Fridge cabinet circulation fan.

[OzeLouie]

Fitting a small circulation fan in marine fridge cabinets has become very popular and beneficial as it reduces stratification, improves system performance and efficiency. This is because the system will run harder at a warmer and therefore better COP reaching cut off temperature quicker and cooling product faster.
But there is a down side, the cabinet must be 100% sealed otherwise the cold plate will frost up more rapidly and any performance gains lost as frost restricts the cold plates heat absorbing ability. The best way to check lid / door seal is to put a strong light in cabinet on a dark night and check for leakage. A broad soft sponge seal is best. Any leak is a fail..

This is most important for the very popular combination fridge / freezer cabinets where the freezer is refrigerated and a thermo-fan is employed to 'steel' some cold air from the freezer as required.
With large combination cabinets an SS heat exchange cover to separate the warm (possibly moisture laden) from the fridge side from mixing with the freezer air and cold plates.

Here is a diagram depicting both options: Click to enlarge

[Richard Kollmann]

Adding fans offer many thermodynamic advantages in ice box conversion refrigeration.
Kollmann Marine