46 U.S.C. § 56501: Definition

In this chapter, the term “essential vessel” means a vessel that is—(1)(A) security for a mortgage indebtedness to the United States Government; or (B) constructed under this subtitle or required by a contract under this subtitle to be operated on a certain essential foreign trade route; and (2) necessary in the interests of commerce and national defense to be maintained in condition for prompt use.