This is my reference on the likely ship losses to the French with POWs involved.29 April 1806 'Montrose' packet from St Michaels/Sao Miguel in the Azores for London taken by 'Le Topaze', a French 40 gun frigate, and burnt. The crew was taken as POWs to Brest. 12 October 1806 'La Constance' taken off Brest, captain killed and surviving crew became POWs. 9 December 1806 the 'Adder' was lost near Abreval and the entire crew served eight years as POWs. 18-19 February 1807 big storm affected ships of Channel fleet off coast of France with several driven by stress into French ports where crews became POWs. Other vessels foundered with all hands. Ships whose crews became POWs included Prospero, Inveterate, Ignition and Magpie. There is the story from 4 March 1807 of the crew of the 'Blanche' under Captain Sir Thomas Lavie which was lost off the French coast and the crew marched to Brest- 26 officers, 180 seamen and 25 marines - who "were prisoners of war for seven years, but were released on the abdication of Napoleon". March 1807 45 crew of the 'Cesar' taken as POWs in Verdon Roads. So, Montrose Adder and Blanche seem to fit the bill as do possibly the other four ships lost in February 1807. If Henry Rae was exact in his timing of 7 and a half years as a POW and that time is taken from mid 1814, then the vessels closest fitting late 1806 to early 1807 excludes Montrose, which would be closer to 8 years. I think those are the likely ships.