The Black Galaxy — Murray Leinster

When Stellaris, the first human interstellar ship, unexpectedly hurtles off the earth, it takes the ship's designer, Rob Cantrell, his girlfriend and a skeleton crew with it into the furthest reaches of space. If only that was their biggest problem! With no star maps, killer aliens on their tail and a ship that was only half finished, their journey home is going to be quite the challenge!