The Outcasts by John Flanagan
Hal is the son of the great Skandian warrior, Mikkel. As his dying wish, Mikkel asked Thorn to care for his son. Hal is an outcast, though. He is only half Skandian: his mother is an Araluen slave. And Thorn has a habit of getting drunk. So much for learning his father's skills. But Hal has some friends, and he loves to build ships. When he builds his first ship, the Heron, with new technology, Oberjarl Erak is impressed. Things only get harder for Hal, though.
His enemy, Tursgud, is determined to make Brotherband training miserable for him. His whole team made up of outcasts and misfits, can Hal prove himself not just to his peers, but to the Oberjarl as well?