Abstract: Fore and aft headers are parallely spaced apart and connected by a plurality of tubes or pipes extending normal thereto and forming therewith a grid type heat exchanger. The headers are streamlined and fixed to the bottom of a marine vessel with the tubes spaced outwardly from the vessel's bottom and extending longitudinally therewith. The inboard and outboard sides of the tubes are narrowed to define wider entrances and exits between tubes than the spaces between adjacent sides of the tubes intermediate the inboard and outboard sides. The streamlined headers reduce flat plate resistance and resistance due to turbulent water flow to promote laminar water flow therebetween from forward to aft. The widened entrances and exits between tube tops and bottoms accelerates water flow up, around and between tubes by eliminating the "vena contracta" and increasing the flow around adjacent tube sides, thus increasing the rate of cooling of a coolant from an interior heat source.