Sentence:  ... you must know that when the second iron is thrown overboard, it thenceforth becomes a dangling, sharp-edged terror, skittishly <WSD>curvetting</WSD> about both boat and whale, entangling the lines, or cutting them, and making a prodigious sensation in all directions. 
Meaning of the word marked with <WSD> tags:  ("of an object") To jump, skip, shake.