; (5) stable shipbuilding rates of construction should be maintained for each vessel class, utilizing multi-year or block buy contract authorities when appropriate, until a deliberate transition plan is identified; and (6) prototyping of potential new shipboard subsystems should be accelerated to build knowledge systematically, and, to the maximum extent practicable, shipbuilding prototyping should occur at the subsystem-level in advance of ship design. 4. Procurement authorities for certain shipbuilding programs (a) Contract authority (1) Procurement authorized In fiscal year 2021, the Secretary of the Navy may enter into one or more contracts for the procurement of any or all of the following groups of vessels: (A) Three San Antonio-class amphibious ships and one America-class amphibious ship. (B) Two Columbia-class submarines. (C) Six John Lewis-class fleet oilers. (2) Procurement in conjunction with existing contracts The ships authorized to be procured under paragraph (1) may be procured as additions to existing contracts covering such programs. (b) Certification required A contract may not be entered into under subsection (a) unless the Secretary of the Navy certifies to the congressional defense committees, in writing, not later than 30 days before entry into the contract, each of the following, which shall be prepared by the milestone decision authority for such programs: (1) The use of such a contract will result in significant savings compared to the total anticipated costs of carrying out the program through annual contracts. In certifying cost savings under the preceding sentence, the Secretary shall include a written explanation of— (A) the estimated end cost and appropriated funds by fiscal year, by hull, without the authority provided in subsection (a); (B) the estimated end cost and appropriated funds by fiscal year, by hull, with the authority provided in subsection (a); (C) the estimated cost savings or increase by fiscal year, by hull, with the authority provided in subsection (a)