sail from wherever they are at that moment in time 'to the mark and around it'.  It doesn't matter whether the boat required to give mark-room is the right-of-way boat or not.  Nor does it matter how the boat entitled to mark-room got to that position.  If the boat required to give mark-room is also a keep clear boat under rules 10, 11, & 12, then in addition to giving mark-room, they must also keep clear under the normal right-of-way rules. 

This is how we get to the whole notion of a tactical versus a seamanlike rounding.  A boat can sail a tactical rounding while they have the right of way because the other boat has to both give mark-room and keep clear as a give way boat.  A boat can only sail a seamanlike rounding when they are a keep clear boat because they need the exoneration that goes with sailing within the mark-room the boat is entitled to.  

Other than required by 18.4, there is nothing in rule 18 that requires a boat to sail any particular course.  They can sail slowly, fast, or backwards if they want.  The 'promptly' that Johan refers to only describes how a boat must maneuver to keep clear and does not apply or imply anything else.

In Case 118 the inside boat is a keep clear boat, this is the key fact.  This is why they need to be careful.  They are not required to sail 'promptly' to the mark, they can sail as fast or slow as they like.  However, they will only be exonerated for breaking RRS 11 while they are sailing in the mark-room they are entitled to, that is they get to sail to the mark and around it, which they do.  The outside boat is required to give this much room, but not more.  If the inside boat were to sail lower, no longer sailing to the mark, and the outside boat had to alter course to avoid then the inside boat would be found to have broken RRS 11 and not be exonerated.

@John Christman
You made me doubt (again ;-) )

I was already on the same page that a boat could sail outside her mark-room (from the point of entry of the zone), but I had trouble with the view that you could get a 'new' corridor from another position if you first turn away from the mark (staying in the zone) and then turn back towards the mark.

After your post and re-reading rule 18.2.b, I paid more attention to the word "thereafter." Since the word "thereafter" is in the rule, I can agree that you can get mark-room (the 'new' corridor) from the position at which you decide to sail to the mark.

I stand corrected.