The main question is whether the SIs have to use the word
"obstruction" in defining the area to be avoided, or is the fact that
it is a prohibited area enough to make it an obstruction for the
purposes of Rules 18 and 19?

If you are writing the sailing instructions and you want it to be
treated as an obstruction, then be sure to use the word "obstruction"
in your declaration as well as "prohibited". That way the SIs will
communicate clearly to the racers that it is to be treated as an
obstruction. It may or may not be a legal requirement, but it never
hurts to be explicit.

Ryk

There are a few other problems associated with a SI that indicates the
Finish Line is a 'prohibited area' unless finishing.

(1) Light air starts, class just gets across the line, air shuts down,
current pushes them back through the start-finish line. Now what?

(2) Most of the SI's identify the situation as 'prohibited' or wording to
that effect but do not spell out a consequence of going through the finish
line before a finish. So, off to the Protest Committee, and the Committee
says fine, but what is the penalty -- none spelled out in the SI's.

(3) Due to rough weather, the RC Boat gets moved around and in effect so
does one end of the Finish Line. Along come boats on a leg, trying to
figure out where one end of the Finish Line is so as to stay clear, but the
RC boat is all over the place, perhaps the line 'grew' substantially from
when the classes were started, and boats searching for the outer pin,
unknowingly cross through this new finish line. It happens. Happened to us
two seasons ago, we were in the lead, and we retired once we figured out
where the heck the other end of the line was (behind us and maybe a few
hundred yards further out and back in relationship to the RC boat). Yes, we
could have continued and appealed, but at the time we did not know the RC
boat was pulled off its position. And since the Start-Finish line is at
times in the middle of some legs, you either take it on the pin or on the RC
boat sides.