From @1 to @3 Y, on stbd tack is row to B on port tack, rule 10.  Boats on opposite tacks, rule 18 mark-room does not apply.

@3 Y, the row boat is changing course, required to give B room to keep clear, rule 16.

@3 + delta, before Y passes head to wind, Y changes course to port, giving B room to keep clear.

If B fails to keep clear, she is exonerated because she is sailing within the room to which she is entitled, in accordance with rule 43.1(b).

Until head to wind  Yellow has ROW under RRS 10. While tacking Yellow has to keep clear of Blue. RRS 18.3 does not apply. 

Lets walk through it, considering the Rules as we go.

At P1 and P2 - R10 applies. Blue is KC, Yellow is ROW - there is NO Overlap since boats are on different tacks and not with wind astern (see definition “Clear Astern and Clear Ahead; Overlap”)

At P3, it looks like Yellow has had to take evasive action to avoid Blue. Blue has infringed R10. Yellow has complied with R14. Even if Yellow had been htw (but not beyond) at P3, she would still be on Starboard so still ROW. Blue has still infringed R10. If, however, Yellow had passed htw just after P3, then Blue would have become ROW – R13 and would not be limited by R15 since Blue gained ROW because of Yellow's actions. If that was the case, then Blue was entitled to hold her course round the Mark. But she might have been limited by R16.1 if her change of course failed to give Yellow Room to Keep Clear.

Note that Mark Room does NOT change ROW. And in this case there is no Mark Room for either boat.

Basically, I agree with John, however, as there is no room involved, B would be exonerated in accordance with 43.1(a) 

Bjorn, while it is true that rule 18 does not apply and there is no mark-room involved, when Yellow changes course at position 3, rule 16.1 applies and Blue is entitled to room to keep clear of Yellow.  At position 3, and from the description from  Gijs, Yellow has to take action to avoid Blue and therefore, by definition, Blue does not keep clear of Yellow and breaks rule 10.  Because Blue is sailing within the room she is entitled to from rule 16.1,  rule  43.1(b) exonerates her.

As the picture shows, and maybe the drawing in not fully clear, the Umpire (ISAF official) signalled no foul made. I was on Yellow, and when we were making out turn for mark rounding Blue would have gone clear ahead on straight courses. Since we started to change course (R16) and enetering a tack (R13) I believe the Umpire ruled correcty although in the moment it felt different.... (subjectively)...
What interest me most is to hear from the experts comment on following: Although on Startboard tack in the zone - do you always loose your rights when changing course or tacking/gybing despite initial RoW on R 18.... I guess if this one would have gone into a protest hearing the jury would put down the famous phrase - "suppose there was no mark ..... would Blue have cleared Yellow on straight courses ...." 

Gijs Vlas

Said Created: Yesterday 15:10

So the umpire concluded

Your diagram shows Y not passing head to wind.

For an umpire to conclude that a boat has changed tacks, it must be quite clear to the umpire that she has passed head to wind, otherwise, in accordance with the following definitions:

Tack, Starboard or Port

If Y had passed head to wind things would have got a whole lot more complicated, because rule 18 would have begun to apply.

Initially, while boats were close hauled on opposite tacks, rule 18 did not  apply.

Y did not 'lose her right of way', but by changing course she became subject to the limitation of rule 16 on that right of way, of being required ro give B room to keep clear.

Yes

Boats on opposite tacks sailing  to windward, take the mark away.

@ John - I think you are correct. Especially with committees stating "take the mark away and what about it then..."
This whole thing went down in a couple of seconds - I am not sure if we got to the other tack (guess not) - but even if we had passed head to wind, these boats are old 20 meter classic cargo boats with 150+m2 sail and are inert in response and turning, so I would guess Rule 13 to be in effect until close hauled (we never got there) and if we got close hauled still Rule 15 would kick in and this rule is much applied with these heavy slow-manoeuvring vessels. Had we not cleared away we would have "T-boned" the rear of Blue with 18tons at 7kts.....hence effecting Rule 14 and severe damage and injury risk for people. 

Gjis, thanks for the context on the type of boats. I'm not an expert on rule 16.1 ("When a right-of-way boat changes course, she shall give the other
boat room to keep clear.") But during port-starboard crossings, it means that starboard cannot change course once port is past the "point of no return", i.e., when port cannot tack or duck without hitting starboard. These course changes include starboard "hunting up", meaning a luff to try to make contact with port. 

Looking at your diagram, it's significant that blue's transom is half a boatlength ahead of yellow's bow when you look at ladder runs. Blue would have easily crossed Yellow had Yellow not luffed up.

The point of no return is going to vary by type of boat: an Opti, with a relatively big rudder and narrow daggerboard, will turn much faster than even most dinghies. Thus, the point of no return will be relatively late, maybe when port's leeward side is a boatlength from starboard's bow. With your cargo boats, it sounds like the point of no return is pretty early, perhaps when port's leeward side is three boatlengths from starboard's bow. If true, then for Yellow to give Blue room to keep clear per 16.1, they'd have to be luffing just prior position 2.

@Al Sargent
ST - PT - getting to that point of no return (Zoom-lens is taking away some depth) but we do cross astern with a "beer-cag" room left. Releasing the main is most critical here ;-) 
Fleets are 16 boats, the class itself has A-, B-, C- and small A-league - all handling is manual, no winches

Wow, it's so different looking at the diagram and then seeing the boats themselves!  Was this umpired fleet racing?  

I think we might want to put more emphasis on rule 16.1, here.  When Yellow luffs up, she is required to give Blue room to keep clear, as everyone here has pointed out.  But beyond that, such a luff is an offensive maneuver: Imagine being on Blue and seeing Yellow head up to T-bone you.  True, Yellow bore off and gave room, in the end, but, particularly in view of the handling properties of the boats involved, it would have been much better if Yellow had continued on a close-hauled course until Blue crossed her bow, then tacked.  Not a rules issue, a courtesy and common-sense issue.

@ Rob Overton
Yes this is umpired racing for the National Championship. 
RRS 10 to 16 are all sensitive in terms of providing the other boat room to keep clear and avoid danger. Every skipper and tactician is well aware their responsibilities to sail safe despite "close racing". In this case for Yellow had the first impression that we had RoW, so hailed and commenced to turn and next immediately beared away realising it would lead to danger. Compared to dinghy's and keel yachts these boats turn slower, but they do turn, be it with a radius.