Starbuck discovers, while rinsing the decks of the Pequod, that some of the oil casks in the hold are leaking. Starbuck goes to Ahab to inform him of this, and Starbuck advises that they stop and inspect the casks to find and plug the leak. But Ahab says that it’s not necessary, that the ship will carry on to Japan to find the White Whale, and that the oil isn’t important. Ahab then orders Starbuck away. But Starbuck, who does not dare openly defy Ahab, nevertheless tells him to “beware himself” (“Ahab beware Ahab.”) Starbuck leaves, and Ahab realizes that Starbuck might be right—that Ahab is his is own enemy. Ahab remarks that Starbuck is not courageous enough to defy him and mutiny, but Ahab nevertheless orders to hold up the vessel and inspect the leaks, as Starbuck requested.
Ahab's initial response that the leaking oil doesn't matter is astonishing in that the entire purpose of a whaling ship is to collect whale oil and then sell it. For Ahab to ignore leaking whale oil is essentially to admit that he is not on this voyage for commercial reasons at all. Starbuck's comment that "Ahab beware Ahab" is moral or almost religious . He is saying that Ahab's monomania will lead to his own destruction. What is amazing is that Ahab sees that Starbuck is right , that he is his own worst enemy, but he responds not by giving up his monomania but by deciding that in order to most fully pursue his monomania he must act like a man who care about the financial aspect of this whaling expedition in order to hide the fact that he doesn't.. While Ahab has rightly sized up Starbuck as a man who recognizes Ahab's goals but will not stand against him—a stance that Ahab views simultaneously as loyalty and as cowardice on Starbuck’s part—he realizes that he won't be able to command the loyalty of the crew unless they feel that the whaling voyage has the prospect of enriching them as they hope and expect.