Which job had the legendary pirate Black Bart on board the slave ship Princess, before he became pirate?

Explanation

The Welsh pirate Bartholomew Roberts (1682-1722) was born John Roberts and was only after his time, known as "Black Bart". He plundered ships from America and West Africa in the period 1719-1722. Black Bart was the most successful pirate in the pirates' golden age, measured in terms of vessels caught. It amounted to more than 470 trophies in the pirate's career. Roberts was third mate aboard the slave ship Princess, when they were attacked by pirates, and he was forced to join them. Coercion soon turned to benevolence, when he discovered the benefits of the new lifestyle. He died in combat and was buried at sea according to his own wish.