When can a race be shortened or abandoned after the start?

When (rule 32.1). After the starting signal the race committee may shorten the course or abandon the race:

• because of foul weather;
• because of insufficient wind making it unlikely that any boat will sail the course within the race time limit;
• because a mark is missing or out of position; or
• for any other reason directly affecting the safety or fairness of the competition.
  It may also shorten the course so that other scheduled races can be sailed, or abandon the race because of an error in the starting procedure. However, after one boat has sailed the course within the time limit, if any, it must not abandon without considering the consequences for all boats in the race or series.

How to shorten (rule 32.2). Display flag S with two sounds before the first boat crosses the finishing line. The finishing line is then (a) at a rounding mark, between the mark and a staff displaying flag S; (b) a line the course requires boats to cross; or (c) at a gate, between the gate marks.

How to abandon (rule 32.3). Display flag N, N over H, or N over A, with three sounds.

Source: Racing Rules of Sailing 2025-2028, rule 32, https://cdn.revolutionise.com.au/site/ofeyzh6cn3tumfga.pdf