The two ships differed in detail, Unicorn having a beakhead bow, a unicorn figurehead, two-light quarter galleries and only five pairs of quarterdeck gunports, while Lyme had a round bow, a lion figurehead, three-light quarter galleries and six pairs of quarterdeck gunports.

Lyme was named on 2 August 1748, and commissioned in September 1748 under Captain Charles Proby, while still building in Deptford Dockyard under the direction of Master Shipwright John Holland.

Returning home, she was fitted out at Portsmouth Dockyard from December 1750 to March 1751 (at a cost of £389.6.9d) for bearing the new ambassador to Tripoli out to the Mediterranean.

After her first commission finished in 1752, she was surveyed on 1 July 1753, and then underwent a small repair and was fitted out at Plymouth Dockyard (under Admiralty Order on 4 December 1753, for a total cost of £1,519.6.3d) in February to March 1754.

Back home, and under Vernon's command again, she was surveyed again on 7 June 1760, and then underwent a small repair and was fitted out at Chatham Dockyard (for a total cost of £4,211.6.4d) in May to August 1760, before sailing for the Baltic.