Abstract: The single tube or single tube of a three, five or the like uneven number of tubes extending between a header unit attached to the underside of a boat hull in communication with the water circulating pump of the inboard engine and a return fitting attached to the underside of the hull is centrally divided into side-by-side separate compartments of equal area and volume by a longitudinally extending flat divider plate so that the uneven number of tubes is converted into an even flow number whereby an accurate volume flow can be achieved between the header and the return fitting in terms of the volume capacity of the water circulating pump.