I have no criticism of the modders, the problem was in the original game. Possibly it was toned down a bit in one of the patches, but in the original it took many hours of game time just to get to and from the operating area. Got a job, got kids, got a limited amount of time per week to play computer games, who wants to spend all his spare time for an entire week jumping out of time compression to dive from an air attack over and over and over and over and over and over and over and over again?
"When you look at the first five items on the list, about a fifth of all US losses are basically self-inflicted. Some of the unknowns may have been as well."
And Lockwood estimated that 10 of the 16 sunk by surface ships were aided by the infamous congressman from Kentucky.
http://en.wikipedia.org/wiki/Andrew_J._May
"Any idea how many weren't sunk but holed and had to RTB?"
Or damaged badly enough to have to RTB, to actually hole a fleet sub required detonation within 10-20 feet of the pressure hull. What actually sunk most U-boats was cumulative internal damage rather than one lucky hit. No stats for that, but many stories of heavy damage to systems, electrical fires and battery explosions, wrecked motors and pumps, along with routine failures like the HOR engines. There's one account of a HOR boat that had all four gears fail at sea. One thing the game doesn't cover is the small auxiliary diesel engine in fleet boats normally used to top off a battery charge, that sub used the aux diesel to putt back to base at 3 knots. S-39 (in a different book PIGBOAT 39) started one patrol four times before actually making it to her patrol area, the previous three tries she had to RTB for assorted mechanical failures before going 100 miles.
The amazing part isn't how many had to RTB due to damage, what's really amazing is the really bad ones repaired by the crew and they decided to finish the patrol regardless. One HOR boat lost an engine on the way to the patrol area and went ahead to finish the patrol on three engines, cannibalizing parts from the disabled engine to keep the other three running.