Blue was keeping clear by sailing to pass astern of Red.
Red altered course and blue had to immediately change course to continue keeping clear.
Red broke RRS 16.2.
RRS 18 does not apply at a windward mark between boats on different tacks.
Red was not entitled to room or mark-room under any other rule.
No exoneration available for Red under RRS 21.

How did they manage to change the size of the zone anyway? ;)

Thorsten, no argument with your analysis, but if I was STB I’d argue that if Port was passing that closely that STB couldn’t turn away, that Port wasn’t going to pass astern without some amount of altering course anyway. 

Don’t you think that, unless STB agrees that Port would have cleared, that most PC’s would have a hard time finding the Fact that Port was actually keeping clear prior to STB’s course change away from Port? - Ang

I agree with Thorsten. All Blue has to do was ‘keep clear’ and that included being able to ‘sail her course’. Sailing her course in respect to Red was continuing to sail the same heading.

When Red changed course she was subject to rule 16.1 & 16.2. Blue did not have to anticipate that Red might want to change her course by luffing.

If Blue & Red were sailing converging collision courses, and Red, because of a reasonable apprehension of a collision, changed course to avoid contact, it would have been different.

But such was not so in this case. Red the ROW Boat changed course to shoot the mark. Red has no defence in my book.

Phil .. 

That's what I'm saying (sort'a arguing for sake of exploration here .. ) is that from the drawing, I'd say Red had a reasonable apprehension of collision .. and given Red's course options to avoid, up to STB was the only viable option. 

My point being that 16.2 has an initial condition that must be established .. that ," a port-tack boat is keeping clear by sailing to pass astern of a starboard-tack boat ".  I can see establishing that as a Fact to be a challenge in this situation (unless STB concedes the fact).. and creates the tug-of-war between Case 50 and 16.2.

Well, most of our drawn diagrams are somewhat hypothetical and the distances are exaggerated in either way for demonstration purposes.
Of course it all depends on how the parties present their cases and of course, if we add water, things get less precise.
This is a situation and diagram I saw in several tests so far without ever seeing it on the water or hearing a protest concerning this.
Starboard luffing seems to be the rarer sibling of starboard bearing away, to say the least.. ;)

Yea .. I get that .. just a position 1, Blue does not look like she is " sailing to pass astern of .." Red.

Also good Q regarding the Zone.  Someone must remember when that was a option to choose between 2 and 3.

The option to change the zone to 2 or 4 boat lengths went out at the end of 2012.

You are not "keeping clear" if the other boat cannot maneouvre. I would say that Blue has no case.

Paul ... it's much more subtle than first appears.
 

What you are thinking about is Keep Clear's , "...  if the right-of-way boat can also change course in both directions without immediately making contact. ". 

The problem with applying that is the first part of that sentence .. " when the boats are overlapped, if the right-of-way boat can also change course in both directions without immediately making contact."

Red and Blue are not overlapped .. as they are on opposite tacks and they are closer than 90 degs to the wind.

I think my argument is Red's only hope.  16.2 only applies if the first condition of it applies .. that Blue was "sailing to pass astern of " Red.  If Red succeeds in a claim that Blue's course at #1 was not a course that would pass astern of Red (in other words, Red has a reasonable apprehension that Blue is on a collision course with Red's stern at #1), then I'm making the case that Red can head-up and Blue's line was too close.

The Zone different because the "regatta" is a pretty informal weekly race series, we've tweaked some rules to fit our courses better (and now that I think about it, I could be wrong on the Zone change, it doesn't come up often in our occasional rules situations).
Edit: The Zone isn't different in our fleet, I was mixed up on that.  Updating the original post.

The diagram is meant to be illustrative, not exact, though as usual diagrams are a major source of information regardless.  But "Is Blue passing Red to stern" is the rub.  Blue I think has a hard case to prove in a protest room, but doable in this situation; Red's stated intention in altering course was definitely to shoot the mark, not to avoid Blue.  And the definition of Keep Clear mentions that the second part about being able to change course applies when boats are Overlaped, which the boats were not because rule 18 does not apply, one boat was pointing upwind, and the boats were on opposite tacks; so Red would not be allowed to alter course regardless.

This wasn't tested with a protest hearing, just lively discussion after racing (I was the Red boat, for the record).  Discussion was lively because it didn't "feel" right to many, as 16.2 seems primarily in place to avoid dial-ups, but it does look like the correct interpretation is to chuck Red if Blue can establish that they were initially keeping clear astern.  Interestingly, as I found when researching this, the current Team Race Test Rule for 16.2 would exonerate Red with its specific language about bearing away (http://www.worldsailingywc.org/tools/documents/testteamrule162v2Jan2019-[24555].pdf).  Its not exactly applicable to fleet racing, but it seems noteworthy.

RRS 13?
  "The Starboard tack boat rounds up past close hauled to shoot the mark,"

Rule 13 starts with "after a boat passes head to wind..." and that did not happen here. He just headed up above a beat.

Duh.
Mis-read.
Sorry.

OK, here is the 3 BTL and hopefully the boats are in the approximate position.

To get to 16.2, you have to go thru 16.1.

Since Red is RoW boat, she can change course, provided she gave room to Blue. Blue had all kinds of room here. Blue did the right thing, kept clear of Red. 

Good luck bringing into the room only 16.2.

Oh, one more thing, since Blue later tacked in the Zone, R 18 kicked in, if that mattered.

Interesting that the question was "What should the call be?"  
That implies that it was being umpired and if that was the case the facts are much establish  by the Umpires and what they see and there is a chance that 16.2 could be applied. I see it all the time as an Umpire but have never seen a protest. 
For all our drawings, I would challenge anyone to establish the facts sufficiently clearly in a protest hearing for the jury to give a 16.2 decision.

I understand this, like most of the other topics in this forum, as a discussion about the rules and how to apply them.
A situation is described, either in a diagram or in a description or in both, and the community discusses applicable rules and decision.
To me, this means the described situation, academic as it may be, are the facts.
The conclusions and decisions follow these facts without questioning them.

What possible outcome a hearing about that situation might have, is a completely different discussion.
As mentioned above, this depends on how the parties present their case.
A protest committee will balance the probabilities of the parties’ stories and usually the stronger presentation will win.
In this case, Blue will of course have to battle uphill in a hearing, simply because the chances of Reds course change breaking RRS 16.2 are comparably small.
However, Red having good chances in a hearing does not mean Red did not break RRS 16.2.

The question of how to present a case to a protest committee might be one different topic ;)

Btw: breaking RRS 16.2 does not depend on breaking or ‚go through‘ RRS 16.1, as mentioned above ;)

Cheers
TTT

Kim said .. " To get to 16.2, you have to go thru 16.1. ".

Are you calling attention to the "In Addition, " at the head of 16.2?  I don't think that phrasing is used anywhere else to connect 2 sections of a Part 2 rule that I can see.

While agreeing with much that has been said about the rules, and the outcomes of protest committees,  I still come away with the view that Blue cannot "pin" Red such that she cannot make the mark.

Had this been a zone with 3 boat lengths, and had the incident taken place at the extremity of the zone then I would go with tossing Red. But as presented Red was almost on the mark when she headed up to shoot the mark. If Blue was pinning her, Blue was preventing Red from rounding...a failure in my option to keep clear. 

While it is a different subject than the rules interpretations, always in the room the PC must look at the physics/geometry of the question.
Here I have projected the course of Red, had she not changed course. Clearly Blue would have had to further changed her course anyway. Perhaps even more so.

@Paul Baehr: as mentioned above, the ‚has to be able to change course in either direction without creating immediate contact‘ part applies only to overlapped boats. If Red is pinned by Blue because she cannot luff without breaking a rule, that is tough luck. It is a matter of 1 or 2 seconds delay in luffing, anyway.

@Philip Hubbell: this seems to be a problem with creating a diagram precisely matching the question ;)

The point is that Blue does not have to change course as a result of Red's course change (16.2), because Blue already had to change course, as proven by the geometry.
And as for 16.1, there is no contact, so Red has given Blue the room to keep clear.
The rules do not provide Blue a "gotcha" tactic, and the PC should not condone any.

As mentioned before, establishing whether or not Blue was sailing to pass behind Red is the difficulty in this case - this is the reason Blue will have a difficult case to present in a hearing.
If the answer to that is "Blue was sailing to pass astern" or the question is "can Red break RRS 16.2 by luffing, even when in the zone", which I understood it was, then the answer is "Yes." ;)

The rules of section C were deliberately changed several years ago so that all the other rules of Part 2 applied at mark rounding - not just rule 18. Previously, rule 18 prevailed when there was conflict. So rule 16.2 applies inside the zone just like rules 10 & 11 and all the other rules of Part 2.

Thorsten, a drawing agreed to by both parties ranks as fact. 
We are obliged to draw conclusions from it and use it to judge the validity of statements made.

I'm a longtime Vanguard 15 sailor. I've probably done about 700 races in the boat, and roughly 400 ducks while on port tack (and picked up a national championship along the way). Based on that experience, I don't believe Blue was steering a course to keep clear.

In summary: a Vanguard 15 is very tippy. So, when one boat heels, its mast can extend out as far as half a boatlength from the leeward rail. When two boats heel in the opposite direction (one to windward, one to leeward), their masts can make contact even when the hulls are a boatlength apart.

Here's the detailed math. All this is assuming flat water and no extra margin of error due to boats bounding around in waves.

The Vanguard 15, like many dinghies, is very tippy. A crew or skipper coming in from hiking, or to grab the windward jib sheet to prepare for a tack, or to grab the vang, can heel the boat by around 10-20 degrees. A small puff can tip the boat by 20 degrees; a big puff, by 30 degrees. Going into the wind shadow of another boat (such as during a duck), can lead a boat to heel to windward 30 degrees.

A V15 rig is about 21 feet high. (The mast is 19' 9" long and is deck-stepped.) The beam is 5' 6", with a half-beam of 2' 9" (you'll see why half-beam matters in a moment). In each of the above scenarios, here's how much the masthead will swing:

When you combine 3 and 4 above, Blue should allow a boatlength of gauge (space between their path and Red's) to safely pass to leeward. That's 15 feet.

Now, suppose Red luffed all the way to 1 degree off head-to-wind (not tacking, but almost). That represents a luff of 35 degrees. (45 if it's windy, but let's give Blue the benefit of the doubt.) A V15 has very little weather helm, indicating that it's center of lateral resistance is in the middle of its 15 foot length. So, let's assume that the boat pivots along a seven-and-a-half foot radius when it luffs. A 35-degree luff by Red would swing the stern to leeward by 4.3 feet (= sine(35 degrees) * 7.5 feet). 

So, the most that Red could have swung their stern (4.3 feet) is less than the gap Blue should have to avoid locking rigs is about half of what Blue should allow in order to avoid colliding to due a big puff or heel to windward. 

To sum up, Blue was steering a course that was irresponsibly running the risk of a collision. I'm not a judge, but to me, that doesn't sound like keeping clear. Not making contact with another boat shouldn't be a matter of luck, but intentionally seamanlike sailing.

Math aside, from my own personal perspective, even on big ducks, I've never once been in a position where Red's stern swinging up would have prevented me from keeping clear. The ducking technique most good sailors use in a V15 is to bear away at around 2 boatlengths so that your bow is pointing at the starboard boat's port aft corner, then head up to track that corner with your bow. Eventually, your bow misses the starboard boat by inches, but because you've avoided the risk of locking rigs, it's actually more conservative. It also avoids the risk of planing into the side of the starboard tacker because you couldn't ease your mainsheet enough, and putting a hole into them. (I've been the recipient of one of these holes. Not fun.)

So, in this situation, Blue was not following best practices for ducking and was not taking steps to avoid a collision.

Adding some context to this scenario.  I was racing on the night in question (although not one of the boats involved) and have spoken with both participants about it since then.

First, and this might have been mentioned already, there was no protest filed or heard. This was a casual race, the parties resolved the matter on the water (don't remember exactly how), and the scenario was discussed afterwards primarily just in an effort to better understand the applicable rules in the future.

Second, this scenario occurred in very light wind.  We've debated since then whether it is even possible in heavier air conditions, but regardless it certainly is more likely to occur in light conditions where the swing of the stern is exaggerated compared to forward movement.  I think that Al's analysis of mast swing could potentially be relevant in heavier conditions but is less likely to come into play in light conditions, where there are no puffs coming to cause the starboard boat to heel.  (As for windward heel of the port boat, that is within the port boat's control, at least in a small dinghy like the V15.  If the port boat successfully stabilizes to prevent mast contact caused by windward heel, I don't think it matters that there might have been contact if they had failed to do so.)

Third, Cameron's original post altered the actual circumstances to present what all parties involved felt was a more interesting rules question.  In the actual scenario, there was immediate contact when Red turned to windward to shoot the mark.  At the time, based on a misunderstanding of the definition of "overlap" in part (b) of the definition of "keep clear," the parties believed that Blue failed to keep clear because Blue had not given Red space to alter course without contact.  Using the correct definition of "overlap"  (i.e., does not apply because the boats were on opposite tacks, Rule 18 did not apply, and Red was not sailing more than 90 degrees from the wind), all involved now believe that Red violated Rule 16.1 - as right of way boat, Red changed course without giving Blue room to keep clear.

Both the 16.1 situation that actually occurred and the hypothetical 16.2 situation described by Cameron in the original post are interesting and arguably unintuitive situations where, at least as we currently understand them, a port boat can prevent a starboard boat from pinching to round a mark.