LitCharts assigns a color and icon to each theme in Moby-Dick, which you can use to track the themes throughout the work.
Limits of Knowledge
Fate and Free Will
Nature and Man
Race, Fellowship, and Enslavement
Madness
Religion
Summary
Analysis
Ishmael also states that the harpoon line has not one but two harpoons connected to it, in each boat, and that both harpoons are made to rest in a wooden “crotch” until they are thrown by the harpooneer. The second “iron” or harpoon is used in case the first does not land in the whale’s side; but in any case, one or another of the harpoons usually dangles around the whale and is a danger to any sailors who come in its path. Ishmael states that he introduces these two “irons” here because they will factor later into the narrative.
Just as “the lines” warrant their own chapter, the “crotch” on which the harpoon sits will later relate to Ahab, who, in his final lunge at Moby Dick, will get caught in the lines, hooked around the crotch of the whale-boat, and will be flung from the vessel and hanged “by his own rope.” Ishmael wants there to be no ambiguity about the deadliness of these implements aboard the whale-boat.