OPA 90 – For vessels navigating in US Waters- 46 CFR 8104(a)
An owner, charterer, managing operator, master, individual in charge, or other person having authority may permit an officer to take charge of the deck watch on a vessel when leaving or immediately after leaving port only if the officer has been off duty for at least 6 hours within the 12 hours immediately before the time of leaving.
To cater for the above regulation, navigating officers departing ports in the USA enter in column 'X' the hour at which the vessel departed port (Example: 18.5)
The attention of the master and ship owner / charterer is drawn to the words ' only if ' above.
(i) A person violating subsection (a) or (b) of this section is liable to the United States Government for a civil penalty of $10,000.