What Is The Difference Between A Box Cooler And A Keel Cooler?

The main difference is where the cooler is mounted.

A box cooler is installed inside the vessel's sea chest, within the hull. Seawater flows around the tube bundle inside that sea chest, so the unit is protected from many external impacts and underwater hazards.

A keel cooler is installed externally on the hull below the waterline. The engine coolant circulates through the cooler while seawater on the outside of the hull removes the heat.

In practical terms, that means:

• Box cooler: more protected, compact in-hull arrangement, commonly used with a sea chest.
• Keel cooler: outside-hull mounting, direct seawater contact on the exterior surface, often chosen when an external closed-circuit cooler layout suits the vessel better.

So the simplest way to say it is: a box cooler sits in the sea chest inside the hull, while a keel cooler is mounted outside the hull below the waterline.