Brightstorm : a sky-ship adventure / Vashti Hardy ; illustrated by George Ermos.

Enhanced descriptions from Syndetics:

Arthur and Maudie Brightstorm are devastated to learn that their famous explorer father has died in a failed attempt to reach South Polaris, the southernmost point in the world. To make matters worse, the Lontown Geographical Society finds Ernest Brightstorm guilty of sabotaging the expedition of his competitor, Eudora Vane, a respected and powerful member of the Society. But then a mysterious clue leads the twins to question the story they've been told. Joining the ragtag crew of the Aurora, a homemade sky-ship captained by the intrepid Harriet Culpepper, Arthur and Maudie race to South Polaris to salvage their family's reputation. As the sky-ship sets sail, the twins must keep their wits about them if they are to uncover the truth about what happened on their father's doomed expedition, and to prove themselves worthy explorers. Brightstorm is a propulsive, vividly imagined fantasy adventure set among vibrant landscapes and peopled with dynamic and unforgettable characters.