On 4 September 1780 he was in command of Unicorn when he had the misfortune of encountering, in a fog off Tortuga, a French frigate and two ships of the line that captured him.

The subsequent court martial honourably acquitted Frederick for the loss of his ship, and in October 1781 he received command of the 44-gun frigate HMS Diomede.

He was present with Admiral Lord Hood at the Siege of Toulon, and as a commanding officer of his ship, he saw action at the Battle of Genoa against the French fleet.

While in command he saw action at the Battle of Cape Saint Vincent,[5] where the outnumbered British squadron under Sir John Jervis defeated the Spanish fleet.

[6] After taking a convoy to Lisbon, Frederick joined Jervis, Earl St. Vincent, on the resumption of the blockade of Cadiz.