The Time Pirate by Ted Bell
(Suggested reading level: Grades 5-8)
I read this aloud with Amber (10), and she and I have entirely different opinions of it, and, David, who listened in on most of it, pretty much agrees with her, which makes me think that I just wasn't the right audience.
It's 1940, Nick McIver, a 12 year-old boy must defend his home, a small British island, against the looming Nazi invasion. But the Nazis are not his only enemies; using a time-travel device invented by da Vinci, he also battles 18th-century pirates who've kidnapped his sister. To further confuse things, the pirates, who are equipped with a time-travel device of their own, threaten to change the outcome of the American Revolution. Nick feels compelled to help General Washington and his troops, even though doing so makes him a traitor, because he knows that the support of the United States will become crucial...