In 1764 he and Adam Babcock, another young New Haven merchant, became partners. Using the profits from the sale of the family house, they bought three ships and started trading with the West Indies. Arnold brought his sister Hannah to New Haven to work in the apothecary business when he was not there. He traveled a lot for his business, throughout New England and from Quebec to the West Indies, often running of one of his own ships. On one of his voyages, Arnold fought a duel in Honduras with a British sea captain who had called him a "damned Yankee, destitute of good manners or those of a gentleman." The captain was wounded after in the first part duel, and apologized after Arnold threatened to aim to kill on the second part.