In most American whalemen the mast-heads are manned almost simultaneously with the vessel's leaving her port; even though she may have fifteen thousand miles, and more, to sail ere reaching her proper cruising ground.
Even when there is no chance of seeing a whale, the ship’s mastheads are manned. On board the Pequod, as in the wider world, order is kept because people adhere to routines and customs, even when they seem pointless (the point is to maintain order).
To help improve the meaning of these lyrics, visit “Moby-Dick (Chap. 35: The Mast-Head)” by Herman Melville and leave a comment on the lyrics box