The Z flag is one of several flags that a race committee can use to penalize boats that are OCS (on course side) before or at the start of a race. The use of the Z flag is described in rule 30.2 of the Racing Rules of Sailing. (The other flags used to indicate starting penalties are the I flag, see rule 30.1, and the infamous Black flag, see rule 30.3.)

According to rule 30.2, if the Z flag is displayed, then if your any part of your boat or your crew is in the triangle formed by the ends of the starting line and the first mark at any time during the last minute before the starting signal, then you have broken rule 30.2 and your score for that race shall include a 20% scoring penalty. If you break rule 30.2, no hearing is required to penalize you--it is automatic. The scoring penalty is defined by rule 44.3, and is usually 20% of the number of boats that are entered in that race, rounding 0.5 upward. But under the penalty, you cannot be scored worse than DNF (Did Not Finish) boats.

It is important to realize that once you have broken this rule the penalty stays with you even if the race is restarted or resailed (though not if postponed or abandoned before the start of the race). And if you break the rule again when the race is restarted, you get another 20% scoring penalty added to your score--automatically! This rule applies whether you are OCS at the start or not. If you are OCS under the Z flag, you must still return to the pre-start side of the course before starting, and your score will still reflect the 20% scoring penalty. But even if you are over the line in the last minute and then clear yourself below the line before the start, you will be penalized under rule 30.2.

According to rule 26, the Z flag must be displayed by a race committee at the preparatory signal during the starting sequence. This is typically at four minutes before the starting signal, but not always--sometimes the start sequence is shortened to three minutes, and the preparatory signal is made at two minutes. The Z flag may be displayed by itself or in combination with the I flag. If displayed in combination with the I flag, then you must round an end of the starting line before starting and you will still be given the starting penalty.

Example: You are racing in the third race of a national championship regatta. There are 50 boats entered in the race. After two general recalls, the race committee hoists the Z flag at four minutes before the start. At 42 seconds before the start, as you are luffing on the starting line, a leeward boat luffs you up and the race committee sees the bow of your boat poke over the starting line. However, you duck your bow back below the line and the start is clean--there are no OCS boats. You sail a great race and finish in first place. That night, your celebration is cut short when the results are posted--your score is an 11, which is your score of 1 plus 20% of the boats that entered, 10, equalling 11.