What is needed for a successful Napoleonic conquest of Britain?

[longsword14]

or what if Napoleon went all in on steam powered warships? What if the French navy had their own equivalent to Napoleon?

Steam engines capable of use in ships were really not feasible at the time. It would take a long time for development and a ton of money to do that. The French never put as much importance on their navy as England (being a land power). To get a Napoleon on the seas there must be time to gain experience and a large navy that does not fear losing ships as much, both realistically impossible. France was busy fighting on land.
Any means of changing the French naval situation would have to develop in the long term provided Napoleon gets his aims on the continent.
By 1805, the British had the best navy and the French had the greatest army of their times. The problem for France was that other landpowers were a credible opposition to her because they could reform and raise new armies, unlike Britain who really had no real challenges.

 

[Saphroneth]

or what if Napoleon went all in on steam powered warships? What if the French navy had their own equivalent to Napoleon?

The fact that the first steam powered line-of-battle ship was laid down 1848 (after earlier British invention of the blockship) and used the screw, a technology not made practical until 1835 (in Britain) should indicate how unlikely this is.

For reference of the state of the art at the time, in 1809 Fulton built a steam ship with 19 hp. The Savannah of 1819 (which had an outsize engine for the time) had 90 hp.

Napoleon had 960 hp. Any engine you could actually fit on a ship at the time of the Napoleonic Wars would be able to drive it only very slowly, and given the parlous state of French engineering compared to British (the industrial revolution happened in Britain, after all, not France) suggests that the resultant ship might actually give up more effectiveness in firepower (from the engine and the big paddle wheels occupying mass and ship side real estate) than it gains from being able to slowly move against the wind.

I know it's cool in Total War games, but those very much push things decades ahead of their time.