46 CFR §42.05-30

1. (a)As used in this part 42, for a vessel engaged on international voyages or on domestic voyages by sea, the term existing vessel means a vessel which is not a new vessel. With few exceptions an existing vessel is a vessel the keel of which was laid, or which was at a similar stage of construction, prior to July 21, 1968. (See § 42.05-50 for the definition of a new vessel.)
2. (b)-(c) [Reserved]
3. (d)As used in part 44 of this subchapter, for a vessel marked with load lines for special service on a coastwise or interisland voyage, the term existing vessel means one whose keel was laid prior to September 28, 1937. (See § 44.01-20 of this subchapter.)
4. (e)As used in part 45 of this subchapter, existing vessel in all regulations pertaining to a vessel engaged solely on Great Lakes voyages before April 14, 1973, means a vessel whose keel was laid before August 27, 1936. The regulations pertaining to these vessels that are in effect after April 14, 1973, do not use the term existing vessel.
5. (f)As used in part 46 of this subchapter, for a passenger vessel marked with subdivision load lines, the term existing vessel means a vessel whose keel was laid or was converted to such service prior to May 26, 1965. (See § 46.05-30 of this subchapter.)