The Walter propulsion system could drive a full size U-boat (submerged) at 25 knots for 3 hours (substantially faster than type XX1), the system would be carried in addition to conventional diesel and electric drives. However, the smallest speck of dirt in the containers or pipes could cause a violent explosion. Following the war, the Royal Navy commissioned a Walter U-boat into service, and later the Vickers company built two improved Walter type boats (Explorer and Excalibur). The development of Walter drives was overtaken by nuclear boats (the Nautilus covered 1400 miles at an average speed of 20 knots during early trials c1954 without surfacing).
Source: Aircraft versus Submarines, Alfred Price
A problem with guiding a torpedo using ASDIC is that the target may return fire with a homing torpedo using the the hunter's ASDIC source as the target point, additionally, ASDIC will illuminate the hunter for attack by other enemy forces. More effective to use a passive homing system and leave the attack area undetected.