The article describes the process of increasing power efficiency of the ship’s main engines along with reducing power losses on the drive of auxiliary mechanisms. The most common ship cooling systems with two circuits of fresh water and seawater have a disadvantage: the power consumed by the external circuit pump drive does not change when the temperature of the cooling seawater changes. The amount of energy taken from the effective power of the engine by the pump is significant. The technical solution proposed allows to regulate the speed of the mounted pump and to reduce fuel consumption. The chart of the variator in the seawater pump drive of the engine 8ChSP13/14 has been shown. The variator with automatic control is installed between the pump and the engine power take-off shaft, which helps to regulate the speed of the impeller for normal engine cooling. The maximal effect of the proposed modernization can be obtained under the vessel's operation with the seawater temperature below the calculated one. The description of the experimental stand developed in the laboratory of Astrakhan State Technical University, which is simulating the cooling system of the marine engine 8ChSP13 / 14, has been given. The graph of the change in torque versus time is illustrated at the impeller speed of 1 800 rpm. It has been stated that the mechanical characteristics of the pump for different operating modes obtained as a result of experimental studies allow to assess the economic effect from reducing fuel consumption. The results of economic design of installing a variable speed drive in the cooling system of a marine engine are presented as a case study of the vessel Moscow-169 (Astrakhan passenger motor ship).