What do the I, Z, U and black flag starting penalties mean?

These are the rule 30 starting penalties. Each is signalled as the preparatory flag at the 4 minute signal (the P, I, Z, Z with I, U or black flag).

• I flag (rule 30.1): if any part of a boat's hull is on the course side of the starting line or one of its extensions during the last minute before her starting signal, she must sail across an extension so that her hull is completely on the pre-start side before she starts.
• Z flag (rule 30.2): no part of a boat's hull may be in the triangle formed by the ends of the starting line and the first mark during the last minute. An identified boat receives, without a hearing, a 20% Scoring Penalty. She is penalised even if the race is restarted or resailed, but not if it is postponed or abandoned before the starting signal. If identified again in a later attempt to start the same race, she receives an additional 20%.
• U flag (rule 30.3): same triangle restriction. An identified boat is disqualified without a hearing, but not if the race is restarted or resailed.
• Black flag (rule 30.4): same triangle restriction. An identified boat is disqualified without a hearing, even if the race is restarted or resailed, but not if it is postponed or abandoned before the starting signal. If there is a general recall or the race is abandoned after the starting signal, the race committee must display her sail number before the next warning signal, and she must not sail in the restart or resail.

Source: Racing Rules of Sailing 2025-2028, rules 26 and 30, https://cdn.revolutionise.com.au/site/ofeyzh6cn3tumfga.pdf