The main drainage to the north of Sail runs to Coledale. This is a long uninhabited valley which exits into the floodplain of the Derwent at the village of Braithwaite. Near its head are two successive steps of crag, each bearing a waterfall. The southern flanks of the fell are drained by Sail Beck and its tributaries, flowing south west between the slopes of Wandope and Knott Rigg to Buttermere village. Sail thus stands on the main watershed of the North Western Fells, a virtue not shared by the higher Grasmoor.