Worldshaker by Richard Harland

Col (or Colbert...) lives on a city ship known as Worldshaker. It's the year 1875 and he lives on one of the Upper Decks with his parents, sister, and baby brother. Col is selected to follow in his grandfather's footsteps and become the next commander.

As the successor, Col knows his actions must be above reproach. But he's already violated their basic rule by not turning in the Filthy who broke into his room. The Filthy--Riff--escaped from the officers forcing her to become a Menial and all she wants is to get back Below.

Col agrees to help her so he won't get caught. He doesn't know why he doesn't just turn her in, but he can't. They break into the lower decks and send her back Below. After she's gone, Col hopes life goes back to normal, but he can't help but think about her. And when it appears his peers are going to teach him a lesson, Col decides he needs her help to save his life.

While he realizes the danger in such a task, he doesn't realize the chain of events he'll start just by falling Below and surviving...

Action-packed and adventurous. Col is a likable, although naive, character. Riff is especially funny, but also a little naive. Col attempts to change the world and his trust in adults is frustrating and enduring at the same time. A great steampunk novel.