According to GovInfo, “the official number of the vessel, preceded by the abbreviation “NO.” must be marked in block-type Arabic numerals not less than three inches in height on some clearly visible interior structural part of the hull. The number must be permanently affixed to the vessel so that alteration, removal, or replacement would be obvious.
If the official number is on a separate plate, the plate must be fastened in such a manner that its removal would normally cause some scarring of or damage to the surrounding hull area.”
The name must be marked in such a way that all characters are not less than four inches in height.
The markings can be any means and of any material, provided that the result is durable markings.
For recreational vessels, the name and hailing port must be marked together on some clearly visible exterior part of the hull. For a commercial vessel, the vessel name and hailing port must be marked on the stern and the name must be marked on the starboard as well as the port bow.
Hailing port markings on a documented vessel must include a place and a State, Territory, or possession of or in the United States. State abbreviations are acceptable.