For the purposes of preserving in the national interest the full availability and usefulness of essential vessels, which, under the provisions of the Neutrality Act of 1939 [
22 U.S.C.
441 et seq.] (or any proclamation issued thereunder), or compatibly with the national interest, cannot be operated in the service, route, or line to which such vessels are assigned pursuant to this chapter, or in which they would otherwise be operated, the Secretary of Transportation is authorized to make adjustments of obligations in respect to such vessels and to make arrangements for the maintenance of such vessels, subject to the provisions of this section and to such rules and regulations as the Secretary of Transportation shall prescribe as necessary or appropriate for carrying out the purposes and provisions of this section. If the Secretary of Transportation, upon written application in respect of any essential vessel, determines after such examination, investigation, and proceedings as he deems desirable, that