I think this is not possible with regard to rule 18 - see  Preamble to Section C and rules 18.1 and 28.1.
For R19 (&R20) it is possible , for example MR CALL B12, MR CALL C1...

Paddy,

As Boris references, the preamble to Section C negates application because your premise includes, " ..  approaching the line to start."

If you are looking to nibble at the edges though, you can explore the question .. "When is a boat approaching the starting-line to start?"

Ang 

.. for instance, assuming these boats are traveling 1 BL every 3 sec's (6.5kts for a 10m boat).. at 30 sec's before the gun ..

Q1) Are these boats "approaching the line to start"? 
Q2) Does the answer to Q1 change if it is under I-flag or P-flag?
Q3) If "yes" to Q1, what if the time before the gun at position 1 is 1min?   2min?

Angelo,

Doesn't MR Call C1 cited by Boris above answer your question?

John, it might .. but I'd ask which answer in MR C1 do you feel definitively answers the questions, given both boats in my drawing seem to be passing the RC on their starboard side (i.e the "required side")?

Angelo
What I said was “before approaching the line to start” 
So section C rules do apply. 
And my understanding is that there is no required side until the start signal. 
Thanks to all who responded. 
Paddy

Angelo 
RRS 18.1 requires that boats have to be leaving mark on same side for the rule to apply. So it never applies at the Start before boats are approaching to start because there is no required side until the start signal. When boats are approaching to start rules of section C turn off. 
So only 19 and 20 apply at the start before boats are approaching to Start
There is also no proper course before the start and that’s why RRS 17 does not apply until the gun. 

Paddy re:”What I said was “before approaching the line to start” 

Doh!  You did indeed!  Crazy how my mind skipped over that.  

“because there is no required side until the start signal. “

Are you getting that language from a case or appeal?

Angelo
Sorry I can’t remember where I read that there is no required side at a start mark before the starting signal. 
I have a memory of the argument. Since there is no proper course before the starting signal there cannot be a required side at the starting marks. 
It might have been from a Dave Perry explanation of rules at the start!
Anyway it’s one of those beliefs I’ve had for many years hope it’s not one of those very annoying common rule misconceptions. 
I’ll research it and let you know. Help anyone else?
Paddy

Paddy, if you examine MR Call C1 that Boris provided, I'd suggest that the language/understanding you used seems inconsistent with Answer's 1 & 2 (below, emphasis added)

If as you state, a starting mark does not have a side until the starting signal, then Answer 2 would imply that a boat can not be approaching the mark before their starting signal. 

But, if you then examine Answer 1, it clearly leaves open the possibility that boats can be approaching the starting mark before the starting signal.

Ang

Paddy,

Could you explain to us a bit more about why you are asking the question and what sort of scenario you have in mind?

Allen
There has been general discussion at our club about the start boat being an obstruction and how boats have rights under RRS of section C before boats are approaching the line to start when rules of section 3 are turned off.
When I started to delve into the nitty gritty of how and when these rules apply I was not confident of my full understanding of all the implications in different scenarios. 
So my op was to start a discussion that would help me to better understand the rules from the preparatory signal until boats start and clear the start line. So far I’ve been helped. 
 
Angelo 

I’ll answer your last email a little later today.
Sorry it’s taken me so long I’ve been very busy over the last few days.
Paddy

John Allan

said Created: 21-Feb-16 20:41

 Angelo Guarino

said Created: 21-Feb-16 20:51

My mistake in looking at Call C1 Answer 2 in isolation from Answer 1.

I understood Answer 2 to be saying that a boat on the course side of the starting line or its extensions.

Answer 1, in particular diagram 1c makes it clear that this is not the case

So, I'm satisfied that @ 30 sec before the starting signal, in your diagram, both boats are approaching the starting mark to start.

About starting marks 'having a "required side"'.  I don't think it's a good idea to treat 'required side' as if it's a defined term.  I'd put it this way.

Until boats are approaching the starting line to start, they are not required to pass the starting marks on any particular side

Except that during the last minute of an I, Z, U, or Black Flag start, when boats on the course side of the starting line extensions are required to leave the starting marks on the side 'outside' the line. 

Another way of looking at this would be to say that under an I, Z, U, or Black Flag start, from the 1 Minute signal, boats should be presumed to be approaching the starting marks to start unless there is evidence to the contrary.

Usually in fleet racing, one would expect all boats to be approaching the starting marks to start with one minute to go.

Angelo

 My statement that there was no required side at a start mark until the gun was wrong. Oops

Having reviewed the Definition of “Sail The Course” I now believe that there cannot be a required side at a Start Mark until the string rule starts “from the time she begins to approach the starting line from its pre-start side . .” 

This means that there cannot be a required side before the “string rule” starts.

If true your quote
"A2 [..]  A boat cannot be approaching a mark to start until approaching to pass the mark on the required side."

seems to say that a boat can never be approaching a mark to start until it is on the pre-start  side of the line.

Allen I think that every boat must have a unique test to determine if it is approaching the line to start. So just applying a time limit would be dangerous.

 
You said;
"So, I'm satisfied that @ 30 sec before the starting signal, in your diagram, both boats are approaching the starting mark to start."

Wouldn't that depend on the performance characteristics of each boat and the tactics employed by each skipper?

You said;
"Another way of looking at this would be to say that under an I, Z, U, or Black Flag start, from the 1 Minute signal, boats should be presumed to be approaching the starting marks to start unless there is evidence to the contrary."

Dangerous to generalize or assume. There could be  so many different situations with boats of different sizes and characteristics.

You said;
"Usually in fleet racing, one would expect all boats to be approaching the starting marks to start with one minute to go."

Most of my sailing is in mixed fleets some boats are approaching the line to start at the 1 minute signal others might not be approaching the line to start until 15 seconds to the start signal.

Paddy

Paddy,

You're raising some very good points which I'll try to discuss in detail later.

You said

I'm having difficulty visualising the 15 second scenario.  Could you perhaps diagram it or describe it in a bit more detail?

John sorry it’s taken me so long to answer. 

I used to sail on a J24 and one of the skippers tactics for a start was to position the boat so that at the one minute gun  it was Starboard of the RCV about 3 boat lengths away where he’d go almost head to wind on starboard sails eased . With about 10  to 15 seconds to go he’d start towards the RCV intending to start just after the boats at the RCV end had started. His aim was to cross the line and tack to port in clear air. He hoped to  come back to the fleet on Starboard. This strategy worked about 70% of the time. Of course sometimes it was a mess. 

Another strategy I’ve observed was at the 1 minute signal to run away from the line for about 20 seconds then tack or gyre towards the line on starboard hoping to find an opening. Again that strategy had mixed results. These examples where from club races in mixed fleets. 

I believe that all boats must be judged on an individual basis to ascertain if they are approaching the line to start. 

Angelo do you agree that starting marks don’t have a required side until boats are approaching them to start and at which time the “String rule” starts. And RRS 18 could not apply before the start

Paddy