Rafael Sabatini 
Captain Blood 
His Odyssey

Dr. Peter Blood, an Irish physician, had a wide ranging career as a sailor and as a soldier, before settling down to practice medicine in the town of Bridgewater. The town begins to fight for the Duke of Manmouth, and though Peter wants no part in the rebellion, he tends to the wounded soldiers. Because of this he is convicted to treason, and sentenced to death by hanging. King James II, for purely financial reason, decides to sell the criminals as slaves, instead of killing them. After being sold, Blood escapes, steals a Spanish ship, and sails away to become one of the most successful pirates/buccaneers of all time, hated and feared by the Spanish.