I hear lots of confusion around the rules before the start but I don’t think they are too complex. The first point is that before the start there is no proper course so any rules about not sailing above proper course do not apply. We are back to the more basic rules, generally meaning windward/leeward, but also key is rule 15 and rule 16

All these situations (and more) can be found on the web at http://game.finckh.net/situat/tit_gbr/start_e.htm

(Screens reproduced by kind permission of Uli Finckh)

15 ACQUIRING RIGHT OF WAY

When a boat acquires right of way, she shall initially give the other boat room to keep clear,
unless she acquires right of way because of the other boat's actions.

Also

16 CHANGING COURSE

16.1 When a right-of-way boat changes course, she shall give the other boat room to keep clear.

This is key because you do not have to anticipate another boat’s action. When a boat gains an overlap from clean astern she has to initially give the other boat room to keep clear. If the other boat is stopped she has to START trying to keep clear which probably means sheeting in and luffing as she gains steerage way. Also if the leeward boat luffs which she is entitled to do she must give the other boat room to keep clear.

See http://game.finckh.net/situat/tit_gbr/l09_e.htm

Note Blue is clear astern of Red – before the start. Rule 12 applies
At this point Blue who is Clear astern must keep clear.

Red is all but stopped, sail flapping so Blue rapidly gains an overlap

Blue sailing faster starts to overlap red. (Note there is a gap between

them – there was plenty of room for blue to gain the overlap) At this

point Red becomes windward boat so Blue gains right of way.

Now Blue starts to luff (he has not passed beyond head to wind)

DOES RED HAVE TO KEEP CLEAR?

Answer YES!

In accordance with the definition Leeward and Windward, both boats are on starboard tack,
therefore rule 11 applies as soon as they are overlapped and the blue boat (leeward) will
have right of way at that time. At the moment when the overlap begins, Red must begin to
keep clear and shall react to the luffing of the blue boat. Blue's course change to windward
does not infringe rule 15 and rule 16. Rule 17 does not apply, because there is no proper
course before the starting signal.

Gareth notes. Blue can’t luff violently, because he gained right of way he has to give Red a chance to keep clear, but equally Red must start trying to keep clear (sheet in and then luff).  If Red is a stopped Topper and Blue a much faster boat Blue can’t just luff into Red , she must give room for Red to get started – that might mean more room because red will initially make leeway as she start to move before the centreboard starts to work effectively. So long as Red does all she can to keep clear she is OK, but doing nothing is not an option. If Blue pushed in so close that Red cannot keep clear then Blue is wrong under rules 15/16.

So now let’s make it a bit more complex by adding a third boat...

http://game.finckh.net/situat/tit_gbr/s59_e.htm

This time we have three boats, Blue and Green are almost stopped sails flapping.

Red is approaching fast from behind.

Red starts to overlap blue and tries to push in. Is he allowed to?

The answer is NO.

Green is right-of-way boat for Red and Blue and therefore obstruction. When Red
establishes an overlap, right of way changes. Rule 11 requires Blue to keep clear.
Rule 15 and because of changing course rule 16.1 requires Red to give Blue room
to keep clear. From the time the overlap begins Blue is unable to give Red room to
pass safely between Blue and Green. Therefore Red is not entitled to room to pass
between Blue and Green (See last part of rule 19.2(b)).

Now here’s one that I had to think about a bit...

http://game.finckh.net/situat/tit_gbr/m12_e.htm

Green is stopped on the line, Blue is coming down the line, Red is closing

from below the line.

Can BLUE ask for room to pass behind Green?

The answer is YES (which initially surprised me)

The green boat is an obstruction, because both Red and Blue must keep clear of Green
under rule 11 and rule 12. Therefore rule 19.2(b) applies and the outside boat (Red)
must give the inside boat (Blue) room to pass the obstruction (Green).

The point is that the rules passing an obstruction are a bit like the rules rounding marks...

19 ROOM TO PASS AN OBSTRUCTION

19.1 When Rule 19 Applies
Rule 19 applies between boats at an obstruction except when it is also a mark the
boats are required to leave on the same side. However, at a continuing obstruction,
rule 19 always applies and rule 18 does not.

19.2 Giving Room at an Obstruction
(a) A right-of-way boat may choose to pass an obstruction on either side.

(b) When boats are overlapped, the outside boat shall give the inside boat room
between her and the obstruction, unless she has been unable to do so from the time
the overlap began.

(c) While boats are passing a continuing obstruction, if a boat that was clear astern
and required to keep clear becomes overlapped between the other boat and the
obstruction and, at the moment the overlap begins, there is not room for her to pass
between them, she is not entitled to room under rule 19.2(b). While the boats remain
overlapped, she shall keep clear and rules 10 and 11 do not apply.

I hope you found that helpful, any comments or questions let me know

Gareth