While I don't recommend this be done at any sort of high-level event, you could write the change on a white board, display Lima with one sound and "take attendance" of the boats that come up to the signal vessel to read it.  If the boats have radios, make an announcement: "Competitors, competitors, competitors - the RC signal vessel is displaying a white board with a change to the SIs - (state the change) - please approach the signal vessel to acknowledge the change or hail by VHF."

It's critical that you make sure all boats are notified of the change or a very long redress hearing is in your future. 

Yes, passing a written note containing the change is permissible.   Keep in mind that you want to be careful what you are changing.   It should be fair to all boats.  

Thank you, Matt and Peter. Situation: SI's have a schedule of two races. Weather today is beautiful but tomorrow will be a washout. The fleet over the radio wants to race a third race today and none tomorrow. How can I change the SI to change the schedule?

That's more of a problem, since usually a schedule change must be made the day before - depending on how the SIs are written.

The best way to handle that is to build in some flexibility in the SIs that allow you to run more races than normally scheduled.  It requires forethought.

"Three races are scheduled each day. However, one extra race per day may be sailed provided that no class becomes more than one race ahead of schedule."   

We’ve done this often at match race events where we needed to issue a new Pairing List, for example. The issue is to track who has received what, especially when there are different groups/fleets. 

I personally find the ‘oral’ aspect of 90.2(c) ambiguous. I have in the past used Appendix LG template para 2.2 adding ‘by displaying code flag L and displaying changes on a board and/or broadcast on channel [number].’ There are varying opinions on this.

Can we refrain from looking for "loopholes" but rather learn how to use RRS appropriately. It ain't just words. It is perceptions and respect for the sport.

Greg, I apologize for the use of the term "loophole". I meant no disrespect for our sport. I was asked as a non-interested participant about a possible solution to a situation. I agree with Alan and the ambiguity of 90.2(c). The reason for this forum is to discuss those incidents where there is uncertainty in the application of a rule. The responses so far indicate that rule 90.2(c) has some flexibility when it is fairly applied and good for all the competitors.

Thanks Con
The deeper I get into the rules and application the more I “perk up” to words. Certainly this group is about quality alternative solutions. For me that one word sent me in different direction. Looks like there are good ways to solve this problem. 

I think that generally, rule 90.2(c) permits SI to be changed on the water in any way other than orally (but if not orally, how else than in writing?), but the race committee has to do someting (like a checklist) to prove that the change has been communicated to all boats..  To change SI orally, you need a procedure in your SI.

So, written notes on the end of a pole with a clothes peg to hand them to boats coming past, or a white/black board are OK, as long as they are communicated to each boat before the warning signal.

As to changing the schedule of racing by adding a race on a day, firstly, your SI can, and probably should prohibit this.  Secondly, whether or not the SI prohibit it, I don't think you should:  you have told boats what your intentions are in the NOR and SI, they have planned their racing, and their post race activities on that basis.  If you wrote SI to limit the number of races in a day, you should stick with it.

On the other hand, I would not willingly put a number of races per day in Si. For a normal 7 race weekend regatta, you have a first warning signal, you have sunset and other local factors, and you have a no warning signal after time.  Let the RO fit the races in as best he or she can.  It doesn't take an act of genius for competitors to work out how many reaces are lilkely to be in a day.

Coming back to the SI/NOR, while you aren't prevented from changing the SI, by writing, on the water, recall that most NOR will include a Schedule of Races which will probably, and should, have the same information about number or races, and time limits as your SI.  For a change in the schedule of racing contained in the NOR, I would not normally consider that a change on the water, shortly before racing was with 'adequate notice' as required by rule 89.2(b).