My take:

Until the overlap is established L (clear astern) has no obligation other than to keep clear. She can sail any course she chooses as long as she does so. 

Once she has the overlap rule 17 restricts L from sailing above her proper course but permits her to sail below it for any reason and alter up to proper course whenever she chooses (recognizing RRS 16). In any case, proper course downwind is generally a judgment call, highly dependent on conditions as well as boat type. 

A boat could easily be halfway down the run and decide that hotter might be better. 

I totally agree.  It is useful to think of rights, responsibilities, obligations and opportunities.  Assuming the boats are on the same tack the clear astern boat L has the responsibility to keep clear but no obligation or restriction on how to do that.   The boat has the opportunity to sail as she pleases to get an overlap to Leeward at which point she has obligations under section B general limitations 15,16 and 17.  I think that Dave is suggesting that evidence for a boat sailing a proper course is that a boat has been sailing it independently of the interaction.  It doesn't place any restriction nor does sailing differently to gain an overlap imply that a boat cannot resume a proper course assuming that they do it in accordance with rules 15, 16.
 

As Brent Draney wrote above, L has no restriction on her course before the overlap. However, if the matter gets to the protest room, one can never know how the PC will find the facts. If W claims that L sailed deeper down wind before the overlap was created and then began luffing up, the PC might find that L sailed abover her proper course and infringed Rule 17. The smaller the course change of L, the stronger its position in a hearing before a  club-level PC or similar, whose members may be less experienced and professional than senior judges.

Once L gained a leeward overlap her "proper course" is to continue to sail the heading she is on through the duration of the overlap since she is obligated (Rule 17) to sail no higher while within 2 boatlengths of W.  For L to have gotten to that position presumably that is the course she was sailing "consistently" (Perry) before gaining the overlap.  L is pinned by W and may not fetch the finish, which she should have anticipated before sailing into the overlap.  She was sailing faster than W and could have steered to windward of W (which would then have set up former W to now become ROW once overlapped, Rule 15).  If the finish is not nearby, then L, sailing faster, may have had time to break the overlap and then head higher to fetch the finish, which would now be her "new" proper course.   Case 46 explanation, "While L was not entitled to sail above her proper course, she was entitled to luff to her proper course, " seems to be contradicting itself.

I agree with Tim and Brett, but the question was "Is there a reason for sailing low?" and if I knew that the boat ahead had the sort of helm who would luff me regardless, I would certainly not risk going to windward of them, that could easily mean sailing much lower than a proper course.

Ron, your interpretations are off the mark.

Proper Course is determined at each moment in time and only applied in the context of the rule that uses the term and in relation to the other boat referenced in that specific rule. 

There simply is no obligation “to continue to sail the heading she is on through the duration of the overlap“.  That is no place in the rules. 

Many factors can go into a course change of a boat that is Proper Course limited. Changes in wind strength, direction, proximity to marks or finish-lines, and also tactical maneuvers relative to other boats not referenced in the rule that uses the Proper Course term. 

Nigel, you are on it along with others you site.  The key is Proper Course is immaterial ... is not determined or calculated for comparison .... before the rule that uses the term applies (17 in this case). 

Before that moment a boat can take any course they like, on purpose or by accident or miscalculation, without loosing their right to sail to their Proper Course when the term applies in rule 17 ... and their Proper Course course can change at any moment due to many factors. 

In my initial post (now edited) I wrote "a PC would define Proper Course in 17"
I meant "a protest committee would define Proper Course in 17"
I hope nobody was confused by this.

Regarding the Dave Perry discussion - in a Rule 17 hearing L is trying to establish that she was not sailing above her proper course. One way ("in part") to establish that a higher course is proper is to demonstrate that she'd been sailing the higher course consistently prior to interacting with W. But that's not the only way. Sailing a course consistently is not definitive.

It would be perfectly legit to say that a boat decided for tactical reasons she wanted to go to leeward of another boat, sailed deep specifically to establish the leeward overlap and then luffed up to a course which gave her a higher VMG. Even if her purpose in getting the leeward overlap and luffing was to disadvantage the windward boat. As long as L can credibly claim that she went no higher than what she reasonably believed was her proper course in the absence of W, no rule is broken.

Tim Hohmann
Created: Yesterday 15:28

While I agree that a boat may seek tactical advantage by sailing below her proper course to gain an overlap then luffing a windward boat up until she reaches her own proper course, I have some problems with this wording.

If L allows the protest committee to believe that 'her purpose in getting the leeward overlap and luffing was to disadvantage the windward boat' she is just begging the protest committee to apply 'in the absence of the other boats ...' part of the definition of Proper Course, and conclude that she was sailing above her proper course.

Secondly, the test of Proper Course, like most tests in the RRS is objective (or perhaps subjective objective).  It doesn't rely on what L 'thinks' was her course to finish as soon as possible, it's what L can persuade the protest committee, on the basis of the evidence of herself and others of facts (shifts, gusts, current, her boat performance etc) was the course to finish as soon as possible: in other words, not what L 'thinks' but what an average reasonable sailor, represented by the protest committee 'thinks'.

John, I think we're in violent agreement on your second point.

On the first point, no rule prevents a boat clear astern from sailing below her proper course, and rule 17 doesn't prevent a leeward boat from sailing below her proper course for any reason, other boat referred to in the rule or no, or from coming up to her proper course when she chooses to do so. Downwind proper course is always a judgment call and, within certain parameters, impossible to "prove." I agree that winning this depends on L convincing the protest committee that the higher course was still proper, but for the protest committee to infer that the lower course must be "proper" because L had previously chosen to sail it is, in my opinion, an incorrect interpretation.

On the water, a boat can't determine her proper course based on what she thinks a protest committee will think based on what other boats might think they think. She can only determine her proper course based on what she thinks and then, as you say, hope she can persuade a protest committee that what she thought was reasonable.