15-20 meters is good and still extremely lethal as the destroyers are very accurate. Jungman's DC_20 mod located at the "U-Boat War-Mart" sets the damage radius to 20 meters, fixes destroyer sonar's lack of a blind spot, and effectively fixes radar cross-section of snorchel heads. (radar cross section was HUGE beforehand... like the size of a car maybe)
Go here and say thanks to Terrapin
http://www.mpgtext.net/u-boot/