Why does whitman call ship "grim and daring" in line 4?

Line 4: “Keel” has multiple meanings. The first is the part of the ship that goes along the underside that gives the ship stability in the water. This usage really doesn’t make much sense, though, because one can’t see a “steady keel” unless the boat has fallen over. The other use of keel here is as a synecdoche. The keel, a part of a boat, is used to represent the whole boat. More importantly, "the steady keel" lets us know that, despite its rough trip, the ship (like the nation) remains steadfast and true.

https://www.shmoop.com/o-captain-my-captain/ship-symbol.html

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