You could also experiment with your technique, if you are insistent upon doing it yourself, by using the "SS02 Artillery" mission file in the Submarine School. It displays on the Submarine School list as "Artillery Training", and puts you on the surface near some merchant vessels that shoot at you, and then some planes that attempt to bomb and strafe you. It is much easier to get through that in Stock than TMO, which is easier to do than RFB, since TMO adds difficulty with a bumped-up AI, and RFB adds "weakness" to your sub's 'armor' level, so you'll be sunk rather quickly (even in wTw) if you and your crew aren't on their toes... and it will give you an idea of what to expect in a career if you don't avoid surface engagements of all kinds - especially with airplanes.
