the position of the boom is the only way to see what tack a boat is on…especially when sailing so deep downwind that one can easily choose either tack…and how do you know  the  precise relative wind direction another boat  may be enjoying?

I don't know that I've ever seen a formal limit on it... I mean if the main is clearly backwinded and being physically restrained are you still on that tack? In the absence of anything else I guess the answer is yes you are still by the lee on starboard.  It's not fast or safe so it's kind of self enforcing? 

It's true the term "by the lee" isn't defined.  The rules-writers try to avoid cluttering up the rulebook with defined terms when the meaning of the terms in ordinary [nautical] English is both clear and appropriate to the rules application.  Happily in this case, it's not necessary to know exactly what "by the lee" means.  A boat's tack when sailing downwind or by the lee (notice that we don't have to decide which of these conditions apply) is the side on which her mainsail lies (emphasis added).  To me, at least, "lies" clearly indicates the side on which the mainsail can be set, with the wind filling it from astern.  This doesn't prohibit the boom from being restrained (for example, by a preventer or a crew member), but it does mean that, if the sail is backwinded, the boat's tack determined by the side of the boat the wind is from, not by where her boom is being held.

A boat that becomes overlapped to leeward of another boat from clear astern sometimes tries to avoid breaking rule 17 by gybing and then gybing back.  That way, the boats are briefly on opposite tacks, and when she gybes back they are overlapped, so rule 17 does not apply.  However, if the leeward boat simply brings her boom past her center-line and then throws it back to the original side, she has not changed tacks because her boom was never lying on the new side.

These TR Calls may be useful. They do not rely on any special TR rules, so the analysis and example diagrams are valid for fleet racing.

Team Race Call G1

 

Definitions Leeward and WindwardDefinitions Tack, Starboard or Port

 

Rule Leeward and WindwardDefinitions OverlapDefinitions Tack, Starboard or Port

If a boat's sail does not fill after a gybe, she is not sailing yet with the sail on that side of the boat, so her windward side determines the tack she is on.

And this old (2013) MR Call might also help

Match Race Call G5

Rule 10; On Opposite TacksRule 11; On the Same Tack, OverlappedRule 17; On the Same Tack; Proper CourseDefinitions; Tack, Starboard or Port

John is definitely is right, TR and MR Calls are often used to get there in terms of when is a boat on a particular tack after a gybe. (although these calls are not authoritative to other disciplines, when the other disciplines are silent on a topic, we call them persuasive arguments). Of course, if the sail is being restrained and is not filling up, then that boat is not on that tack, but on the tack corresponding to the side that is away from the wind.

The key term use in the definition of Leeward and Windward is "lies", as the  term itself is not defined in the RRS we go to the dictionary use. The verb "lie" has a connotation of a "resting state" in its use (interpreted as without outside physical restraint - as supported by the calls John has provided, specially TR G1). Having worked on the translation of the RRS to other languages this has been the subject of robust discussion with native and non-native IJs and IUs.

So a boat is sailing by the lee as long as the main sail remains full in that tack unassisted. 

Cxema re: “So a boat is sailing by the lee as long as the main sail remains full in that tack unassisted.”

Once the mainsail lies in an established by-the-lee state (filled unassisted), would we consider the boat to have ‘gybed-back’ if the boom is restrained by crew or preventer when the sail momentarily became slack or backwinded due to wind change or movement/rocking of the boat?

If I understand your question correctly, I would go back to TR Call G1 Q&A 2:

Question 2
The boat now bears away further, with her boom still out over her port side, because now her crew is holding it there while the wind is backing her mainsail. Which tack is she now on?

Answer 2
Port tack. She is now neither running directly downwind nor sailing by the lee. A boat is on the tack corresponding to her windward side, that is the side towards the wind. There can be no doubt that this side is her port side and therefore she is on port tack.

Cxema .. yea, I get the TR Call, but I'm addressing the difference between a momentary state and a steady state I guess.

A boat is on a steady-state BTL condition and a powerboat wake comes buy and rocks the boat side to side, back-winding the sail and a crew or preventer keeps the boom from flopping over as the boat rocks a few times.  Or, they sail into a lull and it takes a moment for the helmsman to respond but the sail is restrained for that moment.

During those moments, are we swapping tacks on this boat? .. or can we think about the TR application as referring to a longer-duration condition when it uses the word "holding"?  In other words, does "holding" imply a longer duration of time?

Cxema Pico

Said Created: Today 07:46

Very kind of you to say so, but regretfully I disagree with some of the rest of your analysis.

 
That may be one connotation of "lie", but it is not the only one.  My Shorter Oxford gives one meaning of "lie", in respect to things as "to be situated in space".

It is possible to "lie restlessly"

I believe that the correct interpretation of "lies" in the context of fleet racing is no more than "is" or "is situated", that is, the reference to the sails position in space, not it's state, or restfulness or fullness or otherwise while in that position.

If a mainsail is filled on one side of a boat that is a sufficient but not necessary condition to determine her tack.

That may well be, but while MR rule C2.6, the 'bring back gybing' rule refers to " ... her mainsail has filled ... ", the word "filled" or "fill" does not appear in the RRS applicable to fleet racing or team racing.

MR Call G5 referred to the mainsail being filled, which is quite proper for Match Racing.

I think the TR Calls have inadvertently picked up the MR terminology.

TR Call G1 refers to the mainsail being filled, but this is descriptively, not prescriptively.

TR Call G3 says

If a boat's sail does not fill after a gybe, she is not sailing yet with the sail on that side of the boat, so her windward side determines the tack she is on.
 
MR and TR Calls often adopt assumptions and simplifications to achieve quick and consistent umpiring, and I think this is one of those.

Thus I think that the better way of stating Cxema's conclusion is

So a boat is sailing by the lee as long as all of the main sail remains full in that tack unassisted. 


So, Angelo, I don't think the state, filled, unfilled, flapping, is irrelevant, it's just the side of the boat that the sail is on.

A key feature of the Calls is that artificial assistance is used to move the boom and sail across the boat.  The MR Call, is looking specifically at an artificial double gybe.

Once the sail has moved across the boat without artificial assistance, I don't think there's any problem with keeping it there, and a boat certainly doesn't change tacks because its mainsail fills and flaps.

There's nothing wrong with the seamanlike use of a preventer, but I think the sail has to be got onto that side of the boat without artificial assistance.

First, I'm staying out of the match racing and team racing discussion as I have been a competitor and trained for it but never been an on the water judge and there are experienced judges on this forum who are already discussing the related issues above.  Just dealing with W/L issue in fleet racing.

In fleet racing this is a W/L and mainsail position judgement.  There are many dinghy one design fleets that sail significantly by the lee, they are usually cat-rigged or main-jib boats, no spinnaker.  Some boats such as the Laser can sail 25 degrees by the lee if trimmed and balanced correctly and the competitor retains starboard rights while doing so.  This can be observed in national, international, and olympic level events with on the water judges present.  Since I believe the original poster was discussing fleet racing and not match or team racing, I believe the relevant information is the definition and the rule below:

 

I asked this question here in 2019, there were three answers that Rule 11 is determined by mainsail position as defined in the definition W&L (see link below).  Since then I asked a pair of on the water judges at a national level Laser event and they confirmed this.
Traditional W/L (Rule 11) or does the OD fleet racing style matter?

Thanks everyone for the thoughtful replies. I had initially been asking about fleet racing, and it didn't occur to me that the answer could be different in team or match racing. From reading all the replies carefully,  it seems there is not a consensus on whether the sail can be held there and not full, or whether the sail must be full. I'm surprised there is no appeals case on this. 

Jim, Although not official interpretations of the rules the match racing and team racing calls can be used for guidance when the rules being interpreted are the same as the fleet racing rules.

I would use TR Call G1 to provide your answer, which is consistent with the way International Judges and Umpires I work with interpret the rules..
https://www.sailing.org/tools/documents/20212024CallBookTeamRacing-[26940].pdf

Question 1
A boat sailing downwind on starboard tack bears away until she is sailing by the lee, with the wind continuing to fill her mainsail. Which tack is she on?

Answer 1
Starboard tack. The definition states that 'when (a boat is) sailing by the lee or directly downwind, her leeward side is the side on which her mainsail lies'. Since her mainsail remains filled on her port side, the port side is her leeward side. She is on the tack 'corresponding to her windward side', i.e. starboard tack. 

Question 2
The boat now bears away further, with her boom still out over her port side, because now her crew is holding it there while the wind is backing her mainsail. Which tack is she now on?

Answer 2
Port tack. She is now neither running directly downwind nor sailing by the lee. A boat is on the tack corresponding to her windward side, that is the side towards the wind. There can be no doubt that this side is her port side and therefore she is on port tack.

Question 3
A boat on starboard tack bears away until she heads directly downwind. To slow her speed, her crew holds the boom along the centreline. The wind is hitting the port side of her mainsail, although only her leech moving towards her starboard side indicates this. Which tack is she on?

Answer 3 
Port tack. As she is sailing directly downwind her leeward side is defined as the side on which her mainsail lies. This is her starboard side. She is therefore on port tack.