I have found out that two torpedoes will sink the carrier - start four Mantas loaded with torpedoes, first guide them away from the enemy to avoid AA fire, then attack in a row. Fire the torpedo manually if the enemy carrier is in sight, then dock. Take control over the second Manta, fire the torpedo manually - and that should be it. If not, try the third Manta and so on. Great fun, even if the game crashes after this. ;-)