Sec. 11.
That during a period of five years from the enactment of this Act the board may annually set aside out of the revenues from sales and operations a sum not exceeding $25,000,000, to be known as its construction loan fund, to be used in aid of the construction of vessels of the best and most efficient type for the establishment and maintenance of service on steamship lines deemed desirable and necessary by the board, and such vessels shall be equipped with the most modern, the most efficient and the most economical machinery and commercial appliances. The board shall use such fund to the extent required upon such terms as the board may prescribe to aid persons, citizens of the United States, in the construction by them in private shipyards in the United States of the foregoing class of vessels. No aid shall be for a greater sum than two-thirds of the cost of the vessel or vessels to be constructed, and the board shall require such security, including a first lien upon the entire interest in the vessel or vessels so constructed as it shall deem necessary to insure the repayment of such sum with interest thereon and the maintenance of the service for which such vessel or vessels are built.