Hello Catalan,

Your diagram shows boats Red and Blue, both in the zone and required to leave the mark on the same side.
This situation satisfies R18.1(a).
The two boats are not on opposite tacks.
Neither boat is leaving the mark.
The mark is not a continuing obstruction.
Therefore, none of the exceptions (1) to (4) are valid.
Mark-room has not yet been given, so Rule 18 applies.

If Red and Blue were overlapped at the time the first of them reached the zone, then R18.2(a)(1) requires the outside boat to give the inside boat mark-room.
John Christman shows a simple solution to the question of which boat is inside.  
The diagram does not show a wind direction.  It may be possible that the boats could round the mark without needing to tack.  In that case, it is easy to see that Red is the inside boat.
Whether the boats are on a reach or close-hauled at the position shown, does not change the fact that Red is the inside boat.
 
If one of the boats tacks, Rule 18 no longer applies due to R18.1(a)(1).
If both boats tack onto port and are overlapped, Rule 18.2(c) applies and the inside boat at that time is entitled to mark-room.

Robert "My review of RRS 86.1(a) indicates no issues with this change."

An argument against that assertion might be that 86.1(a) forbids changes to rules of an appendix that changes Part 2 and Def's and that doing what you suggest .. in effect ... changes those Appx-rules by inserting a "however-clause" exception and then inserts the starboard language.  

What I am saying is that it is more complicated than simply having a gravitational force component parallel to the local surface of the water.  Surfing, as a general phenomena, is defined and understood.  Where it isn't defined is in the RRS.  Unfortunately, this forum is not the place for the very long and very deep dive explaining the physics of surging, surfing, and planing.  But to me, the take away is that we need to understand the difference between surging, planing, and surfing and when each of these is possible to be able to properly interpret and apply RRS 42.

Wow Paul ... that must have been a lot of work (and moving forward as the working out the kinks phase begins). 

Congratulations!

Ang

Thanks Ben ... yea .. you asked how the reasoning would go and I obliged :-)