Upon completion of the construction of any vessel in respect to which a construction-differential subsidy is to be allowed under this subchapter and its delivery by the shipbuilder to the Secretary of Transportation, the vessel shall be documented under the laws of the United States, and concurrently therewith, or as soon thereafter as practicable, the vessel shall be delivered with a bill of sale to the purchaser with warranty against liens, pursuant to the contract of sale between the purchaser and the Secretary of Transportation. The vessel shall remain documented under the laws of the United States for not less than twenty-five years, or so long as there remains due the United States any principal or interest on account of the purchaser price, whichever is the longer period. At the time of delivery of the vessel the purchaser shall execute and deliver a first-preferred mortgage to the United States to secure payment of any sums due from the purchaser in respect to said vessel: Provided, That notwithstanding any other provisions of law, the payment of any sums due in respect to a passenger vessel purchased under section 1737(b) of the Appendix to title 50, reconverted or restored for normal operation in commercial services, or in respect to a passenger vessel purchased under subchapter V of this chapter, which is delivered subsequent to March 8, 1946, and which
(i)
is of not less than ten thousand gross tons,
(ii)
has a designed speed approved by the Secretary of Transportation but not less than eighteen knots,
(iii)
has accommodations for not less than two hundred passengers, and,
(iv)
is approved by the Secretary of Defense as being desirable for national defense purposes, may, with the approval of the Secretary of Transportation be secured only by a first-preferred mortgage on said vessel. With the approval of the Secretary of Transportation, such preferred mortgage may provide that the sole recourse against the purchaser of such a passenger vessel under such mortgage, and any of the notes secured thereby, shall be limited to repossession of the vessel by the United States and the assignment of insurance claims, if the purchaser shall have complied with all provisions of the mortgage other than those relating to the payment of principal and interest when due, and the obligation of the purchaser shall be satisfied and discharged by the surrender of the vessel, and all right, title, and interest therein to the United States. Such vessel upon surrender shall be
(i)
free and clear of all liens and encumbrances whatsoever, except the lien of the preferred mortgage,
(iii)
in as good order and condition, ordinary wear and tear excepted, as when acquired by the purchaser, except that any deficiencies with respect to freedom from encumbrances, condition, and class, may, to the extent covered by valid policies of insurance, be satisfied by the assignment to the United States of claims of the purchaser under such policies of insurance. The purchaser shall also comply with all the provisions of section
868 of this Appendix.
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