Yes, you're right.  If P can't cross S without S having to take avoiding action, P should tack away and then tack back after S has rounded the mark.  However, if the situation is as you describe except that P is crossing ahead of S, S can't luff up unless she gives P room to keep clear, even if the luff is a normal action to round the mark.

Super interesting in the last part... what's the rule I should be looking at there?

Rob, thanks for weighing in on a common situation that we see on the San Francisco cityfront.

Let's assume the situation above, plus one more detail: P tacks to avoid S. In that situation, when is the soonest that S can legally luff up to start to round the weather mark? Is it when:

Seems to me that it's option #1 above, here's why:

The relevant rule here seems to be 16.1, "When a right-of-way boat changes course, she shall give the other boat room to keep clear." Meaning, S can luff above close-hauled to start her turn as soon at P luffs above close-hauled as well, because that maneuver increases the distance between the two boats, and gives P more room to keep clear. 

So, what are your thoughts?

Dan, if I can tag onto Rob’s comment, be aware that there is no rule in the situation you describe that invokes “proper course” for the starboard boat.  Therefore, though Port would be wise to leave room, there is no rule limiting how long STB can delay her tack (I.e., there is no rule like 18.4 for this windward mark situation). 

In otherwords, Port is a sitting duck. STB could delay her tack to 1/2 BL of Port, tack and claim she had reasonable expectation of collision.  If asked why STB didn’t tack within the ample room Port provided, STB could say she didn’t want to because she wanted to force Port to tack away to gain and advantage against her. 

As Rob said better to do 2 tacks to keep clear .. and leave no doubt. 

Angelo and Rob: I'd suggest a different approach. Rather than doing two tacks to keep clear, P might be better off slowing down by easing their sails when they were a few boatlengths from the weather mark, so they can do a small duck below S and still lay the mark. That way, S has to continue past the layline before initiating their tack in order to comply with Rule 13 (keeping clear while tacking). S would round the weather mark behind P.

in answer to the separate questions and suggestions:
(a) The constraint on S luffing to round the mark is rule 16.1.  Once P has begun the crossing, she is, as Angelo says, a sitting duck -- there's usually no way for her to avoid S other than to bear off at the last moment, which only works when she's almost clear even after S luffs. That means S's luff breaks rule 16.1.  My point in making the comment about S luffing was that S is not exonerated for breaking rule 16.1 just because it's her proper course to do so.  Rule 18 does not apply, so there's no exoneration for S for breaking rule 16.1, no matter what.
(b) Al is right -- it might be much better for P to slow when she sees S coming, so that S can tack and round the mark ahead of her.  A lot of sailors forget that slowing is part of their arsenal.  

Rob .. to be clear .. your 16.1 discussion above relates to the alternate scenario you added (not part of the OP), where Port was crossing clear of STB prior to STB turning to windward to start her tack/round maneuver … and not the OP scenario where Port was not crossing clear.   Correct?

Sure, 16.1 applies, but we have that issue of how can a boat be given room to keep clear if they weren’t already keeping clear in the first place?

We have talked about Port’s strategy… but what about STB? IF STB really wants to put a penalty on Port, I think starboards move would be to duck Port, protest, then tack and round behind Port.  I think STB’s duck of Port’s stern would be a much easier ‘avoiding action’  case to make to a PC then tacking close inside of Port and claiming that was the avoiding action (assuming the boats would have intersected at the starboard side of Port’s stern as in the OP.)

Angelo, you are absolutely right.  I was addressing my alternate scenario in which STB "hunts up" in order to put PORT, who was keeping clear, in a position where she's no longer keeping clear.  And definitely,in the original scenario STB should bear off to pass astern of PORT and protest.  If STB doesn't luff up first, there's no way she should ever lose that protest (see Case 50 -- https://tinyurl.com/yc8xup6k).

You're also right, if PORT is already failing to keep clear when STB luffs to round the mark, then PORT is not exonerated for breaking rule 10.  And clearly there are circumstances when the reasonable action for STB to take to avoid PORT is to luff and tack around the mark -- for example, when PORT is on a course to T-bone STB.  In that case, STBD had to take avoiding action and PORT breaks rule 10 -- the fact that STB wants to tack and round the mark is irrelevant.  

Rob, Angelo... thank you, this is great... I get the meaning of the additional comment. Obnoxious hunting by S after P starts ducking is prohibited as it usually is, but P needs to take action well before putting S into the drivers seat, as Rob's comments make clear. And the whole reason I'm making sure I'm looking at it correctly ;)

And, Al is right... this scenario happens regularly at Blackhaller Mark on the SF cityfront. A big flood can give the port side of the course a big advantage. The leave-to-starboard is standard given that the rest of the course is either downwind or to starboard of the mark.

Thanks again!

Given S must keep clear while tacking (rule 13). It seems that the winning move for P is to slow down or duck just enough to cross the stern of S as she is attempting to tack around the mark to be left to starboard and then luff up. As S falls off onto port, she shall be required to keep clear of P (rule 13), and once S is on port P is leeward and ahead, S must keep clear (rule 11), and P is free to luff her up without fouling her (last sentence of rule 17). P is not required to give S "room to keep clear", as S acquired right of way due to S's tack (rule 15).

Given the OP described the boat on port as approaching on the layline, and will be slowing down and/or ducking to allow S to cross her, she could have more boat speed than S and will have plenty of momentum to come back up to or above the layline to the mark.

Judges, have I got this right?