33 CFR § 181.29 - Hull identification number display.
Two identical hull identification numbers are required to be displayed on each boat hull.
(a) The primary hull identification number must be affixed -
(1) On boats with transoms, to the starboard outboard side of the transom within two inches of the top of the transom, gunwale, or hull/deck joint, whichever is lowest.
(2) On boats without transoms or on boats on which it would be impractical to use the transom, to the starboard outboard side of the hull, aft, within one foot of the stern and within two inches of the top of the hull side, gunwale or hull/deck joint, whichever is lowest.
(3) On catamarans and pontoon boats which have readily replaceable hulls, to the aft crossbeam within one foot of the starboard hull attachment.
(4) If the hull identification number would not be visible, because of rails, fittings, or other accessories, the number must be affixed as near as possible to the location specified in paragraph (a) of this section.
(b) The duplicate hull identification number must be affixed in an unexposed location on the interior of the boat or beneath a fitting or item of hardware.
(c) Each hull identification number must be carved, burned, stamped, embossed, molded, bonded, or otherwise permanently affixed to the boat so that alteration, removal, or replacement would be obvious. If the 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. A hull identification number must not be attached to parts of the boat that are removable.
(d) The characters of each hull identification number must be no less than one-fourth of an inch high.