Oh yes I didn’t notice this was in respect to Global 1940, so both the BB and CV have two hit points and a sub would need to hit a Carrier twice to of course remove it from the board.
However, I don’t understand your comment “no aircraft aboard”… there isn’t ever a situation where aircraft are “aboard” a carrier in combat… saying this gives the impression a Carrier can’t be lost with planes on board which is not true. The Carrier and planes are seperate entites once combat has begun, they are only “onboard” the carrier when landing in the non-combat move/air units land phase of the game turn.