Danger Money — Mary Chapman

When Bob Thomson joins the crew of the Admiral, all he can think about is how much he's going to earn. His mom doesn't want him to go. Because this is World War I, the Admiral is an armed smack, and defends itself against German subs. But Bob doesn't care. The first time out, they sink a sub. It's money for jam, he thinks. Until the day the Admiral meets two German subs, and Bob really has to earn his danger money. Shades is a highly successful series of exciting, easy to read, short fiction books with storylines designed to appeal to young people who want high-interest material but who find full-length novels too daunting.