Free verse refers to poetry that does not have rhyme, rhythm or meter. All three are true of "Abroad at a Ship's Helm". If you look to the end of the lines, there is no rhyme scheme that runs through. In addition, each line has its own arrangement of hard and soft syllables, and so they lack a musical cadence. Without rhythm, there can be no meter.
What is more important to note is that the poet using free verse creates his/her own rules. Look at the formation of the poem. Each line varies in length. Repetition is used, but inconsistently. The point of view is also inconsistent, starting in the third person, and then slipping into second person ("O you give good notice indeed, you bell by the sea-reefs ringing,"), and then back into third person. Whitman did not adhere to any traditional poetic rules, but he still created a beautiful piece of poetry.