Boxcooler is tube-type heat exchanger invented and built in the Netherlands. The first boxcoolers were used for river vessels. Having a good experience with this, applications grew slowly for bigger ships like coasters and fishing vessels. Gradually more type of ships (dredgers, tugs) were supplied with boxcoolers. The principle of a boxcooler is quite simple; cooled water is circulated by force through a U-tube bundle, placed in a sea chest with inlet and outlet grids. Cooling effect is obtained by natural circulation of the seawater in the sea chest or by forced circulation due to the speed of the vessel. The outboard water is warmed up and rises by its lower density, thus causing a natural upward circulation. Boxcoolers can be made of galvanised steel, aluminium brass or HiResist.