Catalán, on what basis do you think Rule 15 applies? - Ang

YES! But 15 don't apply !

I agree there's no rule 15 component. Y went from stbd ROW to windward ROW, she initially acquired ROW earlier.

But how much mark room must Y allow, given that B's obligation with respect to rule 31 (as changed by B3. 31) allows her to touch but not hold onto the mark? If B hits the mark but on the correct side, has Y given sufficient mark room? 

I have no experience with board sailing,  However, Appendix B changes the definition of mark room to be the "room to sail her proper course".  If the inside boat would normally touch the mark when sailing her proper course then I would consider that sufficient room.

I seem to recall a comment that "it is never seamanlike to hit a mark" Not that I can find it anywhere now.
However, Case 118 explains room and says it is room to sail to a position close to the mark. 
I don't think the fact that they "may" touch a mark is relevant.

I'm not sure you can apply Case 118 since it deals with the standard definition of Mark-Room, which is replaced in Appendix B. 

Nigel, I think you are looking for Q-A #4, Case 114.

Also MR Call D1

To the question, I would interpret it this way.

Though “mark-room” changes in Appx B, “room” does not. Seamanlike way lives in “room” which is part of “Mark-room” under Appx B.

Rule 31 always only speaks to the consequences of the boat touching the mark and not other boats.   If the Inside boat touches the mark (by force or by choice), she does not break rule 31 under Appx B.

Though touching a mark no longer breaks a rule for the boat touching it under Appx B, being forced by another boat to collide with a mark could be significant evidence that the inside boat was not given “room” within “mark-room” that the Outside was obliged to give her.   

I think the Outside boat still needs to give the inside boat sufficient space to round the mark on the proper side without touching it. 

Supposing the lay-lines in the diagram are meaningful downwind lay-lines, then B's proper course after @1 is to sail to the layline then gybe.

Equally arguable that Y, not constrained by other boats is sailing what would be a proper course for boards of that type so that also may be B's proper course.

@2 minus delta, Y forces B to gybe prematurely and sail deep towards the mark, thus losing boatspeed.

I'd say that by forcing B to gybe when she did, Y does not give B mark-room and breaks rule 18.2(b).

Mark-room

Ordinarily mark-room includes room to avoid touching a mark in accordance with rule 31, in a seamanlike way.

The seamanlike way to avoid touching a mark (or another boat) is to manoeuvre so as to leave some space as a margin of safety between the boat and the object.

This margin may be small, less than half a metre, but I don't think any less than .3 of a metre.  It will vary, of course according to the existing conditions and the characteristics of the boats.

So mathematically, the space a boat required to give mark-room needs to leave between herself and the mark is the beam of the other boat plus 2 x the margin of safety.

If rule 31 does not apply so as to prohibit boats from touching marks, then, nevertheless seamanship still requires a boat to not hit a mark and to leave a margin of safety to do so, so, the space required will still be much the same, maybe a little less, maybe a little more if it is true that boards do not turn around marks as precisely as keeled boats.

A protest committee would need to be advised by an experienced board sailor about the characteristics of boards and the actual space needed.

John, by my (board-unfamiliar) reading of B2.18, Blue (port tack, then windward) has not yet earned mark-room at position 1.
Hence she is not entitled to her proper course. Nor is she even entitled to sail TO the mark.
Mark room begins when one of the boats is ROUNDING the mark. There is no Zone.

Thanks Phil

I'm obviously equally unfamiliar with Boards and Appendix B.

So mark-room entitlement begins when at least one of them is rounding or passing the mark.

And Rounding or Passing is defined as

So, assuming the diagrammed laylines and Y's course meaningful, B would only reach that point on port tack when she reached the starboard tack layline, which is the very place that Y is preventing her from reaching.  So I agree that Y does not break rule 18.2 by forcing B to gybe.

@2, B now needs to begin to bear away to round the mark (at a safe and seamanlike distance, however small that may be), so rule 18 begins to apply, and B, overlapped inside is entitled to mark-room to sail her proper course to round the mark.

There's about 1/3 of a board length between B's boom and Y's 'hull', and Y is bearing away, so Y is giving mark-room.

What happens next?

Board-savvy comments welcome!