I couldn't find enough info on HMS Manxman in the books it is described as a 80 gun 3 decker , I couldn#t find any equivalent on the british or french navy lists. Were there any 80gun 3 deckers or is it pure fiction?
There were 80-gun three deckers, but they were obsolete by the 1750s, if not earlier. They were tall (having three decks), but short, which made them both relatively slow and leewardly (because their higher hull was more easily pushed downwind by the wind). Yet they were expensive ships to build and operate. The 74-gun ships were almost as powerful, much better sailing vessels in most states of the wind, and cheaper to build and operate, needing smaller crews.
The Royal Navy continued to build small numbers under the old 'establishments', which were only finally abandoned during the Seven Years War. The
Cambridge of 1755 and the
Princess Amelia of 1757 were the last two to be built. The
Princess Amelia would be the last to see action, at the Battle of the Dogger Bank in 1781.
There is an invaluable database at ThreeDecks.org that will tell you a lot about their dimensions and histories. See
I hope that helps.