Ishmael notes that the forehead of the sperm whale is so thick and so strong (resembling “horses’ hoofs”) that the whale can use it quite effectively as a battering ram, without even feeling any pain. Ishmael begs the reader to believe that the whale can do this, as, later in the novel, there will be feats of “battering” the reader might not otherwise take to be truthful, if he has not seen the power of the whale’s forehead.
Again, Ishmael makes plain to the reader that it would be possible—indeed, likely—that a large sperm whale, like Moby Dick, could use its head to destroy an entire whaling vessel. Of course, this is exactly what will happen to the Pequod at the close of the novel. Ishmael's tale continues to layer on the foreboding, continues to ramp up the feeling of fate.