Abstract: The valve is inserted in the conduit that carries ambient water to the engine for cooling it. The valve has the form of a T or a Y. Cooling water enters port A against the urging of a first spring-loaded check valve and flows out through port B on route to the engine. The stiffness of the spring is carefully selected so that the first check valve functions as a flow restricter, whereby at low engine speeds the flow is reduced to prevent overcooling of the engine. When the boat is out of the water, flushing water or an antifreeze can be drawn in through port C which is provided with a second spring-loaded check valve. If the flushing water or antifreeze is supplied under pressure, the pressure must be great enough to overcome the urging of the second spring-loaded check valve.