The entirety of the poem is an extended metaphor. It uses figurative langue to compare seemingly unlike things. for example; The United States, The Civil War and Lincolns assassination. Int his metaphor, the captain is the president, the voyage is the war and the ship is USA.
(I just tried to reword from a previous question which was similar)
Sample Response: Whitman begins by comparing Lincoln to the captain of a ship and his army to a boat. The metaphor honors Lincoln and the army for helping defend and win the idea of freedom. Whitman ends the poem by saying that Lincoln, the captain, has died. The army and the United States have lost an important leader.
