No doubt others will offer their opinion.
My only experience with RRS vs CoRegs was with long distance races when the SIs specified when the CoRegs would replace the RRS.
My memory was from 30 minute before sunset to 30 minutes after sunrise.  Sunset and sunrise times are easily found for any local.
There might have been an additional condition to cover fog, but I can't remember the wording.

ColRegs are always in force.
Others will join in to enhance this post however the basics are, when you agree to race, you agree to finer collision avoidance rules.
For example, paraphrased, ColRegs say that if there is risk of collision, the stand on vessel shall continue and the keep clear vessel shall alter her course early and substantially.
When you agree to race, the others who are racing are anticipating that the keep clear will keep clear.  In addition ColRegs require that stand on vessel to take action if it appears that the keep clear vessel does not "appear" to be "giving way" as apposed to a racing vessel does not have to anticipate that a keep clear vessel will not keep clear.
I have seen SIs written that cancel the RRS at Dusk and revert to ColRegs thought civil night so you may want to consider that.
I would not want to round a busy leeward Mark while governed only by ColRegs.

Hi,

Been involved with a number of Offshore Races. Didn't find Col Regs helped much. We now use Appendix RV. Our SI is below:
APPENDIX RV (Reduced Visibility)

Except when otherwise stated in the Sailing Instructions, the rules of RRS Part 2 shall not

apply between the times of sunset and sunrise as stated in the Sailing Instructions and shall

be replaced with RRS Appendix RV or by Government right-of-way rules. For the purposes of

this event the time of sunset shall be considered 18:30 and the time of sunrise 06:30. A boat

may take a time penalty as specified in  the Sailing Instructions when she may have broken

this rule while racing.
Government right of way rules cover the local government's rules about sensitive areas!! Basically you have no rights!!
Appendix RV is easily understood and works well for us in very congested areas. 

Hi Peter, 
yes, we use Colreg from local Sunset time to local Sunrise time. Specified in SI. Say=ing that Colreg replace RRS part 2 for competitors
Usually during coastal/offshore races .
And of course Colreg are always in force with other boats on the water

There is an interesting relationship between the RRS and IRPCAS or local government rules.  Historically, the COLREGS are now the IRPCAS as adopted by the International Maritime Organization (IMO).  The IRPCAS only apply outside of demarcation lines.  Inshore of that it is the local jurisdiction and whatever rules they have.  In the US that is either the USCG rules or state rules, but could even be county or city rules, depending on where you are.

For brevity, IRPCAS here will mean the IRPCAS or other governmental rules that apply.

When you are racing under the RRS (the NoR states that the RRS apply and on the entry form you have agreed to be bound by the RRS) you have entered into a private contract between yourself and any other boats that are also racing under the RRS that the RRS will replace the IRPCAS for determining the rights and obligations of the two boats.  They are not additive.  Because vessels not racing have not entered into the contract, the IRPCAS apply between a non-racing vessel and a racing boat.

The purpose of the IRPCAS is to keep vessels away from each other to minimize the chances of a collision.  They don't work well at all when boats are converging on a single point or in close quarters, like at starting lines, marks, and finishing lines.

One way to test the effects of this switch would be to look at the WS Cases and see how the situations would resolve themselves using the IRPCAS instead of the RRS.   I think you will find that the results are not what you want.  One example is that, under the IRPCAS, a boat on starboard that is overtaking a boat on port must keep clear.  This is the reverse of what we have under the RRS.

IRPCAS (Colregs) do not apply between two boats both racing under RRS. This is very well established legally, with cases going back to the 19thC, so its always one or the other, never both.

As for whether IRPCAS would better serve your particular race, I think one has to consider that their intention is to keep boats sufficiently far apart that they cannot hit each other, and for the give way boat to be confident that the stand on boat will not change direction. If your event will not normally have boats in close proximity, and there is no problem if a stand on boat has to maintain her course even if it prevents her rounding a mark, starting correctly or whatever, then you may well find IRPCAS suitable. But if you envisage competitors being in close proximity then Colregs simply won't work, which is why RRS' predecessors were formulated in the first place.

Thank you all for your insights into the when, why and where to use RRS and IRPCAS (Colregs).

I have been driving since I first posted here and will be much better informed at my morning meeting where this will be the topic of discussion.

Good morning from Greece.

Here generally we are following WS guide line (Notice of Race Guide), to use IRPCAS or Appendix RV, from sunset to next sunrise, in replacement of RRS Part 2. Actually, I haven’t seen any race not to use them.

There is an issue though with appendix RV, which I realized in an overnight race to HYDRA Island, where boats came close enough to one-two boat lengths, due to lack of wind. Into RV there is a restriction of 40 meters between the boats for those situations, IRPCAS haven’t.  Also, IRPCAS PART B Steering and Sailing Rules, Rule 12 (Sailing Vessels) and 13 (Overtaking), along with the rest of the Rules of course, are covering the safety sailing at night or during the day due to lack of visibility (fog). So, for local level races I prefer to use IRPCAS and for higher level races, anybody may overcome this inconvenience and take permission from WS to alter these distances to the demands of the race.  

Appendix RV was discussed in a previous forum (Appendix RV).  I've not come across any use of it since then; all clubs I know that run offshore races overnight use IRPCAS between the hours of sunset and sunrise.  

I would consider if the boats were likely to come across none racing boats.
Would there be ias so the boats racing can identify each other?
Is ias mandatory for the boats?
Are there pinch points or market where you need rrs 19 or 18 which are not in the colour regs.

Offshore/overnight races are generally run under WS Category 3 or higher, which requires boats to have an AIS transponder, and SIs typically include a requirement that it is switched on and working at all times and displays the boat's racing name (WS misses that bit!).  But for the Rolex Fastnet Race, you would be going through a list of 450 entries to see if the boat you identified on AIS was entered or not; a boat that is entered may not still be racing (retired and motoring in the direction of the course or already finished and returning); on other weekends there could easily be more than 1 offshore race in progress and you're not likely to know the entries in another race or whether their SIs invoke Appendix RV.  So, AIS is very useful for collision avoidance but only if all boats are applying the same rules, which I think have to be IRPCAS.  

Meant to comment about pinch points/marks as well.  You are right that IRPCAS cannot cover mark rounding (the 'stand on boat' must do exactly that, they are not allowed to turn a corner).  But Appendix RV doesn't work well either (refer to quote in the previous thread) and is positively unhelpful at other times.  Bearing in mind that offshore races are always started in daylight and there is usually a few hours of daylight before dark, I think the onus is on the race course setters to avoid mark roundings in the dark at early stages in a race when a fleet is still bunched; at later stages, the fleet would typically be more spread and mark rounding less of a between-boat issue.  

That isn't safe.  Sometime ago whilst cruising under sail, we gave way very late when we realised that the boat with a steaming light was using it to illuminate a spinnaker!  

Not until much closer - the lights are required to be visible at a distance, spin aren't.  That's why we have rules about showing lights.