Symbolism, Imagery, Allegory
Even as a rust rat, Nailer lusts after the clipper ships. They are everything that he doesn't have in his life—new, shiny, radiant. They are the ships of the wealthy. At the beginning of the novel, when he imagines himself aboard a clipper ship, this happens:
He tried to imagine standing on the deck of one of those ships, leaving the beach and light crew behind. Sailing fast and free. (5.54)
Clearly, the clippers represent freedom and opportunity for him—everything that is out of reach, and can only be afforded by money. The clippers are very much a marker of economic divisions in society, and the people working the clipper ships are the haves, while Nailer and his crew are the have-nots. Leave it to Pima to make this abundantly clear:
They were all looking at the water now. Hungry.
"You think they even know we're here?" Moon Girl asked.
Pima spat in the sand. "We're just flies on garbage to people like that." (5.69-71)
Even though Nailer's crew desires what the clipper owners have—wealth, security, safety, leisure—they, namely Pima, cover up their longing with scorn for the upper classes. And there's some truth in what Pima says; swanks are equally as prejudiced against rust rats as Nailer's crew is against swanks.
It becomes even more interesting when we compare images of a clipper ship to images of an oil tanker. Check it out:
Ahead, the gull-white hull of the wreck gleamed in the sunlight, beckoning […] Even destroyed, it was a beautiful thing, utterly unlike the rusting iron and steel hulks they tore apart every day. (7.84, 8.1)
The clipper, an upper-class ship, "gleamed" and is "beautiful"; it's also white, which in literature, often signifies purity and wisdom. As the clipper is, so is the upper class—or so the upper class fancies themselves, anyway. But the oil tankers are "rusting" and things that Nailer and his crew tear apart. Dirt and violence are the name of the game, and so, too, are they associated with the lower class.
Because the ships represent their respective classes, it's pretty significant that, at the end of the novel, Nailer finds a future for himself aboard the clipper ship Dauntless. He may be the only person to cross the line from poverty into possibility, and the length has goes to in order to achieve this make it abundantly clear that upward mobility is about as rare as a unicorn in this world.