The situation is a mixed HC fleet, where a 20ft boat and a 40ft boat are approaching a mark, with the 20ft boat traveling faster and breaking an overlap near the mark.
The longer boat argues the zone began at 120 ft and the boats were overlapped when entering the zone, the shorter boat argues the zone began at 60 ft and the overlap was broken before entering the zone.
Definition state that the Zone is "The area around a mark within a distance of three hull lengths of the boat nearer to it."
A strict reading of the definition suggests the zone begins 60ft from the mark and that's when Rule 18 switches on.
But if the boats were even, or if the longer boat were slightly ahead, would the Zone be from 120 ft?
A prudent race committee will want to resolve the ambiguity in advance, wither through reference to a case, or through local practice, but the question is how?
RRS 86.1 doesn't appear to permit an SI to be written to change the definition of Zone, but perhaps allows a changes to 18.1(a) or 18.2(a).
Any recommendations, please?
If the 40 ft boat reaches the 120 ft zone, overlapped, but before the 20 ft boat, then the is the first of two boats to reach the zone, RRS 18.2(a)(1) applies and whichever boat is outside is required to give the inside boat mark-room, and RRS 18.2(a) last sentence applies
When a boat is required to give mark-room by this rule, she shall continue to do so for as long as this rule applies, even if later an overlap is broken or a new overlap begins.
If the 20 ft boat then becomes advanced and nearer the mark, then, theoretically the zone contracts to 60 ft, but this does not change the entitlement of the boat, overlapped inside at the 120 ft zone, to mark-room.
If the 20 ft boat becomes advanced and nearer the mark at the 120 ft mark, then the zone is defined by her length, as 60 ft, and at that point neither boat is in the zone and RRS 18 does not apply.
If the bows of the boats were level at the 120 ft zone, the protest committee would apply the last point of certainty: Immediately before the 120 ft zone boundary, the bow of the 20 ft boat, overtaking from astern was further from the mark than the bow of the 40 ft boat, so the 40 ft boat was nearer the mark and the 120 ft zone would apply.
There us nothing for a prudent race committee to resolve.
This is a straightforward application of the Definition of zone and RRS 18.