Abstract: A closed cycle vaporization cooling system for an underwater vehicle's in-to-outer hull heat transfer having a low pressure freshwater circulating loop means for collecting and cooling waste heat from inside the vessel by an evaporator means located inside the vessel and so configured to operate under all conditions, an adiabatic zone within the loop for conveying vaporized working fluid to a condenser means located outside the underwater vehicle's pressure hull utilizing a chimney effect free-flooded seawater heat sink wherein cold seawater flows from bottom to top of the heat sink over the condenser means, and a condensate means for condensing and returning the condensate to the evaporator means.