(People who haven't read Moby Dick, especially those who expect it to be tedious, should skip this post).
What do you think the function of the whale chapters is? Explanation and clarification, like footnotes, for the story- making everything more concrete, more specific, more visual, more realistic? You can argue that, for example, in chapter 61, Ishmael describes a whale-killing scene. Then in chapter 62, he describes the process- the dart, the harpoon and the lance, the change of places. Then in chapter 63, he writes about the crotch, where the harpoon is kept. "Out of the trunk, the branches grow; out of them, the twigs. So, in productive subjects, grow the chapters."
Or do you think the whale chapters are there for themselves, because they're interesting in themselves? You can say, part of it is that Ishmael wants to grasp the whale, to know it inside out, from head to tail, from blanket to case. That is his quest. His feelings are mixed, the whale is so many things at once: whale and leviathan, God and Satan, a vast, noble, majestic creature and a victim, etc. The attempt to understand stresses the elusiveness of the whale and proves how silly, pointless and unrealistic such a "project" is. Apparently Ishmael's interested in whale facts also because he likes whales, not just because by understanding the whale he can make sense of Moby Dick and the hunt.
Now, starting to read chapter 93, I've just had a theory that Melville ultimately only cares about whales, that Moby Dick is really an encyclopedia about whales and whaling disguised as fiction, that the story serves the infodumps more than the other way around. For example, Melville wants to compare 2 kinds of whales, so he lets Stubb and Flask kill a right whale after a sperm whale; he wishes to show different methods and techniques in whaling, so he introduces them in chapter 61 "Stubb Kills a Whale" (throwing a harpoon and stabbing the whale repeatedly with a lance until breaking its heart), chapter 84 "Pitchpoling" (throwing a spear made of pine that is furnished with a small rope called a warp) and chapter 87 "The Grand Armada" (among gallied whales, using druggs- thick pieces of wood attached to a harpoon). So now, to tell us that from whales we can get not only oil and spermaceti but also ambergris, Melville creates the story about the Rose-bud and 2 blasted whales in chapter 91.
No wonder some readers feel cheated, not getting the novel they want, and put down Moby Dick.