Ishmael here writes that, although Ahab is singularly devoted to the catching and killing of Moby Dick, still he is “not unmindful” of the normal purpose of a whaling ship, which is to kill as many sperm whales as possible, for the collection of sperm oil and the enrichment of the crew. Ahab, Ishmael states, understands that even though members of the crew are “for” his quest at the moment, they might just as easily turn away from Ahab’s command if left to their own devices for too long, and try to take over the vessel in a mutiny, since Ahab is so demonstrably maniacal.
Ahab’s rational deduction here—that he must entertain the crew as the Pequod makes its way to find Moby Dick—demonstrates that Ahab is not completely seized by his madness. He is, in other words, aware that others on the vessel might not share his unbending desire to kill the White Whale. Thus, Ahab’s “insanity” takes on a peculiar rationality and logic—he is so devoted to killing Moby Dick, he can will himself into a kind of sanity in order to protect himself and continue the chase.
Thus, Ishmael writes, Ahab knows he must keep his crew occupied, just as knights on a quest for the holy grail were occupied with intermediate adventures along the way. And Ahab also knows that any whale the Pequod can spot and kill is an opportunity to employ the crew in the catching of the White Whale, which still forms the central purpose of Ahab’s tortured life.
Ishmael once again compares the whalers to heroic figures from the past—here, knights—and Ahab, head of those knights, would then be none other than King Arthur himself.