I'm aware of this rule and agree that the committee boat and the pin end mark are obstructions but as far as the imaginary start finish line between them I find hard to believe can be defined as an "obstruction".
This is a good question as I wonder what the penalty would be for a boat which did go through the start finish line on a downwind leg, when the S.I's state that this is not permitted. I guess the PRO could disqualify them for not sailing the proper course, another competitor could protest them, or could they return back over the line and go around the ends to exonerate themselves? It's not quite so clear cut as the Black Flag Rule.
Grafham have this for some opens (Hence why I know the rule as I helped QA the SI's <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" /> )
At Grafham you are allowed to "unwind"; or you get DSQ'd as the course sailed goes thru an "obstriction";
Best way to imagine it is that each end of the line is a mark of the course that you must pass either PORT or STBD side of both of them, but not "inside" both of them.
The easiest way to word the SI's is to make the LINE an obstriction, and so by definition you have to sail either side of it. You could say something like "on the down wind leg, the CTB and the pin must be passed to PORT
or the CTB and Pin must be passed to STBD (i.e. you cannot cross the line between CTB and Pin and still finish).
Simpler to rank it as an obstruction and job done.......
Texel SI's are Similar in that (I Think) the finish line ranks as an obstruction once you have finished and you are not alllowed to re-cross it - At Texel you get DSQ'd if you do.
In the SI's anything can be defined as "obstructions" and so must be avoided. Some clubs define the cruiser moormings (or a line along the outside moorings). as obstructions; I did the Nab-tower race a few times and I think the Sub pens were defined as obstructions as were a couple of other things.
