Such contract shall provide, except as the parties should agree upon a lesser amount, that the amount of the operating-differential subsidy for the operation of vessels in an essential service shall equal the excess of the subsidizable wage costs of the United States officers and crews, the fair and reasonable cost of insurance, subsistence of officers and crews on passenger vessels, as defined in section
1183 of this Appendix, maintenance, and repairs not compensated by insurance, incurred in the operation under United States registry of the vessel or vessels covered by the contract, over the estimated fair and reasonable cost of the same items of expense (after deducting therefrom any estimated increase in such items necessitated by features incorporated pursuant to the provisions of section
1151
(b) of this Appendix) if such vessel or vessels were operated under the registry of a foreign country whose vessels are substantial competitors of the vessel or vessels covered by the contract: Provided, however, That the Secretary of Transportation may, with respect to any vessel in an essential bulk cargo carrying service as described in section
1121
(b) of this Appendix, pay, in lieu of the operating-differential subsidy provided by this subsection (b), such sums as he shall determine to be necessary to make the cost of operating such vessel competitive with the cost of operating similar vessels under the registry of a foreign country. For any period during which a vessel cruises as authorized by section
1183 of this Appendix, operating-differential subsidy shall be computed as though the vessel were operating on the essential service to which the vessel is assigned: Provided, however, That if the cruise vessel calls at a port or ports outside of its assigned service, but which is served with passenger vessels (as defined in section
1183 of this Appendix) by another subsidized operator at an operating-differential subsidy rate for wages lower than the cruise vessel has on its assigned essential service, the operating-differential subsidy rates for each of the subsidizable items for each day (a fraction of a day to count as a day) that the vessel stops at such port shall be at the respective rates applicable to the subsidized operator regularly serving the area.