A coolship is an open vessel similar to a shallow pool, which allows wort to cool overnight and collect wild yeast and bacteria. The idea is to make sure it is open to the environment, rather than protected by brewery walls. The large surface area allows for as many wild yeasts to innoculate the wort, as well as to aid in cooling. 10-15 hours is preferable (according to a conversation I had with Jean van Roy of Cantillon.)
I built a small coolship by cutting a sanke keg in half. Technically, I guess I built two coolships. It is smaller so the cooling takes place “too fast” (JvR) so I may not be getting as much yeast/bacteria if the wort had a longer chill time.
Thus far, I have had four different brews go through this process. Two of them I am currently tasting every few months, and the other two are still fermenting/aging.