46 U.S. Code § 12502 - Identification numbers, signal letters, and markings
Subsection (a) of this section requires the Secretary to maintain a unique numbering system for vessels of the United States. The Secretary may use existing vessel numbering systems such as the hull identification number for this purpose, but must ensure that no two existing vessels of the United States have the same number. The Secretary should consider and propose the use of a 17-digit number to implement this requirement. This subsection also requires the Secretary to maintain a system of signal letters and to record vessel names for documented vessels. The Secretary is authorized to establish other vessel identification markings for vessels of the United States, such as hailing ports.
Subsections (b), (c), (d), and (e) contain other requirements and restrictions concerning vessel identification markings.