After the British troops were driven out of Boston in March 1776, the port became an important centre for activities against British shipping. The business house of Philip Moore &amp; Co. financed a number of privateers, including Day. A 370-ton British merchant vessel, the "Isaac" of Liverpool, which had been captured on the way home from the West Indies by John Philips in the privateer sloop "Warren", and taken to Salem, was converted to a fighting frigate with twenty 9-pound guns and six 4-pounders, and rechristened the "General Mifflin". On 1 November 1776, the Council of Massachusetts issued a State Navy commission to Day as commander (the document also notes that his Second Lieutenant was William Day Jr.). For its first cruise, the "Mifflin" had a crew of 120, five tons of shot and two tons of powder, plus eight tons of bread and flour, and 120 barrels of meat.