Ooooh .. that's a good one.  Let's see what folk think "above" and "below" means in a sailing sense :-). 

I think the purpose of switching off RRS 18.4 in TR is to allow mark traps.

This looks like it deserves a RR Call.

But searching the TR Umpires Manual for "rule "17", in particular 11.5 (f) indicates that umpires must decide at zone entry whether 17 on or off, so presumably they are going to apply it.

As for the meaning of above and below, again searching th TR Umpires Manua for "layline" will give a few clues.

I'd say that while the meaning may be in doubt when a boat is sailing exactly square downwind or by the lee, the moment she is anything above dead square, she's above her proper course.

My (very not authoritative) opinion is that a boat who's proper course is towards it's leeward side is sailing "above" her proper course. She should keep bearing away until she either reaches her proper course or gybes. I believe this is how the rule is applied in umpired team races, but I also totally get the confusion.

Here is the call from match racing when we still had rule 17.  I think it pretty well covers it.  By not gybing at the layline, Blue is sailing above her proper course and has broken rule 17, even though she must gybe to do it.

As soon as the leeward boats proper course is to mark 4 she has to gybe as she is sailing above her proper course. 

When the leeward boat has luffing rights the windward boat usually luffs quickly calls overlap broken and when the tthen bear off they call the leeward boat to gybe. This happens all the time. 

Thanks JC. It would be great to have a call in the TR call book OR a case about this. The (erstwhile) MR call G8 is illustrative of a similar situation in fleet racing that I've never known how to deal with. I had it happen to me two days ago in fact.
ADD: Michael B's common example would be sufficient.

I think the reason there isn't a call about this specifically is it is the inverse of the scenarios laid out in J5, J6, J8. In those calls 17 is off, if 17 was on then her proper course is to gybe towards the next mark once mark-room is given.
Deletion of 18.4 under D1.1(c) only turns off the obligation to gybe, not the obligation to sail their proper course when 17 applies.

This is such a team race specific thing!  Mark room is not an issue since Blue doesn't need protection from it, so we revert back to standard ROW rules. (You don't have to take the mark room if you are a ROW boat, and we are absent 18.4)   But for team racing we still have Rules 17, is there is nothing that compels you to jibe? Or could have to very carefully be dead downwind (or equivalent) and not jibe. When you think about it, your proper course is to jibe (turning further down wind) so I would say it's probably not permitted under the 2021 or 2025 Rules. Rule 17 resets as soon as you do of course.  The key thing being proper course is not a VMG course it's literally downwind to the point of jibing, or you are above proper course. It's a razors edge though someone else could certainly interpret it differently (what if I'm by-the-lee?) and I'd certainly like to see a case on it.
Much better to do if you had luffing rights and were inside in the same scenario, you could definitely do this maneuver as a kind of trap. 

Agree with Michael Butterfield. Blue is restricted to proper course and must gybe. 
A very common occurrence in team racing. 

Hi All, and again thanks for everyone's contribution.

Additional thanks to John Christman, as the MR Call does answer my question with 'authority'. (Given the official nature of the Call book). I'll make sure i read it before reaching out again

I read a few laws and regulations outside of sailing and there is concept of adhering to the Intention of the law and not just the law as written.
However normally the Intention of the law is pretty clear. In this case, it is not obvious to me that the necessity to gybe is either what is written nor an obvious intention of the rule.
A lot of comments above simple say "Proper course" and drop the "above" word.

Does Rule 17 need to be reworded?

Eg "....... she shall sail her proper course if otherwise her course was to interfere with the other boat's proper course  ......."      (Dont take my wording too exactly, it is there to paint a picture.)

I'll leave it for others to discuss, if they wish. 
Thanks again,
Ross.

I'm with Michael na Murray. G'day guys.

TR D7 refers to the windward mark but I think it's quite clear. If you overstand the mark you are above your proper course.

TR-Call D7

11 On the Same Tack; Overlapped Rule
16.1 Changing Course Rule
17 On the Same Tack; Proper Course

Question 1

B and Y, close-hauled on port tack, approach the starboard tack layline near the windward mark. Y is subject to rule 17. Both boats sail beyond the layline, and are clearly overstanding the mark. Y luffs and is then forced to bear away to avoid contact with B who has held her course. There is a protest. What should the call be?

Answer 1

Penalize B. When Y luffs it is clear that her proper course is to luff in order to tack towards the mark. Y gives B room to keep clear as required by rule 16.1. Y is then unable to sail her new course without the need to take avoiding action. B does not keep clear and breaks rule 11.

Question 2

What should the call be if due to shifty winds and/or rough seas it is not clear whether the boats are over-standing the mark?

Answer 2
 
Penalize both. It is not yet clear that Y's only proper course is on the other tack. Y breaks rule 17 when she sails above her proper course (close-hauled) without promptly sailing astern of B. If B could have responded to the luff and kept clear, but either did not respond or did not respond enough, she breaks rule 11.