Can anyone tell me the title of this book, I read it at age 12. It is set in either the 18th or 19th centuries and has something to do with being on board sailing ships and then being on an island (I think).

At the story's end a boy around the age of 14 or so is being held captive underground with a chained shackle on his wrist, and in order to escape he makes the decision to chop his hand off ( a friend does this for him) and they escape together.

That's honestly all I can remember. Please help!