What constitutes an appropriate tonnage total depends on a number of factors:
- The realism level you are playing at
- The type of boat you have and what equipment it has
- Where your patrol area is
- What year you are in
The more torpedos you have, the more damage you can do, so I tend to think in terms of tonnage per torpedo. If you get assigned to patrol an area with a lot of traffic (or you're directed through one) , you should do better than if you get sent to a backwater.
I play with 100% realism settings, so manual targetting, no map updates, no external camera. I don't run on the surface at a TC setting of more than 32 in areas patrolled by enemy aircraft. When submerged outside of my patrol area, I will often run at TC 1024. The reduction in hydrophone contacts compensates for the fact that there are too many ships in the game. I am beginning to wonder if there are also too many aircraft.
In 1939 to 1941, I figure I am doing well if I get 3,500 tons per torpedo. In 1942 that's down to 2,500 tons per torpedo. From 1943 onwards, I figure I am doing well if I return alive.
That's doing well. Most of my early careers averaged about 2,000 tons per torpedo. (My top career score per torpedo was in a Type IIA with 2,511 tons per torpedo.) It drops a couple hundred per year as allied ASW outpaces u-boat advances. My 1939 and 1940 scores might be higher now if I went back and started some careers at the beginning of the war, but I am playing through, from start to end.
Most of my early careers survived to retirement. In 1943 the opposite is true. In 1939-40 I started 10 careers, lost three subs, and retired 4 commanders, The other 3 also survived to retire later. In 1943 I started 8 careers, lost 5 subs (all to aircraft, I think) and retired only 2.
In 1943,
IF I can get to a patrol area, I can usually still manage to sink > 2,500 tons per torpedo. But all the trips where I have to turn back because of damage or malfunction, or those trips which I don't survive, really cut into the average. I've only had three patrols in 1943 (out of about 20) where I returned with my hull undamaged. Once was because I turned around less than one day out because of engine trouble, once I lost a diesel to aircraft, without damaging the hull, and once I was just lucky I guess.
Almost all the damage is from aircraft. Other than losing subs going through the Strait of Gibralter, I don't think I have lost a sub to surface ships since August of 1940, and that was a maniac who did a harbour raid each patrol.