I recently had a gybe mark "incident" which has left me somewhat confused, and having re-read the relevant bits from Bryan Willis Rules In Action, I'm still none the wiser  Also, although boat C in the situation described below did scream PROTEST at me, he subsequently decided not to proceed.

Picture if you will a gybe mark in force 5, gusting 6, conditions.  Boat A has stuffed in it on the gybe and is swimming some 3 or 4 boat lengths outside the mark, leaving a decent size gap.  Boat B elects to sail outside the capsized boat, but whilst coming up a few lengths astern, I decide to go for it and bear off into the gap.  Boat C is close behind me, but does not have an overlap (or try to dispute that he had one afterwards).

Just as I start to bear off, there is another absolute munter of a gust, so I stay on starboard for roughly 5 seconds whilst the boat accelerates, then carve a nice clean gybe.  In this time and at this pace, the mark is now probably 4 or 5 lengths astern.  Boat B also delayed her gybe, but went a fraction earlier and is now heading back towards the leeward mark, so if I had left my gybe any later we would have been on a collision course at a high rate of knots.... 

The problem is boat C who in the meantime has followed me into the gap and come charging up astern in the same gust.  Her helm screams "Starboard" immediately after we gybe, then "Protest" as he broaches into a capsize, later stating he had to luff hard to avoid me. 

As you can appreciate, this was all over in seconds and there really was very little time to think about it.  From reading the book, my understanding is that I was obliged to gybe at the mark at the earliest opportunity in order to sail my proper course (and avoid B!), and that because Boat C was clear astern going into the mark, he should have kept clear.  However, boat C's argument is that because I delayed my gybe in the gust and hence had left the two boat circle, this was a port / starboard situation and as the boat changing course, I should have kept clear.

Thoughts please?

Boat C has to sail its proper course, Rule 18.4, if they are still rounding the mark and should gybe if that is what it would do if you weren't there.

The first part of 18,  "When This Rule Applies. Rule 18 applies when boats are about to round or pass a mark they are required to leave on the same side, or an obstruction on the same side, until they have passed it." could be used to argue that you had not yet passed the mark so Boat C should still be keeping clear.

Rule 18 (if applicable) overrides rule 10 (port/starboard). "Rule 18 applies when boats are about to round or pass a mark they are required to leave on the same side". There is nothing in the rules that says it automatically switches off if you exit the 2BL circle. Nor are you obliged to gybe as early as possible unless another boats overlaps you on the outside, which wasn't the case. The question revolves around whether you were "about to round or pass a mark" or whether by delaying your gybe, rule 18 had switched off. If I were on the jury, in the conditions you had described, I'd say that rule 18 was still in operation and boat C was required to keep clear.

However if you exited the circle, you and C both gybed and C had an inside overlap as you both reentered the 2BL circle, C would have rights. I appreciate that isn't what happened; I'm indicating how rule 18 functions if you exit and reenter the circle.

Stefan, that hits the nail on the head.  Given the conditions, I don't think my delayed gybe was unreasonable (seaman like?), so you are probably right about rule 18 still being in effect.  And as boat C decided not to proceed with his protest, there might have been some doubt in his mind also.

My reading of the incident is that none of the boats involved had passed the mark. A diagram from the OP would help.