In addition to his power to recondition and repair vessels under section
871 of this Appendix, the Secretary of Transportation may remodel and improve vessels owned by the United States and in his possession or under his control, so as to equip them adequately for competition in the foreign trade of the United States. Any vessel so remodeled or improved shall be documented under the laws of the United States and shall remain documented under such laws for not less than five years from the date of the completion of the remodeling or improving and so long as there remains due the United States any money or interest on account of such vessel, and during such period it shall be operated only on voyages which are not exclusively coastwise.