The Secretary of Transportation is authorized and empowered to sell to aliens, at such prices and on such terms and conditions as he may determine, not inconsistent with the provisions of section 5 [1] (except that completion of the payment of the purchase price and interest shall not be deferred more than ten years after the making of the contract of sale), such vessels as he shall, after careful investigation, deem unnecessary to the promotion and maintenance of an efficient American merchant marine; but no such sale shall be made unless the Secretary of Transportation, after diligent effort, has been unable to sell, in accordance with the terms and conditions of section 5,[1] such vessels to persons citizens of the United States, and has determined to make such sale; and he shall make as a part of his records a full statement of his reasons for making such sale. Deferred payments of purchase price of vessels under this section shall bear interest at the rate of not less than 51/2 per centum per annum, payable semiannually.