In my blogs on the new Section C, I've gone through a number of changes in the rules that, in my opinion, will change the fleet-racing game in a major way, in a minor way, or not at all.  I think it might be useful for me to summarize what I think the changes are, with a brief description of each major rules change and an assessment of how I think it will affect the game we play.

Rule 18 and associated definitions

Definition Zone and when rule 18 begins to apply.  The zone around a mark is now 3 lengths rather than 2, and rule 18 now applies when the first of two boats is at the zone rather than when they are "about to pass" it.  Game changes: (1) A larger zone requires an earlier "inside move" and overlaps have to be established or broken earlier.  This is a major game change.  (2) In very heavy air, the old rule may have applied more than 3 lengths from the mark, because boats were "about to round" earlier; under such conditions rule 18 may actually apply less far from the mark than before.  On the other hand, boats are unlikely to luff each other or otherwise carry out aggressive maneuvers in such weather, so I rate the impact of this game change as minor.

Definition Mark-Room.  The new definition defines much more clearly what is meant by room at a mark:  room to sail to the mark; and then room to take a proper course around the mark.  No game change during the approach, but at the end of the rounding an inside boat with room now gets more room, because her optimal radius turn is much larger than her "seamanlike" radius turn.  This game change is not, in my opinion, major.  Note that Butch disagrees.

Room to tack.  The definition of mark-room says, "However, mark-room does not include room to tack unless the boat is overlapped to windward and on the inside of the boat required to give mark-room."  I interpret this to mean that if a boat is overlapped on the inside and to windward, she does get room to tack, and I think this is what we already thought the rule said (even if it didn't), so no game change.  

No change in right of way.  The old rule gave right of way as well as room to a boat that was clear ahead (or inside and became clear ahead) at the zone.  We used to say she "owned the zone".  The new rule 18 does not change right of way, so, for example, a boat on starboard tack still has right of way over a boat on port tack (though if she owes the port-tack boat mark-room, her right of way is severely limited).  In practice, this makes little or no difference to the game – it just makes the rules easier to understand and remember. No game change.

No point where rule 18.1 "turns off" rule 18.  The old rule 18 applied "until both boats have passed [the mark]."  The new rule deletes these words, so, just as with other rules like port/starboard etc., rule 18 no longer applies when the situation no longer requires it.  In this case, since "mark-room" is only defined when the boat entitled to it is sailing to the mark and when she is at the mark, all parts of rule 18 dealing with mark-room effectively "turn off" when the boat entitled to mark-room is past the mark.  In practice, I don't think the difference between this and the old rule's "both boats" criterion causes a game change.  There may be a game change with respect to rule 18.3, Tacking When Approaching a Mark because mark-room is not an issue in that rule.  So rule 18.3 presumably continues to apply until both boats leave the zone.  For example, even if you are the leeward boat with luffing rights, if you tacked in the zone you can't luff the other boat above close-hauled, even after you have passed the mark, as long as you or she is in the zone.  This change was clearly unintended, and will no doubt be reversed in 2013.  Until then, I don't think it will have much impact on windward-leeward courses, where boats rarely luff each other even to a beam reach after they round the mark, let alone above close hauled.  If the next leg is, say, a beam reach, a boat that tacks in the zone and then luffs after passing the mark might get in trouble.  So let's say this is a moderate change.

Rule 18.1(c).  The provision that rule 18 does not apply between a boat approaching a mark and one leaving it is new, but it only clarifies the sentence of old rule 18.1 that said rule 18 began applying only to boats that were [both] about to round."  (A boat leaving the mark and a boat approaching the mark are almost never "about to round" it at the same time.)  So, little or no game change.  

Rule 18.2(c) turns off 18.2(b).  Rule 18.2(b) is the "lock-in" rule, which says that if a boat was clear ahead or overlapped at the zone, then she gets mark-room throughout the rounding, regardless of her overlap status later.  Rule 18.2(c) makes that clearer, and then says, "However, if either boat passes head to wind or if the boat entitled to mark-room leaves the zone, rule 18.2(b) ceases to apply."  As far as leaving the zone is concerned, this is what we all thought the rule said anyway (even if it didn't).  But the words about tacking are new, and could, in certain circumstances, cause a huge game change.  Suppose a boat approaches a leeward mark clear astern of a pack of other boats, in light air.  Rather than granting them mark-room as required by rule 18.2(b), she quickly luffs up, passes head to wind, and bears off for the mark.  According to rule 18.2(c), 18.2(b) no longer applies, so that means rule 18.2(a) applies.  As soon as she gets an overlap on the other boats, she's entitled to mark-room.  Of course, some of them will have gone around the mark by then, but maybe not all.  To those boats, this would be a big change in the game.

This change was inadvertent, and will probably be reversed by 2010, if not sooner, on an "emergency" basis.  Everybody makes mistakes, and this time, we did.

Rule 18.3, Tacking When Approaching a Mark.  The old rule said that for it to apply, the tacking boat had to "complete her tack" in the zone.  The new rule only requires that she be subject to rule 13 (i.e., between head to wind and close-hauled) in the zone.  In almost all cases, this is equivalent; to see this, sketch a zone on a piece of paper and look at how boats enter it to round a windward mark.  You'll see that boats which complete their tacks in the zone were subject to rule 13 in the zone, and vice-versa.  So why the change?  Well, under the old rule a boat could start to tack, realize she was about to break rule 18.3, and simply fail to complete the tack.  This gave her room at the mark, and because the rules of Section C overruled those of Section A she wasn't breaking rule 13.  So, the new rule is a game change – but only if you knew about this problem in the old rule 18.3, which I'll bet you didn't.

Rule 18.4, Gybing.  This is the rule that requires a right-of-way boat to jibe at a jibe mark, or at a leeward mark where she has to jibe to round it.  The change is that now this rule does not apply at a gate mark.  This means that a boat can enter the zone of, say, the left-hand gate mark on starboard tack, continue more or less across toward the right-hand gate mark, force an oncoming port-tacker to jibe, and then turn and go back to the left-hand gate mark – without breaking any rule.  Under the old rules, if the starboard-tacker's course before she jibed took her farther from the left-hand gate than her proper course around it would allow, she would have had to continue on to the right-hand mark to avoid breaking rule 18.4.  Now, not so.  This is, I believe, a substantial game change.

Rule 18.5, Exoneration.  The effect of this rule is to replace the old Section C preamble, which said that when rules of Section C conflicted with those of Sections A or B, the Section C rules took precedence.  There are two effects of this change: (1) Rule 14, Avoiding Contact, is not mentioned; now, if you break rule 14 you are not exonerated unless you were forced to do so by another boat breaking a rule (see rule 64.1(c)); and (2) breaches of rules 15 and 16 are now only exonerated when the boat is at the mark and sailing her proper course.  I think most of us thought that the old preamble really didn't apply to rule 14 anyway, so I'm rating (1) as an insignificant game change.  On the other hand, old rule 18.2(d) used to exonerate boats from rule 16 if they were "changing course to round or pass [the] mark."  The only places I can think of where this is different from sailing her proper course around the mark is as boats approach the mark (under the old rules, a leeward boat presumably could have luffed an inside leeward boat as hard as she liked, as long as she was turning toward the mark) and in certain cases as they leave it, where a leeward boat entitled to mark-room can now only luff to her proper course.  For example, at a leeward mark, if a clear-astern boat goes inside a boat that was clear ahead at the zone, the outside boat can only luff up to approximately close-hauled to "shut the door" without worrying about rule 16.  Under the old rule, it was unclear whether she was similarly restricted by rule 16, or whether she could go head-to-wind without breaking rule 16; it all depended on how you interpreted "course … to round the mark".  So I'd rate this game change as moderate.

Rule 19

Rule 19.2(a).  This rule simply answers a rules FAQ – it's implied by the definitions of "keep clear" and "right of way".  No game change.

Rule 19.2(b).  In the new rules, there are no zones around obstructions unless they are also marks.  In general, this won't affect the game much – except in match racing, we never put zones around obstructions, anyway; we simply gave room to boats overlapped inside us.  But now there's a move that works at fixed obstructions such as sea walls and shoals:  Suppose two boats are on the same tack and barely overlapped, with the leeward boat almost clear ahead and about to be outside boat at the obstruction.  Two lengths before the mark, she luffs and then bears off sharply, breaking the overlap.  If she does this right, it's difficult for the other boat to reestablish the overlap, so there's a good chance there won't be an overlap before they reach the obstruction, and so the boat ahead doesn't have to give the other boat room.  Even if the clear-astern boat establishes an inside overlap at the last moment, it's possible that the other boat can't give room at that point, in which case she doesn't have to.  I'd say this is a substantial game change.

No special exoneration/precedence over Sections A and B.  As I mentioned above, under the old rules if there was a conflict between rules of Section C and those of Sections A or B, the Section C rules took precedence.  In the new rule 18, this is replaced by rule 18.5, Exoneration (see above), but there is no such provision in rule 19.  So if a right-of-way boat fails to give room when she's required to, the other boat must try to solve the problem while still keeping clear – which might mean tacking or going the other side of the obstruction – and protesting.  Only if she has no other options may she simply push in where there's not enough room and foul the outside boat – under those circumstances she'll be exonerated under rule 64.1(c).  This is a major game change.

Rule 19.2(c).  There's only one change here from the old rules – under the new rules, the prohibition against "going in there" at a continuing obstruction only applies to boats that are required to keep clear before they go in.  That's because a boat with right of way can simply steer at the inside quarter of the boat ahead and force her to move away from the obstruction in order to keep clear.  Thus, the restriction is moot in such cases.  No game change.

Rule 20

Hailing when the other boat is fetching the mark.  Under the old rule, it was not clear what was supposed to happen at an obstruction that was also a mark, if a boat hailed another boat when the hailed boat was fetching.  Most experts taught that the hailed boat had to tack and protest, but they had to resort to rule 2 or rule 14 to get to that answer.  The new rule says exactly what those lecturers and writers were saying, so I'd say there's no game change – at least if you read those books or went to those talks.