How many naval bombers do you need to actually sink ships?

I had 400 naval bombers doing patrols and bombing over the English Channel and they only destroyed a couple of conveys and one destroyer over the course of years. Any time they went up again a fleet they almost never sank anything and a couple of them would get shot down.

I use small planes with the a single torpedo and the best engine possible. Is there a better plane design I should be doing? Should I not have them doing both missions at the same time?