To be in compliance with the USCG, a documented vessel must be marked properly. Specifically, “NO.” (“N” and “O” followed by a period) must be marked before the vessel’s assigned official number. This must be marked in Arabic numerals that are at least three inches high and in block type. Additionally, these must be placed on some interior structural part of the hull where they’ll be clearly visible and, they must be permanently affixed. In terms of “clearly visible,” use common sense.