46 U.S.C. § 57303

(a) Utility Value .— The utility value of a new vessel to be acquired under this chapter for operation in the domestic or foreign commerce of the United States may not be substantially less than that of the obsolete vessel acquired in exchange under this chapter. (b) Tonnage .— If the Secretary of Transportation finds that the new vessel will have a utility value at least equal to that of the obsolete vessel, the new vessel may be of lesser gross tonnage than the obsolete vessel. However, the gross tonnage of the new vessel must be at least one-third the gross tonnage of the obsolete vessel.