When Blue reaches the zone the boats are on opposite tacks so 18 does not apply. When the boats do become overlapped both boats are in the zone so 18.2(a) applies.

the question may be is Yellow entitled to mark room at all. And certainly when she passes head to wind at position 6 they are again on opposite tacks, and Yellow is subject to 13...so Blue is no longer required to give mark room. From the diagram it looks like Blue never restricted Yellow's course. As Yellow when tacking keeps clear of blue, she breaks no rule. So no rule has been broken, and Blue has given mark room until she was no longer required to do so.  Green and White Flag.

In position 3 blue enters the zone before yellow pases head to wind (yellow is head to wind while blue is already half a hull length inside the zone) so the boats are not overlapped at that point according to the definition and as they are on opposite tacks no part of rule 18 applies.

In position 4 blue  was not clear ahead when she entered the zone and yellow was not overlapped when the first boat entered the zone so 18.2(b) does not apply but as the boats are now overlapped in the zone rule 18.2(a) does and yellow is entitled to mark room See WS case 2.

In positions 5 an 6 yellow luffs head to wind and blue keeps clear as required under rule 11. If yellow breaks rule 16.1 she is exonerated under rule 21 but as blue is able to keep clear yellow clearly hasn't broken rule 16.1.

In position 7 yellow shows that the only way she can pass the mark is by passing head to wind and so loses her right to mark room under 18.2(d) and 18.1 (a) and/or (b). Yellow must keep clear of blue under rule 13 until she is on a close hauled course and rule 10 thereafter. She appears to meet these requirements.

Rule 18.3 does not apply because when yellow passes head to wind she is outside the zone and in any event is not fetching the mark after she has tacked so the prerequisits of that rule are not met..

No rule broken by either boat.

We need to establish what happenes right after pos.3 -  Is Yellow will pass head to wind next second after what we see ? I beleive -Yes. So both boats will be overlapped between pos.3 and pos.4 (where Yellow looks as finished tack).

So I suggest no rule 18.3 will apply. Then we use 18.2 a ? and b.  Then Blue is given mark room till pos 6., where she started to bore away, and I am in favour of she may broken 18.2 b.

Do I beleive Yellow tack on port avoiding possible contact with Blue ? In some of sea and wind condition -Yes, strong wind , for example and fact that both boats are still overlapped at pos 4,5,6.  Blue .Closing the door at pos 6 , may force yellow to tack, avoiding Blue. 

So my answer - Blue failed 18.2 b, if we accept 18.2 a

and No rule broken if at pos 3 ,when Blue reach zone,boats not ovelapped yet.

Here's my take:

Yellow did not pass head to wind inside the zone. Accordingly, rule 18.3 did not apply.

Similarly, the boats were on opposite tacks (Yellow had not passed head to wind) when Blue reached the zone and no part of rule 18 applied [rule 18.1(a)].

However, by position  3, Yellow had established an inside overlap and was entitled to mark-room by rule 18.2(a).

Whereas under rule 18.2(b) an inside boat's entitlement to mark-room specifically continues "thereafter", such is not the case under rule 18.2(a), because 18.2(a) is not included in rule 18.2(c) - whose limitations only apply to rule 18.2(b) mark-room. This then raises the question, "can an 18.2(a) right to mark-room be turned off and on as an inside overlap is established, broken and re-estanlished?" Case 2 answers the question making clear that rule 18.2(a) only applies while the boats remain overlapped.

In the diagram, it appears that Blue may have become clear ahead at position 5, but resolving reasonable doubt in the negative [rule 18.2(e)], Blue continued to be required to both keep clear (11) and give mark-room [18.2(a)]. If on the other hand, the overlap was broken at position 5, it was certainly re-established by Blue'sown action of bearing away between 5 and 7.

If Blue's bear-away between positions 5 & 7 caused Yellow to alter course by tacking to avoid contact with Blue, Blue broke rule 11 and 18.2(a).

If on the other hand, Blue's bear-away had nothing to do with Yellow's tack between positions 6 & 7 - that Yellow tacked to avoid hitting the mark, then no rule was broken.

No overlap when B reaches the zone - 18,2(b) not applicable

Inside the zone 18.2(a) applies and B is keeping clear.

When Y tacks (7) no more 18.2(a) applies.

Y keeps clear of B.

NO RULE WAS BROKEN.

We all agree No RuleBroken...next question please.