The Oxford Companion to Beer definition of
is the name given to the shallow, open vessels traditionally designed to cool hot wort prior to fermentation. The use of “ship” in the name probably refers to the medieval practice of cooling the boiled wort (or mash) in a hollowed-out tree trunk—not dissimilar to a primitive boat. Before the advent of refrigeration and chilled water, hot wort was cooled by transfer to these shallow, open vessels, where the wort was allowed to cool slowly. These open coolers or coolships with high surface area-to-volume ratios had three functions: cooling, aeration of the wort, and separation of the cold trub. However, these open vessels were fully exposed to the air and consequently subject to microbial contamination. Inevitably, with larger volumes of wort to cool, the coolship dimensions necessary to achieve effective temperature reduction became unwieldy and alternative methods of cooling (usually vertical coolers in which the hot wort flowed continuously as a thin film over a vertical metal surface cooled with chilled water) were introduced.
Coolships still have application in the brewing of traditional Belgian lambic beers for which the large surface area of cooling wort lends itself to spontaneous fermentation (whereby yeasts and bacteria present in the atmosphere, and often in the fabric of the brewery, ferment the wort to create a rich assortment of interesting and challenging flavors). These lambic coolships are often located in the roofs of brewery buildings where louvered shutters are opened to allow entry of the surrounding atmospheric microflora (and sometimes microfauna!) of Belgium into the coolship room.