Duff was a proud Scotsman as well as a strict disciplinarian. He enforced cleanliness parades every week, and made every effort to make sure that as many Scots as possible served on his ships, although he never gave them preferential treatment over their English comrades. He was also a devoted husband and wrote lengthy letters to his wife during every journey. He and his wife felt that the letters were so personal that they destroyed them after reading, and so the only survivor is his final letter home, which Sophia reportedly was unable to destroy.