A keel cooler is a type of internal combustion engine cooling system used in marine engines.
In this system, engine coolant is circulated through a system of tubing outside the vessel's hull, using the lower temperature of seawater to reduce the coolant temperature via heat exchange before it is recirculated through the engine.
[1] Simpler designs take the form of a tank within the hull adjacent to the vessel's hull plating.
[1] For maximum efficiency, manufacturers design keel coolers with baffles and others means of slowing and disrupting the flow of coolant and/or seawater in order to maximize heat transfer.
[2]