Answer:
Whitman begins by comparing Lincoln to the captain of a ship and his army to a boat.
Step-by-step explanation:
From the very start, Whitman calls Lincoln 'captain', and when he says, "our fearful trip is done," we understand that he means "mission accomplished". The line "The ship has weather’d every rack," is an obvious reference to his army. That's how he starts building up his extended metaphor.