Could someone explain in depth the shipwreck and the captain in part 33 from "Song of Myself"?

It is important to realise that in Part 33 of this
excellent poem, the speaker takes on the voices of people that he sees as unsung heroes.
Thus we are presented with the voice of a skipper in a storm, a mother burnt for a witch
in front of her children and a firefighter crushed by a falling building, to name but a
few. The stanza you are referring to was inspired by a real-life incident that occured
in 1853. A ship was hit by a violent storm in the middle of the sea, washing lots of
passengers overboard. The captain of another ship helped rescue the survivors. The
heroism of the captain in saving the people from the first boat is highlighted in this
stanza, by focusing on how the Captain "knuckled tight and gave not back an
inch":

And
chalk'd in large letters on a board, Be of good cheer, we will not desert
you;

How he follow'd with them and tack'd with
them three days and would not give it up,

How he saved the
drifting company at
last...

Thus the Captain is a
suitable figure to be identified as an unsung hero in this section of the poem, given
his bravery and his faithfulness to the task at hand, even in the face of danger. Note
too how Whitman identifies with the Captain, as he says in one of the most famous lines
of this poem:

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I am the man, I suffer'd, I was
there.

This phrase somehow
makes the scene that much more immediate and shows the compassion and empathy of the
speaker, whilst highlighting the bravery of the Captain.