46 U.S.C. § 56501
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.