It's an act or omission so the rc should seek redress to alow the position crossing the line to stand

RRS 64.3 Decisions on redress. If the finishing times were taken from the same point on the signal boat for all boats the PC can decide that it is fair for all boats to accept the times taken

Sure, but unless and until there is a hearing by a protest committee:

Have any boats finished?

How should the race be scored?

I would abandon the race under rrs 35 and ask for redress for all boats.

Michael B, what rule or case allows PC to grant this? (I ask because I was once RC in a similar situation and while there was no protest, there was lots of muttering. Would love to clear my conscience).

Ok, similar situation except now 32.2 shorten course applies. Does RC need to fly blue as well as stated in SIs?

Definitely not.

See rule 32.2, which redefines the finishing line.

32.2.

If the race committee signals a shortened course (displays flag S with two sounds), the finishing line shall be,

Agree with Mike Butterfield.  Rule 62.1 (a) Race officer should seeks redress for the fleet due to the improper omission of the race committee.  Protest committee should uphold the request for redress and in accordance with rule 64.3 give a result which is as fair an arrangement as possible for all boats affected.  In this case all boats considered to have finished in the order they crossed the finishing line.  

In the present situation, have the competitors made a mistake when crossing the line between the Race committee finishing Vessel (FV) and the buoy scheduled as being the finishing mark?
 NO
 Fully agree with Mickeal B : " It's an act or omission so the rc should seek redress to alow the position crossing the line to stand"
 And for the "old gaffers", I would like to remind the arrival of one race for the windsurfers at the Olympic Games of BARCELONA. 

The mooring of the finishing mark upwind to the finishing vessel (the last leg for boards was square to the wind direction) went being wrapped around the propeller shaft of a TV boat. Both linked, they started to drift towards the finishing vessel, then alongside the FS, then to leeward of the FV. 
 If the firsts boards to finish sailed according to the rule, some later sailed upwind to the FV as no finishing mark was on place according to the rule, and afterwards when the mark appeared to leeward of the FV, some sailed to windward of the FV and others to leeward. Everyone has been ranked according to his position in the fleet and the race has been validated.

Consider a similar situation a little more complicated:

1.  A windward-leeward course with upwind finish at a line above the windward mark, with each leg being .75 to 1 nm long.  
2.  An RC boat acting as the Finishing Boat fails to put up a blue flag.  The two lead boats, far ahead of the rest of the fleet round the windward mark and continue back to the leeward mark even though the course description did not describe such an additional leg.
3.  The RC boat at the supposed finish line radios the PRO (on the Signal Boat at the bottom of the course) that he is on station.  PRO asks, "Do you have a blue flag up?"  Response:  "Uh oh.  Now we do."  This is after the two lead boats have rounded about which the PRO is now informed.
4.  The rest of the fleet (about 12 boats) finishes properly, continuing past the windward mark (though technically not a mark on this leg) to the finish line consisting of the RC boat, now with a blue flag, and a nearby buoy.
5.  PRO sees the two lead boats approaching the leeward "mark" and instructs another RC boat to inform them not to go to windward because they will miss the start of the next race as all the other boats are coming back to the bottom of the course to the Starting Line Boat (the Signal Boat), and that they finished when they rounded the windward mark.
6. PRO files a request for redress for the two lead boats.

What should the PRO have done?  What did the Protest Committee do?

Joe,

Assuming that there is an absolute cut off time limit imposed by a SI similar to:

Boats failing to finish within 15 minutes after the first boat sails the course and finishes will be scored Did Not Finish without a hearing. This changes rules 35, A4 and A5.

 And assuming that the RO, by some sort of interpolation has scored the boats with places as he promised them.

Checked that the finish boat was displaying the blue flag earlier.  On a .75nm leg, he could see the finishing line vessel from the start.

Scored the boats DNF as the SI required.

If there was no SI like the above, then the fact remains that the boats have not finished, and therefore should be scored DNF, despite the promise of the RO.

If the above SI was in place and if it was clearly not possible for the boats around the leeward mark to return upwind and finish within the time limit, the advice of the RO not to attempt to do so is unproblematic.  If, on the other hand, there was time for the boats to finish within the time limit, the advice was an improper action that may have prevented them from finishing.

There is no way that a RO or a race committee can deem a boat to have finished other than by crossing the finishing line or as may be prescribed in the SI.  It is an improper action for the race committee to give the boats a score other than DNF.

However the boats were scored, the RO was right to request redress for the boats.

What did the Protest Committee do?

Not displaying the blue flag as boats were approaching the finishing linen to finish was an improper omission by the race committee which may have made the places or scores of the boats significantly worse, so one requirement of the stem of rule 62.1 and the requirement of rule 62.1(a) are met.

But by sailing away from the finishing line after sailing the course was a fault of the boat's own, so the 'no fault of her own' requirement of rule 62.1 is not met.

Boats are not entitled to redress.

If the RO had given the boats place scores, and no other boat protests there is no power of the protest committee to alter those scores  (Rule 63.1 a boat shall not be penalised without a protest hearing) or to protest them (Rule 60.3 a protest committee may ... not [protest a boat] as a result of information arising from a request for redress).  However, the protest committee might draw the attention of the race committee to the requirement that the boats should have been scored DNF and suggest that the race committee correct their mistake in accordance with rule 90.3(c).

If it would have been possible for boats to return upwind and finish within the time limit, the advice or instruction from the RO not to do so was an improper action, different from the failure to display the blue flag.  Complying with that advice or direction can not be said to be a fault of the boat's own.

This improper action did not cause the boats to sail away from their leading positions when they previously approached the finishing line.  Nothing can give them back those positions.

It may have made the place or score of the boats worse, but the protest committee would need to carefully consider whether this was significantly worse:  is the change from DNF, scoring in accordance with RRS A5.2 Entrants plus 1, significantly different from scores for last or second last place?  Protest committees are generally wise to treat any decrease in a boat's score as significant, and in terms of perception of fairness, it is probably a good thing to give a boat, entitled to redress, something if there has been an improper action or omission by race officials.  So redress of last and second last places would be appropriate.

John,
Thanks for your insightful analysis.  But one factual correction:  the PRO could not discern a blue flag on the Finishing Line boat, against a dark green forest about .5 mile behind it.  That's why he hailed the boat.  Even when it was up, it was not easily seen.  Perhaps we need a re-think on blue flags as finishing marks.
The Protest Committee granted the two boats redress.

Joe,

Checking for the blue flag was a counsel of perfection after the event.

Hands up anyone who hasn't snuck the blue flag up half way through a finishing sequence.

I wouldn't want to tackle the flag being blue.  If you have a local visibility problem, perhaps you could find a lighter more prominent shade of blue.