Mike a couple threads here that discuss this  closed-line topic as well ....

https://www.racingrulesofsailing.org/posts/145-closed-line-and-rules-18-and-19

https://www.racingrulesofsailing.org/posts/141-closed-line-on-a-course

Thanks Angelo.    I did search the forum before I posted, but didn’t find anything.   The word CLOSED would have found threads, but it,s not a word I’ve heard used in my part of the world.  
Thankfully my opinion agrees with the balance of forum opinion.  The line should be defined in the SI,S as an Obstruction.  

Currently the search tool only searches the published content on the site (RRS, Cases, etc).   

I know that making the forums part of the search tool (or having a dedicated forum search tool) is on Paul's "suggestion list" ... (which is quite long and he only has 10 fingers to type with).

This is the terminology that we are using in some of our SI's... using a one turn penalty for Cats.
1.1.        The start and finish lines shall be considered as an obstruction at all times while racing.  A boat crossing the S/F line and breaking this rule shall not correct her error. However, she will be exonerated by completing a one turn Penalty, otherwise she shall be scored DSQ without a hearing. This changes rule 28.1 and A5. The starting and finishing lines and the marks that form it are obstructions for boats subject to this rule and therefore rules 19 and 20 may apply between boats while they are approaching and passing it.

Overall I am not a fan of "Closed" or "restricted" S/F lines. When I do use them I put in the description of the limited S/F that this line is defined as an Obstruction. This is allowed by the definition. Absent this wording a boat may a force competitor to break this line and therefore be DSQ. There are no "turns" available to do a penalty.

I often see the reference that a S/F line is restricted but there is no description of what the limit is. People always tell me that it is common usage and should be understood as meaning do not cross. The word rstriction only means that here is some limit. Perhaps the sentance means don't take pictures, must wear a hard hat or some other confine. I suggest a description of the limitation be includedwhan using this stricture.

Paul must see some really interesting races.
Everyone starting and then doing a one turn penalty to exonerate their breech of the SI! They are racing from the preparatory signal.
Seriously, it illustrates the difficulties of writing SI's, some of the suggestions in the previous threads were probably better examples but may be out of date now.

Dick Rose wrote a column 9-years ago in Sailing World on this very topic and suggested the following wording for the sailing instructions:
"After completing the first leg of the course, a boat shall not cross the starting line. A boat shall not cross the finishing line until she is completing the last leg of the course and finishes. A boat that breaks this rule shall not correct her error. However, she will be exonerated if she takes a Two-Turns Penalty (see rule 44.2). This changes rule 28.1. On a leg of the course from W to L or from L to W, boats may leave the line between S and F either to port or to starboard.

Now we need a definition of "cross."
If only the bow pierces the line but backs out and the boat does not sail through, the dictionary would say she has not crossed.

Philip, in regard to “cross”, I think it’s universally understood in the racing-sports world be it running, cycling, horses, nascar, skiing, etc, that a competitor “crosses the line” when any physical part of the competing entity breaks the plane of the line. I can’t think of a single sport which measures “crossing” as when completely across. (Photo finishes don’t look at the back of the horse!) ;-)

That does bring up the question of if there is a word or phrase that conveys that meaning. In sailing we have the concept of crossing and then “clear” of the line, but that can be accomplished by crossing with the bow and backing away.

We could maybe say “traverse” the line? or talk about being “fully across” or as I did earlier “completely across” the line to covey that meaning. 

How about "keep clear" of the line?

when I agree with others on this thread that generally speaking it's a bad idea to restrict the starting or finishing lines, as doing so needlessly restricts the available options for boats racing.  However, it's almost a necessity when the start/finish is in the middle of the course and starts might be happening when boats are racing up or down legs that would take them through the line. 

Paul, your terminology doesn't work, as Nigel cryptically pointed out.  Boats are racing from the preparatory signal until they have finished and cleared the finishing line, so the rule you quote forbids starting and finishing!  Of course, one could add, "... unless starting or finishing", but that opens up other cans of worms.  Suppose for example that a boat crosses the starting line just before her starting signal.  Then according to the definition start, she was not starting when she crossed the line.  She was, however, racing, so in addition to returning to the line she owes a turn penalty!

I also have a problem with John Super's solution, or any solution that simply designates the line as an obstruction.  I know this weird, but the definition obstruction doesn't actually require that the object obstructs anything, and there's no rule prohibiting a boat from sailing through an obstruction.  If the line is designated as an obstruction and a boat sails through the line and is protested, under what rule do you penalize her? 

There's another way to handle this entire question -- make the start/finish marks marks of any leg on which boats might cross the line and you don't want them to do so.  Here's a sailing instruction from the Morgan Cup, a New York Yacht Club team-race event (slightly modified by me):

This strongly discourages boats from crossing the start/finish line, without actually forbidding it, and avoids all the issues with "cross" and with disqualification or alternative penalties.  Note however that if the leg is upwind, a boat approaching the starboard-end mark on port tack is pretty well screwed, and might end up crossing the line to avoid a starboard-tack boat.  But there are problems when boats are racing upwind during a starting sequence, anyway -- simply forbidding crossing the start/finish line doesn't keep boats from sailing close by and interfering with the start in progress. 

With respect to "cross", the word is used in many sailing instructions when crossing a certain line would be dangerous, and seems to cause no problems.  For example, a standard sailing instruction for any race run in San Francisco Bay where boats might sail along the City Front near St. Francis Yacht Club is:

Generally, the only penalty for this infraction is DSQ.  Because there are strong currents in the area that can induce boats to tack up the shoreline, this SI is actually broken occasionally and the offending boat disqualified.  

Rob:
Wouldn't your suggested language mean that if a boat does cross the starting/finishing line while sailing from M2 to M3, they would have to "unwind their string" to correct their error and thereby compound the problem?

I like Dick Rose's language (above) better.

Here is some wording I used in a recent regatta where I did not want the finish line crossed. 

For reasons of safety, the Finish Line shall not be crossed except when finishing and when clearing the line after finishing. The Finish Line is defined as an obstruction.

If a boat breaks this rule she can be DSQ for breaking a rule in the SIs.

If you don't want to be too harsh you can make the penalty discretionary.

BTW, if we get a couple versions that are pretty bullet proof, we made a Forum section for SI/NOR language that solves specific issues. We could start a “closed-line” thread and copy a couple options there to make it easy to find later.  - Ang

Rob, in regard to the rounding direction language. ...

A while back I came up with concise language which I think works going either direction.   Assuming the marks of the line are P and MB, W and L are the windward leeward marks. 

Course: start -  W - L - W - L - finish. 

This doesn’t solve the “keep-sailing/shall not correct” goal in some solutions though. 

PS:  Actually, with the language “on legs bounded by marks W and L,”, I prob don’t need the starting/finish language. So it reduces to ..

"Except as necessary to sail the course, boats shall stay clear of the (starting) (finishing) line after starting."