David … rule 18 does apply at finish marks. If you read the preamble of Part 2 Section C you will see that it is the starting marks that are specifically excluded from Section C (which includes 18) under specified circumstances.

So, if it’s a windward finish, 18.1’s application restrictions apply.  If it’s at the port finish-mark, 18.3 might apply too.

Downwind, the rules at the finish marks operate much like they do at leeward gate-marks.

The zone is defined from the mark … and an RC boat at either end of the line can be a mark of the line as stated in def: mark and as stated in the SI’s.

Do you have a specific scenario that is confusing you?

PS: Also see Case 132

I was reading several Forum posts about downwind finishes and they kept referring to "the zone" (perhaps incorrectly).  If either end is part of the line defines the finish line, then three boat lengths from "the zone" becomes a complex function of where each boat is relative to the closest part of the line.  If it does matter, then at the boat end, is it three boat lengths from the stern of the boat or the flag position?  So I guess the question I am asking is whether there a scenario where entering the "zone" on a downwind finish has any relevance to ROW?  For instance, if an overlap is established outside vs. inside "the zone" near the finish does it matter?  

Hi David,
you are over thinking this - the finish line extends between the finish marks, and the zone exists around each mark. The zone comes into play when the first boat touches the zone - just like at any mark.

John

David my understanding is that RRS 18 applies specifically at finishing marks, the line itself has no zone.  
The definition of zone is three hull lengths from the mark. We tend to think of the zone as a three boat length circle, but that's not what the definition says. If the finishing mark is a large craft then the zone is all points within three boat lengths of the craft.  Also consider that a finish line may have only one or even no finishing marks.

I think the mistake David is making is thinking the zone is defined with respect to the entire finish line.  That is not the case.  Zones only apply to marks.  There is a zone around the pin that only affects boats at the pin, there is a zone at the boat, and only affects boats at the boat.  If the line is more than 6 BL long, then the middle of the line is not in any zone. 
Edit: Jim beat me to it

David, there's a problem with your original question that I'm not sure is just a typo or reflects a misunderstanding of rule 18: You refer to "room to cross the line (ie. finish)."  Rule 18 is all about passing marks, not crossing finish lines (though of course a boat might be finishing while passing the mark).  So the "zone" is an area that is within 3 lengths of the closer boat, around the mark.  There's no "zone" around the finish line itself. 

In your second question, you ask about the zone's relevance to ROW.  A boat's right of way is not affected by rule 18, but boats don't have to worry about right of way if one of them is entitled to mark-room.  So if, for example, you're in a boat that's overlapped to windward of another boat on the same tack when the first of the two boats enters the zone around a finish mark (which might be a race-committee boat), and you're the inside boat, then you're entitled to mark-room, which means you can sail to the mark if your proper course takes you near it, and then pass the mark on its required side.  If you do those things, and if that results in you not keeping clear as required by rule 11, then you're exonerated for breaking rule 11 (see rule 43).  The same is true for all the other right-of-way rules (rules of Part 2, Section A), as well as rule 15 Acquiring Right of Way and rule 16 Changing Course.  In effect, rule 18 "trumps" the right-of-way rules and rules 15 and 16.  

Consider this diagram. Green on starboard and Red on port are approaching the middle of the line, neither boat is in the zone of either finishing mark. Red must keep clear of Green.

Blue must keep clear of Yellow, but since they're in the zone of the pin Yellow must give Blue mark-room to pass on the correct side of the pin. If Blue contacts the pin or causes Yellow to avoid she breaks 31 or 10 but is exonerated by 43.1(b). Note that Blue is not protected by rule 18 or 43.1(b) if she sails outside of the corridor from her position to the pin. but the mark-room room Blue is entitled to includes room for her boom to miss the mark.

Ha!  Something I've never noticed until now: Racing rules diagram software doesn't draw zones correctly. Note that the zone around the RC boat is shown as the same size as the zone around the mark, whereas under the RRS it should be much bigger. The zone around a mark is defined by 3 boatlengths from the nearest point of the mark, not from the center of the mark. The zone around the RC vessel should be an oval, not a circle, and should be bigger than shown in the diagram, by more than a boatlength in its longer dimension!

Rob, I suppose that’s true, but then to really do it correctly you’d have to be able to scale the committee boat icon to properly reflect its size in relation to the boats. I’m not sure that would be a super value-added feature.

True.

Thanks all who answered my question and especially Tim for the diagram that clarifies the "zones".   I think I get it now (if only I can remember it).  

> The zone around the RC vessel 

Funny, that just occurred to me for the first time too. Now I think of it I (and I expect just about all of us) have raced on a course with an island as a turning mark, and the size of the zone there... Of course typically one doesn't want to get within 3 boat lengths of an island's shore, but I must go and think out the implications for fun! [answer, presumably it will be a continuing obstruction so the zone doesn't come into it].