《天工开物·中篇·舟车》
如果不遇上逆风，仅一昼夜顺水就可行四百多里，逆水也能行驶一百多里。明朝的盐税中，淮阴和扬州一带征收的数额很大，也要用这种船来运送税银，所以就称它为 课船 。
不遇逆风，一昼夜顺水行四百余里，逆水亦行百余里，国朝盐课淮、扬数颇多，故设此运银，名曰课船。
If there is no headwind, it can sail more than 400 miles downstream in just one day and night, and it can also sail more than 100 miles upstream. During the Ming Dynasty, a large amount of salt tax was collected in Huaiyin and Yangzhou, and such ships were also used to transport the tax silver. Therefore, they are called official ships.