 As captain of HMS "Stirling Castle", he took part in the first Battle of Ushant.In 1781, Sir Charles became Captain-of-the-Fleet for George Brydges Rodney, 1st Baron Rodney, and was with Rodney on his flagship, "Formidable", at the Battle of the Saintes off Dominica, where on 12 April 1782, they defeated the Comte de Grasse by breaking the French line.Douglas is credited by many, including Sir Charles Dashwood (a midshipman present at the time who later became an admiral himself), for having the idea for the manoeuvre, but it is a subject of much debate.