Ishmael, in Melville's Moby-Dick, at one point calls the novel the "draught of a draught." How is this comment relevant to the novel's structure?

This is a very intriguing question! The passage you quote
occurs near the very end of Chapter 32:

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But I now leave my cetological System standing
thus unfinished, even as the great Cathedral of Cologne was left, with the crane still
standing upon the top of the uncompleted tower. For small erections may be finished by
their first architects; grand ones, true ones, ever leave the copestone to posterity.
God keep me from ever completing anything. This whole book is but a draught -- nay, but
the draught of a draught. Oh Time, Strength, Cash, and
Patience!

Moby-Dick
might be called the “draught of a draught” (that is, the “draft of a draft”)
in several ways.

In the first place, the novel is highly
unconventional in structure, especially because of the inclusion of the lengthy,
detailed account of whaling and of whales as a species. In some respects, the structure
of Moby-Dick is epic or encyclopedic: it touches on a great deal
that is not strictly or obviously “relevant” to the main plot, and the chapters on
whales and whaling are the obvious examples.

Melville
implies that he is not attempting to offer a smooth, polished, perfectly structured
book. He implies that truth is complicated and ragged and cannot be fit into neat little
prefabricated boxes. One’s perception of truth, he implies, is constantly evolving,
especially if one has a capacious, probing, thoughtful mind, as Melville did and as
Ishmael also does. The idea that the novel is merely the “draught of a draught,” then,
implies Melville’s respect for, and openness to, the full and genuine complexity of
reality.

At the same time, Melville realizes that he has
taken on a literally gigantic task in trying to write a book such as this. He is proud
of his effort, but he is also humble in his recognition that nothing constructed by man
– certainly nothing as ambitious as this novel – can ever be finally perfect or
completely finished. Moby-Dick is deliberately
not the "final word" on anything it discusses. A man like Ahab
would try to have the "final word"; a man like Ishmael would not and does
not.

Indeed, if the quoted passage is seen as a reflection
of the personality of Ishmael, it shows him expressing his typical humility and his
typical thoughtfulness. He is well-read; he knows about both whales and cathedrals (and
much else); he is constantly hungry for new knowledge; and he never assumes that at any
point he has reached the complete truth about anything.

The
phrase “draught of a draught” implies that the structure of
Moby-Dick is a bit of an improvisation, invented as Melville went
along rather than fully outlined and pre-planned before he began writing. Melville
recognized that he was writing an unusual book, unlike anything, really, that had ever
been written before. He recognized, too, that he was even departing from many of the
more conventional structures that had made so many of his early books so successful. He
knew that in writing Moby-Dick as he chose to write it, he was
running the risk of producing a book that the general reading public would not
comprehend or appreciate. His willingness to write such a book shows his courage, his
integrity, his inventiveness, and his dedication to depicting expereience in all its
messy complexity.