Section 173.21 - Certificate of number required(a) Except as provided in §§ 173.13 and 173.17 , no person may operate a vessel to which this part applies unless they have on board in hard copy or digital form: (1) A valid certificate of number or temporary certificate for that vessel issued by the issuing authority in the State in which the vessel is principally operated; or(2) For the vessel described in paragraph (b) of this section, a copy of the lease or rental agreement, signed by the owner or his or her authorized representative and by the person leasing or renting the vessel, that contains at least:(i) The vessel number that appears on the certificate of number; and(ii) The period of time for which the vessel is leased or rented.(b) Section 12304(a) of Title 46, United States Code, states in part: The certificate of number for a vessel less than 26 feet in length and leased or rented to another for the latter's noncommercial operation of less than 7 days may be retained on shore by the vessel's owner or representative at the place from which the vessel departs or returns to the possession of the owner or the owner's representative.CGD 72-54R, 37 FR 21399, Oct. 7, 1972, as amended by CGD 77-117A, 43 FR 17941, Apr. 27, 1978; CGD 89-048, 54 FR 27002, June 27, 1989; USCG-2003-15404, 68 FR 37742, June 25, 2003; USCG-2003-14963, 77 FR 18699, Mar. 28, 2012