I agree - Yellow had no obligations at all in relation to Blue. No need to give space. 

But if I understand correctly, both boats were sailing close-hauled before the mark, and are supposed to continue sailing close-hauled after the mark?  In that case Yellow could have chosen not to tack at all at the mark...  In any case, if Blue gained something it was just because Yellow did a poor tack (your drawing seems to confirm this, as Yellow did not sail close-hauled after the tack)

Yep. And after passing head-to-wind, Y becomes a tacking keep clear boat under 13. Shooting up to the mark and after B goes behind, tack around (keeping an eye on the stern, so it doesn't touch B), would be an option. But even then B could make things difficult, e.g. by coming so close to Y's stern, after Y has passed head-to-wind, that it prevents Y from turning any more (no wiggle room, since the boats are on different tacks). So for sure Y is not in a great position, even if she is a bit ahead.

Correct. Once Yellow tacks on port, she is bound by 10. 

But in the end, like Thomas said, if the beat continues close hauled after the mark, then the fact that the mark is there has no real impact on anything. As long as B stays a bit to windward and Y can't cross B after the tack, the situation is the same as on an upwind leg without any marks :) 

From the comfort of an armchair, if yellow wants to be ahead after the mark

Yellow should Luff on entering the zone so as to be sailing close to the mark or even higher. Her aim is to get blue to become overlapped. If blue becomes overlapped to windward, she either passes the wrong side of the mark or denies yellow the mark room to which she is entitled. If blue becomes overlapped to leeward, yellow then has room to tack. Blue’s aim is to remain clear astern but close enough to prevent yellow tacking. 

Clarification:
This is not a W/L race, but a point to point.
The beat doesn't continue after the mark.  The next mark of the course requires tacking at the mark and sailing hard to weather on port tack.

Yellow didn't have a bad tack.  Yellow chose to sail that course shown as they were not confident in their rights.  They let Blue in on purpose.
The question is in hind sight, did they have to?  What should they have done.

To my gut feel - Yellow cannot tack if she cannot keep clear from Blue - In many protest rooms with mark situations a good jury will take the mark out of account - If there was no mark - yellow would not be allowed to tack !  Changing course always changes explanations of rules - that's how I approach most of my tactics on the water - when I change I loose rights, the other way around for opponents - I am not a rule expert by the rule no's since most of my calls I do have to make under pressure in the heat of the game.... situational awareness ahead is usefull.

Blue is not entitled to room but Yellow is.   Yellow "should" have shut the door by sailing closer to the mark, tacked at the mark and headed for the next mark close hauled attempting to remain to leeward of Blue and in control with ROW.

Not a judge and don't even play one on TV, but I think Y did the right thing by carrying on. Approaching the mark, they were on a beat to weather (it was a "weather" mark), thus RRS 18 does not apply. Had Y tacked to pass the mark on the way to the next one and fouled B they would have broken RRS 13 or 10, depending on how far they were in the tacking process. To me, B could have kept Y on starboard tack as long as they wanted to, a classic match racing tactic. That the next mark was a port tack, close hauled, fetch would be an argument for a proper course claim, but the layline to the next mark carries no rights (not an obstruction) and the prospect of a righty shift would have negated that argument pretty quickly anyway. Y was pinned on starboard tack. Been there, for sure!

Case 15 is relevantly identical.

I agree the outside leeward boat should have used her right of way and mark-room entitlement to close the gauge and get positive control on the windward boat.

In the presented scenario, the Y boat did just what she needed to do to stay out trouble.
Yes, Rule 18 applies. Y is in the Zone and none of the a,b,c,d work here. Y is clear ahead when she reaches the Zone, so B has to give Y mark-room. That works till Y tacks and under rule 18.d, she looses her mark-room protection. Throw in the Definition of Mark-Room and unless the condition of the definition existed, she better sail the way she did in the example.
Kim

Hi Carl
Just a clarification rule 18.2(b) does apply it’s only when boats on a beat to windward are on opposite tacks that 18 does not apply. 
Purely from a tactical point of view Y should pinch up to get close to the mark and lock B out. B would still be obliged to give Y mark room. A win for Y within the rules 

Good thing I'm not a judge! I read RRS 18.1 and see that it does not apply when (a). The question was if should Y tack or not. If they did, either Y would be Tacking (i.e. with no rights) or on opposite tacks, which is 18.1(a).
Lots of ifs in the answers here. Agreed that Y should have been trying to pinch B off for a long time before entering the zone, but that wasn't part of of the facts one way or the other.
I hold my piece and defer to those above. And I'll stay out of the room!

Our club does many starboard roundings and these kind of situations have been involved in several complex protests and collisions.

I agree with your analysis that Yellow loses rights to mark room whilst tacking and is then the keep clear boat once tack is completed.

A further complication to situations like this is if Yellow thinks she can cross (or has rights) and does tack. Then Blue has to hold course and not try to round the mark, else by #16 she will be changing course in a way to prevent yellow from keeping clear.   If Blue starts to round the mark and then they hit then it is anybodies guess who is at fault, because it is really difficult to say if she would have crossed had Blue held course.

A couple of wrinkles:

At position 2, Yellow is still entitled to mark room having been clear ahead when she reached the zone. But it's not possible for her to take advantage of the entitlement without tacking so per Case 62 Blue may take advantage of the space given at her own (minimal, in this scenario) risk.

Rule 18 switches off when whichever boat tacks first passes head to wind (boats on opposite tacks) but turns back on again if they become overlapped after the second boat passes HTW and they're both on port tack. At that point I believe Blue is inside boat becomes entitled to mark room (and also leeward ROW).

I agree that a better move for Yellow would be to pinch up to the layline, as long as she doesn't slow down too much and can remain clear ahead until she reaches the zone. If it's close in that situation I think it would be prudent for Blue to bear off and give Yellow mark room, keeping rule 18.2(d) in mind - if Blue can't definitively demonstrate that she established an overlap before Yellow reached the zone she likely loses in the room.

18.2 - "......  If a boat is clear ahead when she reaches the zone, the boat clear astern at that moment shall thereafter give her mark-room."
RRS . First R stands for Racing. racing it is. so If no overlap entering the zone, blue is not entitled for room at mark, and Yellow should have freedom to close the gap btw mark and tacking point that blue is not entitled to, as much as she has a right as if there was no mark. if there is no overlap entering zone, zone is not an issue, and ww and lw rule does not apply. yellow positioning herself to tack right at mark. whereby blue should give yellow room to travel to proper course even if sb. 

Hi Tim
My thought was that once yellow established rights as clear ahead boat at Zone she should immediately start pinching up to the mark. Blue would have to respond when Y gets close RRS 11 and Give mark room RRS 18. 
Blue would have little options but to slow or tack away. 

Yes, Paddy, I agree. I think Yellow could start pinching up earlier (or maybe just take a couple of bites) as long as she was sure not to swing her stern so as to establish an overlap before the zone. If she could stay clear ahead at in the zone or at least get close enough to luff Blue in the zone if necessary I think Yellow would be OK and Blue would be in a bad spot.

OK, I promise this is the last one! B was not overlapped with Y, and Y had doubts that they could tack, mark or no, without fouling B. The definition of mark-room only requires room to leave the mark on the specified side, then directs the reader to RRS 28, specifically 28.2. The definition specifically says room does not include room to tack unless ... (a scenario that does not exist here), and so B is within her rights to push Y past whatever layline to the next (close-hauled fetch) mark. I miss the old "seamanlike manner" rounding language! (18.3 won't apply because the mark is to be left to Stbd. 18.4 grants a right to change tacks at a gybe mark if needed to get to a proper course to the next mark, but only if a gybe is required.) If there's a rule somewhere that grants a right to tack in front of a ROW boat anywhere other than at an obstruction, please point it out to me. So Y had the room to leave the mark on the required side, satisfying 18.2(b), found himself in a pickle, and did the conservative thing - didn't force the issue and sailed on.
Now, as to what Y should have done.... If I knew anything about tactics I'd spend more time racing than managing races!

I think Blue could keep Yellow pinned on starboard as long as she pleased (or forced Yellow to tack and duck). But in a fleet race sailing one competitor well past the mark quickly becomes counterproductive. 

Tactically Yellow's option is to close the door by luffing after entering the zone, see diagram below. Provided Yellow does not pass head to wind, Blue has some serious problems. If Yellow passes head to wind she will break rule 13.

The first question is does Blue break a rule. We need to look at three rules 11, 14 and 18.

The second question is does Yellow break a rule? We need to look at three rules 16.1, 17 and 18.

Mark, I would add that Yellow breaks rule 14 (it was reasonably possible for her to avoid contact by bearing away when it was apparent that Blue was not going to keep clear) but is exonerated if there's no damage or injury.

And Blue is flirting with rule 31.

Tactically, Blue can get herself out of this fix with minimal cost if, at position 2, she bears away and takes Yellow's stern. Blue would round less than a boat length behind and that could put her in a position to roll Yellow to windward after they both tack.

Rule 14(a) allows Yellow to sail her course in the expectation that Blue will keep clear as required, until such time as it became clear that Blue would not do so. The diagram shows that Blue could readily have luffed and avoided Yellow from a position very close to Yellow. For that reason, the time between the moment it became clear that Blue would not keep clear and the time of the contact was a very brief interval, so brief that it was impossible for Yellow to avoid contact. Therefore, Yellow did not break rule 14.

Mark,

I think you are being over-generous to Y on rule 14.  As I see your analysis, you are bring back the hard luff with contact.

I agree that it's a close call, but Y seems to be turning into the contact at the time of impact:  I think that's inexcusable.

I thiink B would have been getting into Y's wriggle room somewhere about @2.5, and at that point, Y could have borne away, avoided contact and protested, with every chance of success.

Thank you everyone for your input.
The responses were pretty much what I was expecting, which is good.
I will certainly use some of the descriptions of the rules situation and tactical points when I respond.

I appreciate the ability to discuss situations here.