As to Caravels, well... they are all-round the best ships in the game. Your only option in my mind is to take advantage of 1.) your imperial riches, and 2.) their inability to flee, and stomp them with large stacks of wargalleys/firegalleys. You will take losses due to galleys' sucky defence, unless you've got a superstar admiral/captain/commodore/whatever they're called.